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This article explains whether or not it makes sense to convert tankless coil used for making domestic hot water into an alternative hot water system such as using it to heat a separate hot water tank. The short answer is probably not. If the quantity of hot water you are receiving from a tankless coil is insufficient, see Tankless Coil Hot Water Increase. If your hot water pressure is declining because of a clogging tankless coil, be sure to see Clogged Pipes / Tankless Coil De-Scale for diagnosis and repair advice. Thanks to Carson Dunlop, a Toronto Home Inspection Firm and Home Inspection Educator, for permission to use sketches shown in this article.

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Converting a Tankless Coil to Indirect Water Heating

Question:

I am looking for information about connecting a circulating pump and hot water storage tank to an existing tankless coil that can't produce hot water fast enough. My boiler is and old one, and I am trying to get the most out of it before replacing everything. - A Rossini, Katonah NY

Answer:

If you converted a conventional tankless coil on a heating boiler to try to use it to heat a separate tank of hot water instead of simply feeding building water through the coil for heating, you'd be constructing an older type of water heater - a range boiler Range Boiler Water Heaters for a discussion of that method.

A similar method, using a "side arm coil" on a heating boiler, is described at Side Arm Coil Water Heaters.

What about converting an existing coil to something like one of those alternatives to get more life out of the coil? We're not sure it's an economical idea. As a primary water heater, feeding water through an existing tankless coil but no longer directly to the building but rather using it to heat a reservoir of hot water in a tank by recycling until the water in the tank is as warm as desired, you'd need to buy and install:

• extra piping, control valves, and check valves
• a new circulator pump dedicated to circulating hot water through a separate water tank and through the coil on the boiler
• a thermostat that controls turning that circulator pump on and off as needed (the temperature of the water in the coil on the boiler would handle turning the boiler on and off as needed)

It would be unusual to convert a tankless coil to an indirect fired water heater setup just to get more hot water - since the cost of doing so is at risk of being wasted when the coil wears out or becomes clogged. When the coil is shot and you're ready to buy a real indirect water heater, almost nothing of what you bought in our list above will be useful: normally those parts come packaged with the new Indirect-fired Water Heaters (that will still work using the existing boiler) or with a new High Efficiency Water Heaters that works on its own.

The total parts and labor cost of doing the temporary conversion of a tankless coil water heater, if we ignore having to later toss those parts away, is probably similar to buying a new indirect-fired water heater system that comes with all those parts. The difference is that an indirect-fired water heater from the factory will have the coil inside the water heater tank rather than inside the heating boiler. See INDIRECT FIRED WATER HEATERS for an explanation of indirect water heaters.

Alternatively as a booster system we sometimes see a plumber feed the house hot water piping straight from a separate water heater but boost-heat water leaving the water heater when needed by running it through the coil.

There are a couple of approaches or variations such as installing an electric water heater (cheap to buy, most costly to operate), leaving it turned off, and letting it simply hold incoming cold water in a reservoir that warms slowly in the utility room overnight. That approach "pre heats" the water into the coil by stealing heat from the basement. Then if hot water demand is high, say from visiting guests, the owner simply turns on the electric water heater.

If you are having tankless coil problems or are not satisfied with it, take a look at these articles:

1. TANKLESS COILS provides detailed articles about maintaining and using a tankless coil hot water system
2. ALTERNATIVE HOT WATER SOURCES compares alternative ways to make domestic hot water.
3. The characteristics of various water heaters such as life expectancy, cost, safety, and capacity are discussed at WATER HEATER PROPERTIES
4. Ways to improve total hot water quantity, pressure, temperature and flow are discussed beginning at HOT WATER IMPROVEMENTS and continuing at HOT WATER QUANTITY IMPROVEMENT.

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TANKLESS COILS
  AQUASTAT CONTROL Functions
  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.
• Thanks to reader A Rossini, Katonah NY for email discussing getting more out of an old tankless coil hot water system - Feb. 2010.

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