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AGE of WATER HEATERS
ALTERNATIVE HOT WATER SOURCES
ANODES & DIP TUBES on WATER HEATERS
ANTI SCALD VALVES
APPLIANCE DIAGNOSIS & REPAIR
APPLIANCE EFFICIENCY RATINGS
AQUASTAT CONTROL Functions
BACKDRAFTING HEATING EQUIPMENT
BACKFLOW PREVENTER, HEATER WATER FEEDER
CARBON MONOXIDE - CO
CHECK VALVES, WATER SUPPLY
CLOGGED SUPPLY PIPING
CROSS CONNECTIONS, PLUMBING
DEBRIS in WATER SUPPLY, Water Heater
DRAIN a WATER HEATER TANK
ELECTRIC WATER HEATERS
FLOODED WATER HEATER REPAIR
FREEZE-PROOF A BUILDING
GALVANIC SCALE & METAL CORROSION
GAS BURNER FLAME & NOISE DEFECTS
GAS FIRED WATER HEATERS
GAS PIPING, VALVES, CONTROLS
HEATING COST SAVINGS METHODS
HOT WATER EXPANSION TANKS
HOT WATER SUPPLY
HOT WATER IMPROVEMENTS
HOT WATER DELIVERY SPEED UP
HOT WATER EFFICIENCY IMPROVEMENT
HOT WATER PRESSURE EXPANSION RATE
HOT WATER PRESSURE IMPROVEMENT
HOT WATER PRESSURE LOSS
HOT WATER PROBLEM DIAGNOSIS
HOT WATER QUANTITY IMPROVEMENT
HYDROGEN SULFIDE GAS
INDIRECT FIRED WATER HEATERS
MANUALS & PARTS GUIDES - HVAC
MIXING / ANTI-SCALD VALVES
NO HEAT - NO HOT WATER: HEATER DIAGNOSIS
NOISE, WATER HEATER
ODORS IN WATER
PIPING IN buildings, Clogs Leaks Types
RELIEF VALVE LEAKS
RELIEF VALVE, WATER HEATER
SCALE REMOVAL, WATER HEATERS
SEWER GAS ODORS
SOLAR HOT WATER HEATERS
TANKLESS WATER HEATERS
THERMAL EXPANSION of HOT WATER
THERMOSTATS, WATER HEATER
TIMERS for ELECTRIC WATER HEATERS
WATER HEATER ALTERNATIVES
WATER HEATER ANODES, DIP TUBES
WATER HEATER AIR INLET
WATER HEATER DEBRIS FLUSH
WATER HEATER DRAIN PROCEDURE
WATER HEATER EFFICIENCY
WATER HEATER FLUSH PROCEDURE
WATER HEATER NOISE DIAGNOSIS, CURE
WATER HEATER PROBLEM DIAGNOSIS
WATER HEATER COMPARISONS, PROPERTIES
WATER HEATER SCALE
WATER HEATER SAFETY
WATER HEATERS for HOME HEATING USE?
WATER ODORS, CAUSE CURE
WATER PIPES, Clogs Leaks Types
WATER PRESSURE TOO HIGH: DANGERS
WATER PRESSURE LOSS DIAGNOSIS & REPAIR
WATER QUANTITY IMPROVEMENT
WATER SOFTENERS & CONDITIONERS
WINTERIZE A BUILDING
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.
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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.
Drawing of a tankless coil for heating water shown here, courtesy of Carson Dunlop Associates.
[Click to enlarge any image]
How much hot water can a tankless coil provide?
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:
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
More details about this single function limit switch are
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.
MIXING / ANTI-SCALD VALVES provides a detailed explanation of how anti-scald valves work and how to use them.
Diagnosis & Repair Articles for Tankless Coils used to Produce Residential Hot Water
How to get more hot water pressure and more hot water quantity from a tankless coil
How to avoid being scalded by hot water from a tankless coil
How to deal with clogged hot water piping that reduces hot water pressure and flow
What to do about leaks at the tankless coil
Tankless coil replacement or new installations
Continue reading at CLOGGED PIPES / TANKLESS COIL DE-SCALE or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Question: should I have my tankless coil cleaned?
(Feb 24, 2014) Gail said:
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.
All those warnings said, it's worth a try.
Details you should read are at CLOGGED PIPES / TANKLESS COIL DE-SCALE
Question: lower hot water temperature when not at home?
(Mar 4, 2014) Anonymous said:
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.
Question: boiler cycles on and off even when not calling for heat
(May 26, 2014) JGreen said:
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
Coils do indeed become blocked with lime or scale; minerals settle out of water fastest at the hottest place in the system - the coil.
Your boiler company is right that scale formation only occurs at problem levels where the water supply is high in mineral content.
Yes a tankless coil can be de-scaled, See CLOGGED TANKLESS COIL or PIPES, LIME SCALE at inspectapedia.com/plumbing/Tankless_Coil_Clogging.htm for details.
But if the hot water flow is not diminished then the scale problem is probably not yet severe on your system.
Watch out: de-scaling can leave the coil interior etched so that scale forms faster than before.
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?
Meanwhile a safe setting is probably 120F
LIMIT CONTROL, SINGLE - at inspectapedia.com/heat/Limit_Controls.htm
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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