TANKLESS COILS - home - CONTENTS: Tankless Coils on heating boilers: a guide to tankless coils for domestic hot water supply: capacity, safety, clogging, adjustment, cleaning, & Repair Guide. How much water can we get out of a tankless coil before running out? What can we do to improve the amount of hot water from a tankless coil? Preventing scalding where a tankless coil is used for hot water - setting the hot water temperature and using anti-scald valves. Use of a Single Function Boiler Temperature Limit switch and Anti Scald Valves with Tankless Coils
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Tankless coils unwound: using a tankless coil for domestic hot water for washing and bathing.
Here we explain the function, use, capacity, inspection, and repair of Tankless Coils on heating boilers: a guide to tankless coils for domestic hot water supply: capacity, safety, clogging, adjustment, cleaning, & Repair Guide
This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
A Guide to Tankless Coils for Domestic Hot Water Production
Tankless coils used on heating boilers: This device, basically a coil of finned copper tubing which is inserted into the heating boiler, is used to provide
domestic hot water to some buildings.
[Click to enlarge any image]
Watch out for leaks at piping fittings or more seriously the coil mounting plate which bolts the coil to the
boiler (leaks at this location can destroy a steel boiler).
Watch out also for missing a mixing/tempering valve which mixes cold in with the outgoing hot water to avoid scalding temperatures
at nearby taps. Some building jurisdictions require a separate temperature/pressure relief valve on hot water piping at the boiler.
The photo shows a pile of tankless coils found in a building basement next to the heating boiler. We suspected that high mineral content
in the building's water supply was causing frequent coil clogging.
Hot Water Temperature Control with Tankless Coil Heating Systems
The heating boiler, hot water or steam, will include a temperature limit control switch dedicated to maintaining temperature in the heater's interior for purpose of heating the coils of the tankless coil.
As the illustration at left shows, domestic hot water is heated by passing cold water through a finned copper coil which is in turn immersed in hot water inside of a heating boiler.
The temperature and effective quantity of domestic hot water received at a fixture in the building for washing and bathing is set, affected, or controlled by these things:
The water temperature inside the boiler and in contact with the tankless coil
The volume of hot water inside the boiler in contact with the tankless coil - larger boiler or larger volume of hot water in the boiler means that there is more heat to transfer into the coil
The thermal mass of the boiler itself - steel boilers have less thermal mass than cast iron boilers of the same size, and modern, small-sized high-efficiency heating boilers are likely to have much less thermal mass than an antique and huge cast iron boiler. Running the small efficient boiler at higher temperatures can grab back a little of this thermal storage.
The rate at which incoming water passes through the tankless coil - faster flowing water absorbs less heat from the boiler. HOT WATER DELIVERY SPEED UP can speed up the water flow, at the cost of drawing heat out of the boiler faster, thus possibly reducing the time duration over which hot water is provided.
The settings of the aquastat or low-limit LO and differential DIFF controls. Keeping the boiler at a higher temperature and setting a higher DIFF will in general store more heat in the boiler and should thus allow a tankless coil to provide a greater volume of hot water before the system runs out of heat.
See AQUASTAT CONTROL Functions.
The settings of the anti-scald valve or mixing valve - setting the anti-scald valve to a lower output temperature means that it mixes more cold in with outgoing hot, drawing heat out of the boiler and through the tankless coil more slowly than otherwise. You get "longer hot water on time" because you draw heat out of the boiler more slowly.
See ANTI SCALD VALVES.
Considering these factors, you can understand that converting a large old cast iron home heating boiler with a tankless coil to a new small high-efficiency steel heating boiler that also includes a tankless coil may result in a new heating system which is more efficient and economical, but it may provide less total domestic hot water than the old system.
Use of a Single Function Boiler Temperature Limit switch and Anti Scald Valves with Tankless Coils
Here is a photo of a more traditional single-function heating boiler limit controls. In this example the limit switch is being used on a tankless coil, and in the enlarged version of the photo you'll see that the temperature limit on the control is set to about 140 degF.
When a heating boiler uses a tankless coil to produce domestic hot water, a third single-function control may be installed for that purpose. In this photo a Honeywell limit control switch is being used to monitor hot water temperature at the tankless coil which is in turn mounted on a steam boiler of an older home in Portland, Maine. You can see the black-handled mixing valve in the lower right of this photo.
Cold water from the building is entering the tankless coil via the bottom pipe (green corrosion) and hot water, heated by the coil is leaving at the upper part of the tankless coil, where it turns downwards to enter the left side of the mixing valve.
Additional cold water is permitted to enter the bottom of the mixing valve, and tempered (non-scalding) hot water then leaves at the right side of the mixing
Anti-scald valve used with water heater limit switch: In this photograph a single function limit switch is in use on a water heater, not on a heating boiler.
Here too you will notice that there is also a mixing valve installed to prevent scalding - the gray-handled device to the right of the control (and leaking we might add).
The use of a mixing valve or anti-scald valve permits us to set the Honeywell 6006 limit control switch to a higher number, keeping the water in the heater at a higher temperature.
The mixing valve in this photo adds cold to the outgoing hot water to avoid scalding, and at the same time we have more total domestic hot water for washing and bathing than if we omitted this pair of controls.
How to avoid being scalded by hot water from a tankless coil
See MIXING / ANTI-SCALD VALVES for details about mixing valves on boilers and tankless coil systems and for an explanation of how an anti-scald or tempering valve can increase the volume of hot water available from a tankless coil hot water system.
How to deal with clogged hot water piping that reduces hot water pressure and flow
(Feb 24, 2014) Gail said:
Could you tell if I get the coil cleaned would that help.
Gail, I don't have a shred of information about your system or the problem that's a concern.
In general, if the concern is reduced hot water flow through the coil because a tankless coil has become mineral-clogged, indeed cleaning the coil can for a time improve hot water flow rate (more "pressure" at the tap).
If that's confirmed to be the problem you may need to install a water softener.
And coil cleaning is not always possible: if the coil is corroded and perforates it'll have to be replaced; furthermore, some sources warn that the acid cleaning etches the coil internal surfaces, speeding the re-clog rate unless a softener is installed.
Question: lower hot water temperature when not at home?
(Mar 4, 2014) Anonymous said:
Can I turn down the hot water temperature during the day when not home?
You could, but if we are talking about a tankless coil hot water supply system and we are talking about winter use during the heating season, lowering the LO or changing the DIFF - the two aquastat controls that have to do with the tankless coil probably won't do a thing since the call for heat in the home is what keeps the boiler hot - that's controlled by the HI limit control on the aquastat.
However in general, lowering the room thermostat settings when you are not at home is indeed a good way to save on heating costs.
See THERMOSTAT SET PROCEDURE
Question: boiler cycles on and off even when not calling for heat
(May 26, 2014) JGreen said:
We have an oil fired boiler/ tankless water heater. This is continually cycling on and off even ( not related to home air temperature) so presumably to keep the water hot. I have received conflicting opinions on wether the coil can or should be descaled. Can you tell me whether sediment build up is possible? And if so can cleaning be done to reduce this oil consumption. thank you.
Further to my question ... , we do not have a problem with water flow, is that the only indicator of a coil that may ned servicing or replacing? We have been told by the annual furnace maintenance person that coils cannot have mineral deposits unless we are in a hard water area. That does not make sense to me. it is a Saturn high efficiency boiler ,Model K 26 is on the plate.
The boiler cycling on and off and oil consumption is a separate issue having nothing obvious to do with coil scaling.
The boiler is controlled (most likely) by an aquastat. Check your aquastat settings - see
Question: limit switch for water to air heating control as well as separate tankless coil control
(Jan 2, 2015) John Dee said:
Correction to the post below: It's actually a Johnson Controls limit switch.
I have an oil furnace that includes a tankless coil for hot water, which has its own anti-scald valve. The furnace also sends hot water to a forced air handling system. This air handling system has one of the "Single Function Boiler Temperature Limit switches" pictured above, connected to the hot water going in to the air handling system. I can't find any information as to what the temperature should be set to! Any ideas?
We need to know what the switch is supposed to be doing. I would expect that located at a water-to-air heat exchanger the limit switch could be in use to call for heat OR to keep the blower from running if the water temperature is too low - to avoid blowing cool air on occupants; but I don't know your system or what other controls are present. Give us the switch model number and the air handler brand and model number and we might find the manufacturer's recommendations.
What is the present limit switch setting? Who set it? When?
Question: long time for hot water to arrive from tankless coil to the bath fixture
27 January 2015 Dick shouted (caps deleted)
Have summer/winter (tank-less domestic h2o)oil furnace. Must run hot h2o for 5-10 min to get hot water. Once it gets hot have plenty of hot h20 for the rest of the shower. What's up with this?
If the boiler itself were not hot you'd not get hot water from a tankless coil since the flow rate through the coil typically uses hot water faster than the burner on your boiler can keep up. (Furnaces, to be technical, are forced warm air heating systems, boilers are hot water heating systems) .
So I suspect that you have long piping runs between the boiler and the point of use and that those pipes are cold. You can improve matters by insulating the accessible portions of your hot water piping.
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[1a] Bosch, "Instrucciones de instalacion y manejo, Calentador de agua instantaneo, Confort / Confort S 6BP / 6BP n 10BP / 10BP N 13BP / 13BP N"
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 U.S. Department of Energy on Tankless Water Heaters - http://apps1.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=12820
 "Tankless or Demand Type Water Heaters,"U.S. Department of Energy, retrieved 10/14/2012, original source: http://energy.gov/energysaver/articles/tankless-or-demand-type-water-heaters [Copy on file as Tankless_Water Heaters_DOE..pdf] retrived anew 2/25/2014
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