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HEATING OIL PIPING TROUBLES
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NOISE, HEATING SYSTEMS
ODORS FROM HEATING SYSTEMS
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PUFFBACKS, OIL BURNER
RELIEF VALVE LEAKS
RESET SWITCH, HEATER PRIMARY CONTROL
RESET SWITCH, ELECTRIC MOTOR
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SAFETY, HEATING INSPECTION
SAFETY RECALLS CHIMNEYS VENTS HEATERS
SOLAR HEATING SYSTEM DESIGNS
SOOT on OIL FIRED HEATING EQUIPMENT
STEAM HEATING SYSTEMS
THERMOSTATS, HEATING / COOLING
VIDEO GUIDES: HEATING SYSTEMS
WINTERIZE A BUILDING
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
ZONE VALVES, HEATING
Guide to aquastat controls on heating equipment:
This article series explains how aquastats work and what the different aquastat controls are, what they do, and how they are set. We define the HI LO and DIFF controls on heating boiler aquastats and explains what they do and how they work. We explain the location and use of the heating boiler reset button found on aquastats.
We discuss relationship among HI, LO, DIFF, and heating burner cut-in, cut-out, and circulator lockout that are provided on the combination control. We explain how the Aquastat controls hot water production via a tankless coil on the heating boiler.
We explain how to disable the LO control when a tankless coil is not in use on a boiler. After explaining the operation of an aquastat's Hi, Lo, and Diff controls we review the newer universal replacement aquastat from Honeywell, the L7224U Universal Aquastat, followed by a discussion of the simpler single-limit control switches.
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Aquastat Guide to the Reset Button & HI LO & DIFF Temperature Settings on Heating Boiler Controls - Aquastats & Primary Controllers
But with the cover on you can't see much. Here at left is part of an inside view of a typical aquastat showing the three controls that need to be correctly set.
[Click to enlarge any image]
If you just want to set the HI LO and DIFF controls and don't care how they work or what they do, see our separate article
This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.
Here we explain how to set and adjust the heating system controls to preserve this cold marriage. CONTACT us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
How to Find and Use the Reset Button on the Aquastat Combination Control on a Boiler
Summary of the Functions of a Typical Combination Protectorelay™ Primary Control and Aquastat® Controller
Our photo (left) shows an older aquastat type primary controller on an oil fired heating boiler. You can see the gray box covering the control in the right in our photo.
Depending on wiring hookup choices, this control operates the heating system oil burner, circulator, and zone circulators. It can also be configured (factory default) to keep the heating boiler hot to provide domestic hot water through a tankless coil even when the building room thermostats are not calling for heat.
The aquastat controller often also includes connections to a flame sensing device, typically a cad-cell (CAD CELL RELAY SWITCH) or on older heating systems a stack relay (STACK RELAY SWITCH) to shut down the boiler if the burner is not operating properly.
There is a "reset" button to re-start the heating system if it has shut down for safety. We discuss all of these controls in detail below.
The coiled "wire" leaving the upper right corner of this control is routed to a thermal sensor mounted in a well inserted into the water of the heating boiler.
Unlike the aquastat shown at the top of this page, poking through the upper-center of the cover of this heating system control you'll see a red "reset button" in the center of the upper portion of the gray control box.
If this heating boiler has turned itself off on SAFETY OFF you'll see that the button will have popped up higher out of the control cover.
If the red "reset" button on the this safety control is sticking up and the oil burner has shut down, the homeowner is permitted to try ONCE to "reset" the system by pressing the red reset button. Please be sure to read our more detailed instructions for using the reset button on heating equipment at our article: CAD CELL RELAY SWITCH.
Watch out: sometimes the red plastic button on older heating equipment primary controls can break off.
At RESET SWITCH, HEATER REPAIR we show how to reset the equipment safely if this happens to you.
Where are all the heating system reset buttons? If you are looking for the main reset button on heating equipment you'll want to see:
Guide to Function & Settings on Combination or Multi Function Primary Controls (Aquastats) for Heating Boilers
Here we explain how to find and use the Aquastat reset button and how to find the HI LO and DIFF controls on aquastats. We explain how the HI LO and DIFF controls function to turn the burner on and off for both heating and for systems where a tankless coil is in use, also for making domestic hot water. This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.
At night we turn down the thermostat to 55°F to try to save on our heating cost. But our hot water is made with a tankless coil. This morning my wife got out of bed and stepped into the shower for a nice hot wake up. She turned on the water, pretty hot, and stepped in to its comforting stream.
I walked over to the thermostat and turned it up to 90 - thinking (in error) that the thermostat is like an accelerator, and the higher I set it the faster the house would reach a comfortable 72 degrees (wrong). I figured my wife would be able to step out of the shower into a nice warm house. Moments later I heard screaming from the shower as my wife got hit with freezing cold water. What happened? -- anonymous.
Basic Boiler Operation & Control Sequence using an Aquastat
On a call for heat from the thermostat, the zone circulator starts when water temperature is above Low Limit setting (if applicable - that is, if this feature is enabled - the default). The heating boiler temperature is checked. The burner starts when the water temperature is below High Limit setting.
When the boiler temperature reaches or exceeds the High Limit, the burner is turned off. The burner restarts when the
When the thermostat is satisfied - that is room temperature has reached the thermostat setting, the circulator and burner are turned off. Stated with a bit of simplification, on most common boiler control set-ups in the U.S. the thermostat turns the circulator on or off, and the temperature of water in the boiler turns the burner on and off. (Heating systems in Canada operate differently: there most techs set up the circulator to run continuously if the boiler is "on" and a call for more heat at the thermostat turns the boiler's burner on until the thermostat is satisfied.)
What are the Three HI LO DIFF Controls & Dials We See Inside the Cover of the Honeywell R8182D Aquastat
Heating Control Operation Sequence: With some simplifications, the room thermostat calls for heat. The circulators start running, hot water leaves the boiler and heads for radiators or baseboards, cooler water returns from the building into the boiler. Boiler temperature drops to the cut-on or cut-in temperature.
The burner turns on and re-heats the heating water inside the boiler until boiler temperature reaches the cut-off or cut-out temperature. The circulators keep running, and the boiler burner may cycle on and off in this manner, until the room thermostat is satisfied. Then the thermostat turns off the circulators and the burner, if running, will shut off when the boiler reaches the HI limit.
In Canada boilers operate a bit differently: the circulator pump may be set to run continuously, and the thermostat just turns the burner on and off. This gives more even heat and helps avoid a frozen heating zone pipe in very cold climates. Our aquastat function explanation below is bases on the "American" model or as the Canadians say, "the Yanks".
How the Aquastat HI Limit Settings and Functions Work to Control Building Heat
HI Limit: Combination Control High Limit:
The HI cut off temperature for the burner: on a typical aquastat or combination control, the "Hi Limit" setting specifies the cut-off temperature for the heating boiler's burner on a call for heat. That means that when the burner is running it will heat water inside the heating boiler until that water temperature reaches the HI or cut-off point. Then the burner is turned off.
The HI cut-on temperature for the burner is hard wired in this HI Limit control at 10 °F below the "Hi Limit" setting.
Example: Hi set to 200. Thermostat calls for heat. Circulators turn on. Hot water leaves the boiler, cooler water returns to the boiler, boiler temperature drops to 190, burner turns on and stays on until boiler temperature reaches 200 F.
The internal view of the Honeywell R8182D (left) can be found along with additional details about this control in the Honeywell R8182D,H Combination Protectorelay™ Primary Control and Aquastat® Controller Installation Instructions (link to copy below at REFERENCES).
Following wiring details in the instruction manual, the R8182D,H can replace other aquastat controls such as the Tradeline R8182B, R8182E, R8182F, R8182H, R8182J, or White Rogers 6C92.
A newer combination control from Honeywell, the HONEYWEELL L7224U Oil Electronic Aquastat Controller can replace more than 40 older hydronic controls.
How the Aquastat LO Limit Settings and Functions Work to Provide Domestic Hot Water & Circulator Lockout
During the summer, because the thermostat never calls for heat, the HI control will never turn on the boiler. The HI limit operates a relay that is turned on by the thermostat and turned off by the boiler reaching the HI or cutoff point.
Out of heating season, your thermostat stops asking for heat, so the HI goes to sleep.
Out of the heating season, the LO gets to work, as we describe here.
What the LO limit actually does: The LO limit is a setting which is intended to maintain heat inside the boiler in order to assure that the boiler can produce hot water when a tankless coil is installed and when the room thermostat's calls for heat are not already keeping the boiler nice and hot.
During warm months when the boiler is not being called-on to heat the building itself, the "Low Limit" keeps heat in the boiler for the tankless coil. So the "Low Limit" is actually a "low range" operating upper limit on boiler temperature that applies out of the heating season or when the thermostat is not calling for heat.
On typical aquastat heating control the LO or "Low Limit" is NOT the "cut on" point for heat (we explained the heat cut-on just above).
It is helpful in understanding the LO limit to call this the "circulator" controller. Provided that DIFF is set to 10 (its smallest value), then
when boiler temperature falls 10 °F below the LO setting, the burner turns on and the circulator is forced to turn off - we call this "circulator lockout" - we are re-heating water in the boiler and we are preventing boiler water from being sent out to the baseboards or radiators - giving priority to heating the tankless coil.
when boiler temperature rises back up to the LO setting, the burner turns off and the circulator is allowed to turn on - we are unlocking the circulator and allowing the building to be heated again - and taking priority away from giving heat to the tankless coil.
We say the circulator is allowed to turn on because if the room thermostat is not calling for heat, the circulator may have permission to turn on, but the thermostat is leaving the circulator(s) turned off (in the U.S., not on many Canadian heating systems).
And note that we say that the burner turns off as the boiler temp rises up through the LO setting. But if the boiler is being turned on from a fully cooled-down "cold" condition and the room thermostat is calling for heat, the burner will turn on and keep running right up through the LO and on to the HI. Why? Because the thermostat was calling for heat.
Circulator Lockout: The LO limit works in concert with the DIFF setting to control the operating temperatures of the boiler when it is being asked to heat the tankless coil (used for making domestic hot water for washing and bathing), and together they also control when the circulator pump should be locked out so that priority is given to keeping the boiler itself hot - presumably because you're in the shower and the tankless coil is in use.
At AVOID CIRCULATOR LOCKOUT we provide more detail about this function.
The DIFF or "differential" setting on an aquastat adjusts the LO range temperature cut-off point when boiler temperature is rising (the burner is on). The DIFF ONLY talks to the LO control, it has nothing to say to the HI control.
Even though the LO and DIFF settings will allow the circulator to run at certain temperatures, the circulator pump(s) still won't run unless the room thermostat is calling for heat.
When boiler temperature is falling: DIFF settings at values other than 10 do not affect the temperature at which the burner turns on and the circulator locks out when boiler temperature is falling.
When boiler temperature is rising: on this control, the "DIFF" or differential control dial specifies the amount above the "Low Limit" to which boiler temperature must rise before the burner will turn off and the circulator will be allowed to run.
We said the circulator pump is allowed to run because if the thermostat is not calling for heat, even though the DIFF + LO are allowing it, the circulators will not run (in the U.S.).
Watch out: as we explain below, realizing that tankless coils on heating boilers, especially modern small high-efficiency boilers, have rather limited ability to deliver much hot water to the building, most users set the "DIFF" control to its smallest number, 10, thinking that means the burner will come on as soon as possible when it's needed to make hot water out of the heating season. As Damian pointed out in email, that's a mistake if you want as much hot water as possible out of your tankless coil.
At this "lowest" DIFF setting of 10, on a boiler temperature rise, the burner turns off (R-B breaks) and the circulator turns on (R-W makes, to deliver heat) at the LO limit (and "circulator setting" setpoint. So if the LO is set to 120 and the DIFF is set to 10, when the boiler temperature rises to 120 the burner turns off and the circulator is allowed to run - that is to circulate water out of the boiler to the baseboards or radiators.
We call the LO the the "circulator lockout" control or the "tankless coil control" to try to make this more clear. In other words, when the LO is satisfied (we have hot water for the tankless coil), then we can go back to delivering heat to the building IF the building room thermostat is asking for heat.
At any DIFF setting above 10, the (R-B make - burner on) and (R-W break - circulator off) temperatures remain the same as we just described above. LO control setting minus ten degrees.
But the (R-B break - burner off) and the (R-W make - circulator on) temperature will now change to be the LO set point temperature plus the difference between the DIFF set point number (for example 20) and ten degrees. Honeywell gives a helpful example: (refer to the yellow area in the sketch to help understand this feature) [we edited the original text slightly for clarity]:
Adjusting the DIFF to numbers higher than 10 does not change the boiler turn-on temperature ("R-B make") nor the circulator turn-off temperature ("R-W break"). But it does affect the boiler turn-off temperature (R-B break) and the circulator turn-on temperature (R-W make) as follows: the new boiler-off and circulator-on temperatures will be equal to the LO setting plus (DIFF-10).
More HI LO DIFF Setting Examples:
LO = 120 F, DIFF = 10 F: when the boiler temperature drops to 110 the burner turns on and the circulator is turned off. As the burner re-heats the boiler and the boiler temperature rises back up to 120, the burner turns off and the circulator is allowed to turn on.
LO = 120 F, DIFF = 25 F: when the boiler temperature drops to 110 the burner turns on and the circulator is turned off, just as before. But as the burner re-heats the boiler and the boiler temperature rises back up to 135 F, the burner turns off and the circulator is allowed to turn on. We calculated the 135F as follows: LO setpoint of 120 is added to (DIFF minus 10) or 120 + (25-10) = 135.
LO= 140 F, DIFF = 25 F: when the boiler temperature drops to 130 F the burner turns on and the circulator is turned off, because the burner-on temperature is always fixed at 10 below the LO, just as before. But as the burner re-heats the boiler and the boiler temperature rises back up to 155 F, the burner turns off and the circulator is allowed to turn on. We calculated the 155F as follows: LO setpoint of 140 is added to (DIFF minus 10) or 140 + (25-10) = 155.
The effect of setting the DIFF up from 10 to 25 is that when the burner is re-heating the boiler (for example while the tankless coil is in use and you're in the shower), the burner heats the boiler temperature up to a higher level before the burner is turned off and the circulator is allowed to turn back on. This gives more heat to TANKLESS COILS and therefore more domestic hot water to the building occupants.
This Honeywell Aquastat Chart Further Explains of How the HI & LO Limit & DIFF Controls Work on an Aquastat
Even many heating service technicians (those who failed to read the installation instructions for the control they are servicing) are confused about the relationship among the three adjustable controls on the aquastat: the High Limit Setting, Low Limit Setting, and Differential or "Circulator Setting".
Our text (below) and our adaptation of Honeywell's sketch (left) explain these functions.
[Click to enlarge any image]
Here we reiterate the explanation above, trying a little different approach that may help some readers.
On primary controls (aquastats like the like the Honeywell R8182D, the Honeywell L8124A, and the Honeywell L8151A) the HI Limit control (and dial) operates at a heating boiler water temperature range defined as follows:
How the Aquastat HI or High Limit Works
HI (High Limit) sets the boiler Cut-off temperature or burner turn-off temperature on a call for heat - the temperature to which the HI dial is set.
For most aquastats such as in the Honeywell R8182D,E,F,H,J Combination ProtectoRelay™ hydronic heating controls, the High Limit works the same: the high limit "opens" (disconnects) a switch to turn off the burner when water temperature reaches the HI set-point. As water temperature in the boiler later drops a fixed value below the HI set-point the control automatically re-sets and turns the burner back ok provided that also a thermostat is calling for heat.
In sum: the boiler Cut-on temperature or burner turn-on temperature for the boiler, AS LONG AS THE THERMOSTAT IS CALLING FOR HEAT is fixed at 10 degF below wherever the HI is set. (Green in our edited version of Honeywell's drawing.) On some controls this hard-wired fixed gap may be 15 °F.
So if HI is set to 200 °F that's the cutout temperature, and the cut-on temperature for the boiler, as long as the thermostat is asking for heat, will be (200 - 10) = 190 degf (or 185 °F on controls with a 15 degree fixed gap.)
See details about the HI, LO and DIFF optimal settings at AQUASTAT HI LO DIFF SETTINGS
How the Aquastat LO or Low Limit & the DIFF or Differential Work on an Aquastat
On combination controls such as the Honeywell R8182-series Protectorelay™ aquastat, the LO and DIFF are used to control the boiler when it must provide heat for a tankless coil used to produce domestic hot water (for washing & bathing). These two controls work in a way that has confused generations of homeowners and even some heating service technicians. We'll give a couple of explanations of how these controls work.
Three Key LO/DIFF Aquastat Function Concepts:
Operation of the R8182-series Aquastat LO and DIFF with DIFF set to its lowest setting: 10 degF
When discussing the LO and DIFF settings, the of the following actions occur in the yellow colored range on our illustration and all of them occur with respect to the LOW LIMIT setting on the aquastat:
1. On boiler temperature rise up to the LOW LIMIT SETTING (e.g. 140F )the circulator is turned on (or is allowed to turn on) and the burner is turned off.
[This "burner-off" may be over-ridden and the burner may be turned "on" IF at the same time the room thermostat is calling for heat.]
On our illustration this is indicated by the horizontal line in the middle of the yellow-area and labeled on the left as "Low Limit and Circulator Setting" and labeled also on the right as "Switch Makes R-W" and Breaks R-B on Temperature Rise.
2. On boiler temperature further rise up to 165 (140F + 25F = 165F where that 25F is hard-wired into the control) the circulator is turned on (or allowed on) AND the burner is turned OFF.
[As in case 1 above, this "burner-off" may be over-ridden and the burner may be turned "on" IF at the same time the room thermostat is calling for heat.]
This is equivalent to saying that the boiler is hot enough to both supply domestic hot water thorough a tankless coil AND to permit heat to be delivered to the heating baseboards or radiators if the temperature is in this range (140-165F).
This point is at the top of the yellow in our illustration.
Honeywell points out in their triangular note 1 that when water temperature reaches 165 degF (LO + 25F) the burner is turned off OR IF there is a call for heat the circulator pump will run.
Note: If heat is being called-for the circulator will continue to run, dropping the boiler temperature as cooler water returns from the chilly building back into the boiler. When the boiler temperature drops the burner will be turned back on as discussed in these notes.
3. On boiler temperature fall to 130F (140F - 10F set on the DIFF) the aquastat will turn ON the burner ("Makes RB"), and will turn OFF the circulator ("Breaks RW").
The purpose of this point is to stop circulating hot boiler water through the heating baseboards or radiators in order to give my family member, standing in the shower with a head full of shampoo, priority for receiving hot water.
Operation of the R8182-series Aquastat LO and DIFF with DIFF set higher than 10 degF
What would be the optimal setting for the LO & DIFF settings for fuel savings with low and with moderate hot water usage in the summer months when the boiler functioning for heat does not come into play. If you would please identify the low usage from the moderate usage settings it would be greatly appreciated. - Mark
This is an excellent article on the DIFF setting as it pertains to using a Tankless Coil. During the summer in North Carolina when outside temperatures are in the 90's and my system never calls for heat, I am only running my Burnham boiler to provide hot water. It is not unusual to burn 1 gal of oil/day (30 gals / month) just for residential hot water. With oil prices well above $3.50/gal, $100/ month for hot water is a little extreme.
I am curious if some combination of LO Limit / DIFF might cause the burner to run less while still maintaining adequate heat to provide hot water. For example, if my current settings are LO 160/ HI 180/ DIFF 10, might adjusting the LO/DIFF settings either up or down lower oil consumption? I suppose another option could be to use an electric hot water heater with a tank during the summer months. - Tony
I have a VERY old system. Have two separate aquastats, one used for LO and one used for High. No Diff. I have a separate hot water heater. Because of the old system the oil company technician feels that I should keep the furnace on during the summer so that there isn't a "shock" to the system when it is turned on in the fall. Question: What are the best HI-LO settings in the summer and in the heating season.? - Steve
Mark and Tony and Steve: For summer use, if you set the LO to its lowest temperature setting (typically 102 F) you will be keeping minimal heat in the boiler to support the tankless coil in summer and will lower your heating fuel consumption by as much as 30%.
For summer use, if you set the DIFF to its lowest setting you will also lower your fuel consumption.
The result of these settings will also be a reduction in the quantity and also the maximum temperature of hot water supplied to the home - which may be OK for cases when hot water use is only moderate.
If you have an automatic tempering valve or mixing valve installed on your hot water supply that will increase the total hot water quantity available by drawing heated hot water from the boiler only at the rate needed.
You can leave the HI setting on your Aquastat alone in summer; since the system will never be calling for heat, the HI will be asleep all summer.
If you set the LO down to a lower number in summer you will be keeping less heat in the boiler, and in that sense burning less oil; but if your mixing valve is a MANUAL one you may need to adjust it so that water at the tap is hot enough.
If you keep the DIFF to its lowest number in summer you're also minimizing the oil burner on-time. Or conversely, if you set the DIFF to a higher number (say in summer), quoting from the article above:
The effect of setting the DIFF up from 10 to 25 is that when the burner is re-heating the boiler (for example while the tankless coil is in use and you're in the shower), the burner heats the boiler temperature up to a higher level before the burner is turned off.
Given that you are not using your heating boiler for making domestic hot water, the reason a service tech would recommend keeping it on in summer is most likely because some older cast iron boilers can develop leaks between the boiler sections if the boiler is allowed to cool off to stone cold. Such a leak may not "heal" on its own when heat is restored, and it can lead to worse boiler damage.
For summer use on such a system, keeping just about any heat at all in the system, say 100 °F should be sufficient. If your LO is currently set above that number, try setting it down for the summer - but don't forget to restore it at the start of the heating season.
I am inferring that the HI on your system is an upper limit on a call for heat and won't be operating in summer. If I'm mistaken, just keep the HI 20 °F above the LO.
Right now I have baseboard heat and I have the boiler set at HI-170 LO-150 and DIFF at 10. Is that a good Setting. I'm trying to save money on oil. thank you - Matt
I'm still not 100% sure on what my settings should be. I have a Weil Mclain boiler that's about 6 years old with a DHW coil installed. I had a 30 gallon electric hot water heater installed after the coil, and during the summer I would shut off the boiler and use only the hot water heater. During heating season the water would be pre-warmed by the coil and the electric heater wouldn't be doing much.
Problem is, we seem to be using a lot of oil. The boiler is constantly running, every time we use the shower, run the dishwasher, wash our hands, etc. It's also constantly kicking on to keep hot water in standby. During the early heating season, it's a waster, since the boiler isn't on very much to provide heat to the house. What should my settings be? Will I save more money by just not using the coil and telling the aquastat there's no coil? - Matthew
I live in Maine and the winter season is here. What would be a good Hi and Low setting for my aquastat? Its is starting to dip below 10 degrees at night. Thanks. - Nick
Nick and Matt:
Under AQUASTAT CONTROL Functions (the article above on this page) you'll find good aquastat HI LO and DIFF setting recommendations in the article titled Optimal LO/DIFF settings save fuel
Matt: you can significantly save on heating costs by lots of other measures including using a thermostat setback to lower temperatures at night or when the home is unoccupied. See HEATING COST SAVINGS for our suggestions of a variety of ways to save on heating costs, and importantly some priorities: what heat cost savings steps should we address first at a building?
Our aquastat shuts off burner at 190 degrees but old and new temp gauge continues to rise to 215 degrees. is this normal? - Warren G.
Warren: it is normal for temperatures inside a heating boiler to continue to rise a bit after the Aquastat has turned off the oil burner at the HI or when the thermostat has stopped calling for heat.
This temperature rise may occur because:
As long as boiler temperature does not rise high enough to cause the pressure/temperature relief valve to spill you should be fine.
I am currently installing a hot water heater to replace a tankless coil in my boiler. Do I need a different aquastat to keep my boiler from trying to heat the coil? Can I replace my present aquastat #l8148a with #l8124a??? - John Haines
We installed an electric hot water heater, do we have to change anything to make the boiler function just for heat and not hot water (for bathing, dishes etc) ? - Ernie
I have replaced the tankless coil in the boiler with a gas water heater thus eliminating the need for the boiler to heat water for bathing,washing etc. question does the aquastat still needs to be connected for my boiler to run to heat the house? - jorge gonzalez
Take a look at the instructions that come with the aquastat and you'll see a list of devices it can replace. But most likely you do NOT have to replace your aquastat with a different model just because you are ceasing to use the tankless coil for domestic hot water.
On the aquastats discussed here, the instructions (at Disabling LO & DIFF) describe a simple process for disconnecting a wire that will disable the low-limit control, thus eliminating that function when a tankless coil is no longer in use.
Ernie, When you abandon or stop using a tankless coil on a heating boiler, you can save on heating fuel consumption by disabling the LO / DIFF feature on your aquastat. The effect is that the aquastat (primary boiler controller) will stop keeping the boiler hot when there is no call for heating the home itself.
If you leave the LO/DIFF working, the boiler will keep heating itself even in summer when there is no call for building heat - because it thinks it's staying hot to support use of the (now abandoned) tankless coil.
Jorge, you still need the aquastat control, since it has to manage the boiler on and off temperatures when the wall thermostat(s) call for heat in the home itself. But as we explain above, the aquastat controls can be changed to stop keeping heat in the boiler to support the tankless coil that is no longer in use.
The procedure for disabling the tankless coil heating system is in our Aquastat Settings article at Disabling LO & DIFF
Dan, thanks for answering my questions on my recent hot water heater installation. I disconnected the differential wire (see Disabling LO & DIFF) on my aquastat and now my system works perfectly. - Tony
Hi Dan. ..Thanks for answering my question from June 28, 2011. I did, in fact, reduce the LO limit setting this summer from 160 to 120 and it reduced my oil consumption for heating hot water by about 30 percent. I left the DIFF setting unchanged at 10. To be honest, I was a little unclear from your response about the effect raising/lowering the DIFF setting would have on overall oil consumption during the summer months. However, adjusting the LO setting definitely had the desired effect. It was not necessary to adjust my manual mixing valve because the shower water was at a comfortable level. Thanks for taking the time to answer my question and for the valuable information contained on this website! - Tony
Thanks for the feedback that lowering the LO setting will save heating fuel during summer months. We have edited the HI, LO, and DIFF functions above to make that text more clear.
Basically, by lowering the LO to its lowest point (120) you reduced that amount of heat kept in the boiler during the summer (when the only function of the boiler is to stay hot in order to heat the tankless coil used to make domestic hot water). Like you I'd have also set the DIFF to 10 as well.
If you are continuing to use a tankless coil for your home hot water supply, you might want to set the LO back up to 20 degrees below the HI in winter because in cold weather the tankless coil is receiving colder incoming water from the building public or private water supply.
Hi Dan. ..I just wanted to clear up that the 30% savings you mentioned in the article were those that I reported in my post as a follow up to the June 28, 2011 post. ...
I can confirm the excellent point you made about setting the LO back up to 20 degrees below the HI in winter because of colder incoming water from the public water supply. It is now late October in NC and I still have the LO limit setting from this summer at 120 with the DIFF setting unchanged at 10. I have not begun to use the system for heat yet and am only running the boiler for hot water.
I have noticed a definite decrease in shower water temperature and will shortly need to reset the LO back up to 20 degrees below the HI. Again, thanks for all the valuable information you provide on your website! - Tony [October 2011]
There seems to be some conflicting info on the site. In some articles, you say that the DIFF is the temperature rise over the LO setting. For example, DIFF 15 and LO 120 would mean you'd have a range of 120-135. In this article, it's explained that there always a 10 degree drop below LO and a rise above LO of DIFF-10, meaning DIFF 15 and LO 120 would give you a range of 110-125. Which is true? And what about aquastats that have a 5 degree DIFF setting? - Brian
Thank you for the clarification request, Brian. We've reviewed our HI LO DIFF setting articles to be sure that the text is now self-consistent.
HI Limit Definition: The HI is the cut off point on a call for heat. You can think of the HI dial as the "heat control" in this regard: as long as the room thermostat is calling for heat, when the boiler temperature falls 10 degrees below the HI the burner will (normally) turn on. That 10 degrees is hard-wired into the control - it's not adjustable.
LO Limit Definition: the LO is a control function added to keep heat in the boiler when the room thermostat is not calling for heat. This feature is important on heating boilers that use a tankless coil immersed in the boiler to produce domestic hot water.
The DIFF Definition: the DIFF is an added control that defines the cut-off temperature (above the LO) during a burner-on cycle initiated by the LO control - that is, we are not calling for heat, and we're keeping heat in the boiler to make domestic hot water. It works as you described.
Circulator lockout: There is an interaction between the LO and DIFF settings and the delivery of heat to the building however. That is that the LO operating range on an aquastat also will lock out the heating circulator pump - stopping delivery of hot water out of the boiler and into baseboards or radiators - if the boiler temperature is too low. (That's why aquastat instructions specify that the LO should always be at least 20 °F below the HI setting).
The intent is to give priority to the tankless coil and thus the person in the shower. For heating boilers at homes that do not use a tankless coil to make domestic hot water, it is sometimes (not always) reasonable (and a savings on fuel during summer months) to disable the LO/DIFF controls - as we explain at Disabling LO & DIFF
See Aquastat control HI LO settings for details about how the HI and LO settings work.
See Aquastat control DIFF settings for details about the DIFF setting and how it works
See Optimal LO/DIFF settings save fuel for some recommended settings to save on heating fuel cost
As Tony said in his comment above, I also was using about 1 gal of oil per day during the non-heating months. In Connecticut, we are paying close to $4.00 per gal now. I installed an electric water heater in parallel (NOT series) with the boiler's tankless heater. Either the tankless or the electric can be used since the water does not go through one and then the other. (Glad I still had the tankless piped up since I could still have hot water during the recent 10 day power outage in Connecticut. The 4000 watt generator could run the boiler but not the 4500 watt electric heater.) I found that we are using about 11 kwh/day for domestic hot water by timing how long the 4500 watt heater elements were on each day - a 240 volt relay activating a battery powered clock.
The cost at $0.17/kwh is about $1.90/day - better than 1 gal/day at $4.00. I turned off the boiler when I turned on the electric water heater. My concern is that when I went to tune-up the boiler for the 2012 heating season, I found about one cup of rust (some of the material was magnetic) on the bottom of the firebox below the cast iron boiler sections. I open the firebox for cleaning each year and have not seen this much rust before. I have brush cleaned the flue passageways twice since new and don't recall seeing this type of material. The boiler is a Dunkirk that was new in 2001. Water leakage does not appear to be a problem. Should I be worried about damaging the boiler (corrosion) by letting it go stone cold during the non-heating season? - Buddy 1/12/12
Buddy, thanks for the important comments & question. Indeed on many boilers, if you leave the unit shut down for months there can be problems with rust and also with solidification of soot and crud deposits that make seasonal cleaning more difficult (and perhaps costly).
If the boiler and tankless coil is left "on" but not in use, it will run only very little, as it runs just to keep itself hot - it's never cooled down to a longer on-cycle by having moved its hot water out into heating zones.
Alternatively at least it's a good idea to have the annual service/cleanout done in the spring at the end of heating season - don't let it sit over the summer.
I am thinking about retrofitting my 15 years old oil burner with new Honeywell electronic aquastat which includes Outside Temperature Reset module. Has anybody done it ? This would allow me to adjust HI water temperature as a function of outside air temperature. They say this would result in 10-20% fuel savings. Has anybody done it ? Is it worth doing it ? Thank you. - Pavel 2/13/12
The retrofit aquastat control you describe will function properly if properly connected. the actual heating fuel savings ... well that depends on quite a few variables. "Up to" claims, in advertising law, need be true only about 10% of the time.
(Feb 10, 2014) Tom said:
Hi - I am trying to better understand the operation of an Aquastat. If have hot water coming out of my gas water heater (actually cold for a while cause it is a long run) does it hold off delivering water to the output connection (to sink) until the water is of the correct temperature? While that is happening, is it correct that the cold water in the feed pipe from the water heater to the Aquastat goes back around into the home's cold water supply?
(Nov 24, 2012) Radhames said:
I have a room that does not get hot enough, the radiators in this room are the furthest in line...so by the time these start to get warm the boiler shutts off. Should I raise the temperature on the Hi and Lo settings, raise the temperature on the thermostat or relocate the thermostat. Thanks
(Jan 3, 2013) Brian said:
I have a unique setup with a boiler and aquastat that heats (3) zones - one is the coil air exchanger to heat the indoor pool room, the next a line for the hot tub and lastly a line for heating the pool. Just this week we noticed the hot tub was to the point of overflowing a few days in a row, and then this morning was pretty much drained, and the pool level higher. Is this a failure of the aquastat or the boiler itself?! Thanks in advance.
(Jan 9, 2013) Cathy said:
I have a 3 zone boiler heating (hydronic)system. Upstair work fine but lower level only works when we bleed the valves.after closing the valves downstairs the system there gets cold again even though the circulating pump is still running. The aquastat is set at 180 degrees and shuts off at exactly 180 degrees when thermostat temperature is reached but on lower level which is on a separate thermostat,it doe not shut off like upper level does, the temperature goes up to 230 degrees.We have been bleeding the system continuously and have had 4 plumbers come and no one can find the problem.
(Jan 15, 2013) PHILIP said:
I juat replaced the aquastat, the furnace is lit NO HOT WATER circulating... OUCH
(Jan 23, 2013) Greg Macpherson said:
Ihave a heat exchanger hooked up to a coal boiler. Iwant an aqua stat to turn the circulating pump on and off, which one do i need?
(Feb 26, 2013) Joe B said:
I am running a Honeywell triple aquastat L8151a with (1) circulator pump and (3) zones. My thermostat is calling for heat, and the zone valve is open. The circ pump doesn't come on line and therefore the boiler does not go online either. I have replaced the pump and have been able to make it run by forcing the circ pump relay closed. Of course as the cooler water enters the boiler, the boiler lights off. I have checked all Hi and Low and Diff settings. All look within the parameters described above. Is it safe to assume that the aquastat needs to be replaced? Is this aquastat suitable for this kind of (3) zone, (1) circ pump operation? This boiler does include the domestic hot water service.
Tom, the aquastats used on a heating boiler, because they have more jobs to do, are the devices shown on this page;
a domestic hot water for washing and bathing, if made in a dedicated water heater, perhaps in your case a hot water tank heated by a gas burner, will also have a limit control, but a simpler one - search InspectApedia for Water Heaters to see those controls.
In any case, no aquastat holds off on delivering water - these are temperature controls that turn a burner (oil or gas) on and off.
For your second question: most residential hot water supply systems (we're not talking about heating water in baseboards, we're talking about hot water at the sink) are one-way systems: cold water input pressure at the water heater pushes hot water out of the water heater on to the building fixtures when someone opens a hot water valve - say at a sink.
There are hot water circulating systems, used e.g. in apartments and other large buildings, that keep the domestic hot water supply circulating through a loop of piping all the time so that there are not long waits for hot water at the fixture; those systems include an additional circulator pump and heater control.
From just your question I can't say what you've got installed. You're welcome to use our CONTACT link to send photos if youy like.
(Feb 19, 2014) Mark said:
My boiler is firing on and off rapidly. It seems that the contactor inside the aquastat that controls the burner is opening and closing when heat or hot water is called for. Can this be replaced individually or does the entire aquastat need replacing?
Mark I am not sure what's going on, but one would think that the problem is one of proper control wiring and adjustment, not a need to replace the equipment. Take a look at our advice on aquastat control settings to see if there is an obvious mistake with the HI LO and DIFF settings on your aquastat
(Feb 20, 2014) Mark Thomas said:
Thanks for the reply, Dan. I guess I should have been more clear. My boiler is 14 years old and this problem just started.
OK Mark. So IF nobody has changed the HI LO DIFF settings and they're at typical numbers, say 180, 160, (we'll excuse the DIFF number) and the control is behaving erratically, it still could be a flame sensor problem, service problem, or indeed there could be a failing relay in the control.
If you hear buzzing or clicking at a relay that'd be a clue.
In a multi zone hot water heating system typically a relay in the primary control operates one circulator and additional circulator controls (and relays) operate each additional circulator. So most likely for an indirect fired water heater there is a separate circulator relay.
One would think that the problem then would lie in the
- the water heater's sensor monitoring hot water heater temperature
- the control wiring for the hot water circulator control
- the circulator control itself
- a loose wire somewhere in that system
BUT if your system is wired as they usually are in the U.S., it is the heating boiler temperature that is sensed by the aquastat on the boiler (not on the indirect fired water heater tank) that turns the burner on and off. If that equipment is working properly the boiler will turn on when boiler temperature (not your indirect water heater temp) drops to the LO or cut-in and the boiler would keep running until boiler temp reaches the HI or cut out temp.
So I'm back to looking for a bad temp sensor on the boiler, bad burner relay, or a loose wire in that circuit. Of course there could be some other issue like a bad relay in the flame sensor safety control.
BUT again, and thinking as typing: if the problem were in the boiler controls it ought to show up on a call for heat not just on a call for hot water.
If the problem is in the hot water control wiring (maybe it's not wired as I'm guessing) then the problem would only show up on a call for hot water.
By turning DOWN the thermostats in the home (so to be sure you're not calling for heat) and running hot water until your water heater is cold and you're calling for hot water you ought to be able to sort out the two.
And this is an example of flying blind: I assume from your comment your "hot water" is an indirect fired water heater; if your system is using a tankless coil then everything is right at the boiler and all the diagnostics are in its aquastat.
(Apr 4, 2014) Anonymous said:
if I eliminate tankless coil and put in electric water heater, what happens with aquastat? I have forced hot water for heat
You don't have to do anything, Anon, but if you want the boiler to stop maintaining temperature when there is no call for heat (which it will do for the tankless coil), then you want to read
(Apr 12, 2014) Jimmy said:
How do I correct the the temputure control with electrical settings
(Apr 21, 2014) Chanthy said:
My boiler won't circulate the hot water. I have changed the pump and the aqua stat has a broken pin where the switch would click. Does that make a difference? The switch is clicking/pulling in but hot water won't circulate. Any suggestion what could be wrong.
First check that the thermostat is calling for heat and the thermostat wires are pulling in the circulator relay.
In addition to checking and repairing the aquastat if its relay is not pulling in to turn on the circulator, check that any zone valves or manual control valves are open (if present).
(Oct 20, 2014) Ray said:
I have a tankless Peerless WB-3 boiler with a Honeywell L4081b 1047 aquastat. I don't need the boiler for domestic hot water because I have a separate electric water tank, so I only use the boiler for heat. Right now the aquastat is set to H 180 L 160 Diff 10. Should I change the settings? Can I disable the aquastat and how do I do it on that specific model?
You don't have to do anything, Ray, but as we noted earlier in this FAQs series, if you want the boiler to stop maintaining temperature when there is no call for heat (which it will do for the tankless coil), then you want to read
(Nov 9, 2014) Blacklab said:
What Honeywell control could be used with a coal fired ,hot water system to CUTOFF power in the event of a No-Fire or Fire -Went Out condition?
Ask your heating service tech about cad cell relays.
(Nov 21, 2014) Don said:
what happens to a l4081b aqua stat when it gos bad
Don there are several failure modes in aquastats:
- a failed relay no longer switches on or off as it should
- circuit board components can burn-up
- a temperature sensor may fail
- wiring connections to the device may be loose or broken or shorted
(Nov 28, 2014) Anonymous said:
my boiler will only go on when i put thermostat to 72 degrees or higher boiler and circulator turn on together
Anon the thermostat has to be set above room temperature for heat to run.
Also, check for dust clogging in the thermostat.
(Nov 29, 2014) Ken keller - email@example.com said:
I have a 3- zone hot water baseboard system. Oil fired boiler, separate electric hot water heater. The other day I had to rplace two old and leaking zone valves.
The Plumber and the electrician wired up the new zone valves and said they won't be able to turn on the furnace with the thermostat because the new zone valves don't work like the old valves. The only way the furnace will heat the water is for the upstairs zone to be set on high. ?? Please help me !!
A separate electric hot water heater - providing domestic hot water for washing and bathing - has nothing to do with an oil fired heating boiler nor its controls. It is a completely separate appliance.
In typical heating boiler operation (for heating your hot water baseboards) when there are three zones, each thermostat is wired to a zone valve. When the thermostat calls for heat the zone valve opens. When the zone valve is fully open, an end-switch in the valve closes a second switch that connects by wires to the boiler's primary control to turn on the boiler.
Traditionally in a 3 zone system there is a slight difference from what I just said: one zone, probably your upstairs zone, is wired directly to thermostat contacts on the boiler's primary control - that thermostat turns on a circulator and when needed, the boiler. The second two zones work as I described above.
So I suspect that the new zones installed are either improperly wired or were not properly chosen. It makes no sense to me that you'd have to call for heat where you may not want it (upstairs) just to get heat where you do want it in a separate zone (say downstairs).
Call the heating service manager at your service company and ask for help from an experienced heating service tech.
Keep us posted.
what would be a good range for the two sensors (Dials) on the honey well L4010B aquastat oil fired boiler w/ coil and circulator 1 zone heat Thanks
forgot to mention ..a lot of info for a layman to absorb
Kris you can see
but indeed a layman should not have to understand heating system controls - you should be able to ask your heating service company for this help at the time of annual service or any time your heating system is not working.
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We provide links just below to several aquastat installation, setting, and adjustment documents in response to reader requests and comments that people sometimes have difficulty finding this information. But readers looking for specific aquastat control information should always first try the control manufacturer.