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I was just wondering if I can use a single water heater for both my hot water and heating throughout the house. Is it safe and legal? Is it efficient? Right now we have a mix of cast iron radiators and baseboards. I can convert the whole house to just baseboards if need be.

Thank you

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    IDK why this was closed. There are water heaters with the capability to heat a small space through a secondary loop (in fact most water heaters can do this). There are home heating boilers with a capability to heat a hot water storage tank through a zone (in fact most boilers can do this) and there are boilers designed to serve both needs, either with or without a storage tank. The limiting factor is economic, and depends on the usage patterns and requirements. "Given all my requirements and parameters SHOULD I combine my heating" would be a different question, not a better one.
    – jay613
    Commented Mar 8, 2023 at 20:59
  • If you use potable-water rated tubing, it can be done without the secondary loop or heat exchanger, even, which is probably what's being asked: radiantcompany.com/system/opensystem
    – Ecnerwal
    Commented Mar 9, 2023 at 0:03

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An indirect hot water heater takes hot water from the boiler and uses it to make domestic hot water, which is sort of in line with what you are describing. The hot water that comes out of a hot water heater is not the same as the hot water that runs through your radiators (the water that goes through the radiators is generally gross).

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The short answer is yes, they are called combi boilers, but they are expensive. Here's an example. They provide both domestic hot water and hot water for heating in separate plumbing "circuits" Zelinka is certainly correct is that you don't want to use water for heating as potable drinking hot water. Usually heating water is in a closed circuit and treated to prevent corrosion, mineral build up, etc. certainly not something you'd want to drink!

So, yes, it's possible, but again, the units are not cheap!

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  • Yup, the water in the hydronic heating loop is deoxygenated, closed loop, and not potable
    – Edward Kirton
    Commented Mar 8, 2023 at 21:42
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The Apollo HydroHeat system did exactly this. It used domestic hot water for space heating. They're fairly common in my area because they were sold by the gas company as an easy way to retrofit homes to gas.

HydroHeat Schematic (Diagram from user manual)

The system utilized a conventional-looking domestic hot water heater, except it had a higher BTU rating for the capacity. What was unique was that it had a side loop, an extra pair of water connectors on the side of the unit where a fan coil with a pump would circulate out hot water.

To guard against legionella, and for decent heating capacity, the hot water heater was supposed to be turned very hot, ensuring returned water was still hot enough to prevent bacterial growth. A tempering valve, that mixed cold water drawn into the domestic supply, was supposed to be installed, but often was not.

Apollo stopped selling the system at some point, likely due to code and safety concerns, but side loop hot water heaters are still sold and so are fan coils.

A big problem experienced with these systems is that heating boiler water is normally around 180 F. Hot water heaters can't reach these temperatures. If no tempering valve was added, the temperature has to be kept around 120 F for safety. The net result is very limited heating capacity.

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