Limit switch on a steam boiler tankless coilGuide to Tankless Coils on Heating Boilers

  • 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
  • POST a QUESTION or READ FAQs about tankless coils used to produce domestic hot water: clogging, leaks, water quantity, water temperature, scale formation and cleaning

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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.

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A Guide to Tankless Coils for Domestic Hot Water Production

Photograph of a multi function combination control on a heating boiler

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

Sketch of tankless coil for heating water

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:

  • 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.

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

Limit switch on a steam boiler tankless coilHere 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

Honeywell 6006 guideAnti-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.

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

See TANKLESS COIL HOT WATER INCREASE and HOT WATER IMPROVEMENT for a detailed guide to improving hot water quantity and flow when a tankless coil is installed as well as using other methods of hot water heating

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

See Clogged Piping / Tankless Coil & Hot Water Flow and WATER PIPE CLOG REPAIR for a discussion of loss of water pressure due to clogged piping or clogged tankless coils.

What to do about leaks at the tankless coil

Please see TANKLESS COIL / HOT WATER COIL LEAKS for a detailed discussion of how to recognize, evaluate, and repair boiler leaks on or around the tankless coil.

Also see BOILER LEAKS CORROSION STAINS for a discussion of leaks on heating boilers.

Tankless coil replacement or new installations

See TANKLESS COIL INSTALLATION PROCEDURE or select a topic from the More Reading links shown below.



Continue reading at CLOGGED PIPES / TANKLESS COIL DE-SCALE or select a topic from the More Reading links shown below.


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TANKLESS COIL INSTALLATION PROCEDURE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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