Guide to Tankless Coils on Heating Boilers & Used to Supply Hot Water
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What is a tankless coil?
How do tankless coils work to make hot water?
Tankless Coils on heating boilers: a guide to tankless coils for domestic hot water supply: capacity, safety, clogging, adjustment, cleaning, & Repair Guide
How to inspect, adjust, & repair hot water supplied by a tankless coil
How to inspect the tankless coil on a heating boiler
Cleaning & maintenance guide for tankless coils & side arm coils
Questions & answers about tankess coils for making domestic hot water: installation, diagnosis, repair, & adjustment of temperatures & hot water capacity, clogging, & coil cleaning procedures

Here we explain the use of Tankless Coils on heating boilers: a guide to tankless coils for domestic hot water supply: capacity, safety, clogging, adjustment, cleaning, & Repair Guide

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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

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.

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. 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 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.

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
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.

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.

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 Tankless Coil & Hot Water Flow and Water pipe clog repair guide 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 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.

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Questions & answers about tankess coils for making domestic hot water: installation, diagnosis, repair, & adjustment of temperatures & hot water capacity, clogging, & coil cleaning procedures

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TANKLESS COILS
  Clogged Pipes / Tankless Coil De-Scale
  MIXING / ANTI-SCALD VALVES
  Tankless Coil Conversions
  Tankless Coil Leaks
  Tankless Coil Hot Water Increase
TANKLESS WATER HEATERS
  Electric Shower Heaters
  Guide to Tankless Water Heaters
  Operation & Characteristics
  Problems & Defects
  Hot Water Delivery Speedup
  Installation Requirements & Costs
  Should I Convert to a Tankless Heater?
  Calculate Water Heater Requirements
  Capacities of Tankless Water Heaters
  Life Expectancy & Applications
Temperature Pressure Relief Valves - Water Heaters

MIXING / ANTI-SCALD VALVES on tankless coils, why and how
BOILER LEAKS CORROSION STAINS for a discussion of leaks on heating boilers.
HOT WATER IMPROVEMENT for a discussion of how to get more hot water from a tankless coil.
WATER SOFTENERS & CONDITIONERS for a discussion of how to install and use a water softener to avoid mineral clogging of the hot water supply piping and equipment

Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Lime-A-Way, Jelmar CLR-CL-12 Cleaner, Vanish DRK cleaner, Simple Green Lime Scale Remover, Eco Friendly lime and scale remover [links to buy at Amazon.com] and other plumbing de-liming and de-scaling products mentioned in this article are examples of products used to remove calcium, lime, other minerals, and in some cases rust or other clogs or scaling in piping and on plumbing fixtures. Other products are available from your plumbing supplier.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

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Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
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A Guide to Tankless Coils for Domestic Hot Water Production

Hot Water Temperature Control with Tankless Coil Heating Systems

How much hot water can a tankless coil provide?

Use of a Single Function Boiler Temperature Limit switch and Anti Scald Valves with Tankless Coils

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

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair