Sketch of a tankless coil tempering valve or anti scald valve Carson Dunlop Associates Tankless Coil Conversions - Should I Use a Tankless Coil to Heat a Separate Water Tank?

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Hot water supply improvements by converting from a tankless coil to something else. 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.

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Converting a Tankless Coil to Indirect Water Heating

What is the difference between an indirect-fired water heater and the tankless coil on a heating boiler?

An indirect-fired water heater uses a separate reservoir tank of "hot water" that is heated by circulating hot water from the heating boiler (photo below-right, short and gray to left of the tall blue water tank) through a heat exchanger "coil" mounted in the bottom of the water tank.

The domestic hot water used by the building occupants is inside the water tank. The quantity of hot water available is a function of the size of that tank, typically 50 gallons, plus the operating temperature range (higher temperature + mixing valve gives more than 50 gallons of hot water).

At INDIRECT FIRED WATER HEATERS we describe indirect-fired water heaters in detail.

Limit switch on a steam boiler tankless coil Indirect fired water heater plumbing hookup (C) Daniel Friedman

A tankless coil hot water system uses a finned copper coil inserted right into the hot heating boiler (photo above right) water to heat domestic hot water as water is run through the interior of the coil.

Heat from the boiler water is transferred through the coil fins and tubing into water passing through the interior of the coil on its way to the user. Unlike the 50-gallon indirect-fired water heater (above left) the tankless coil (above right) has a reservoir of zero gallons of hot water. Instead, it has a reservoir of "heat" - the heat that is inside the boiler, available to be transferred to the coil.

Why do people convert from using a boiler-mounted tankless coil to indirect-fired water heaters?

The specifications for a typical tankless coil / hot water heating boiler indicate that the maximum flow rate through the coil has to be limited, say to perhaps 2.5 to 5 gallons per minute. Running hot water faster than that will mean the hot water isn't so hot - it needs time (slow passage through the coil) to absorb heat, and it means that you will run out of hot water sooner (sucking heat out of the boiler faster).

Those specifications also typically promise only 5 minutes of hot water supply (that's 25 gallons). Actually some happy users of tankless coils get more than that by combining a flow restricter, high boiler set temperature, and a tempering valve or mixing valve (see MIXING / ANTI-SCALD VALVES) at the coil .

At HOT WATER QUANTITY IMPROVEMENT we discuss various ways to get more hot water in a home. One of these

Which gives me more hot water: a tankless coil on a boiler or an indirect-fired water heater?

Question: can a tankless coil on my heating boiler ever provide as much hot water as from a separate hot water tank tyhpe system?

The short answer is probably not. Let's mention the performance limits of a conventional boiler-mounted tankless coil. Keep in mind we're talking about a finned copper coil immersed into the heating water found inside a home heating boiler whose primary design purpose was heating the home, not making domestic hot water for washing and bathing.

The hot water quantity available from a tankless coil:

The hot water flow rate - how much hot water you can run at a time or how fast you can run hot water out of one or more plumbing taps:

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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