Indirect water heaters

The boiler circulates heating water through a coil inside the tank; the domestic water surrounding the coil is heated by contact and never mixes with the boiler loop. A domestic-hot-water priority control diverts the boiler to the tank when its aquastat calls, then returns to space heating. The tank has no burner, flue or combustion air of its own; its energy source and efficiency are the boiler's.

Because a modern boiler can put 100,000 BTU/h or more into the coil, an indirect tank recovers far faster than a direct-fired tank of the same size, so first-hour ratings are high and a 40 or 50 gallon indirect can outperform a larger gas tank. Sizing considers boiler output, coil rating, priority timing and peak demand together, which is a hydronic design question rather than a catalogue lookup.

Indirect tanks suit homes with a condensing boiler or a boiler replacement in progress, homes with high simultaneous demand, and buildings where a single heat source is preferred. They are not an option without a boiler, and adding a boiler for hot water alone is rarely justified. CANRO describes boilers and hydronic heat as its specialty, so indirect tanks are assessed alongside boiler work; WaterHeating.ca does not cover the boilers themselves.

An indirect tank has an anode, a relief valve and often a mixing valve like any storage tank, and the same 60 °C storage and 49 °C fixture guidance applies. Boiler-side maintenance is separate. Expansion control is needed on the potable side where the system is closed.