Is a potable water expansion tank required on an indirect water heater?

Is a potable water expansion tank required on an indirect water heater when the boiler that’s feeding it already has an expansion tank? The manufacturer’s label and information was obstructed by the insulative blanket.

Are there any check valves or devices that can hold pressure between the tank, and the indirect heater? No. Then no pressure tank is needed.

Is there a backflow valve to the city supply? No. Then no pressure tank is really needed.

The water can’t tell the difference. Just make sure the expansion tank is correctly sized for the maximum temperature difference and volume of stored water.

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Regardless of the source of the heating fuel you are still creating water in a pressurized tank that will need to expand. A thermal expansion tank would be my recommendation.

It’s not your job to determine the size of the expansion tank. It’s not your job to oversee the installation. I would just make the recommendations and allow the plumbers to do the installation. They have the license to size of the expansion tank and do the installation.


I believe the water in the boiler and the water heater are two separate systems and each might need its own expansion tank?

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I do not usually check for a backflow valve to the city, because most of the time the hardware that’s used on the supply line to the property is difficult to determine if one is incorporated. For instance, some water meters have a backflow preventer incorporated, but I cannot tell by looking at it.

In my report I recommended installing a thermal expansion tank for the indirect water heater. Agreed that it’s not my job to determine the size of tank needed. I don’t check for a specific size of thermal expansion tank during my inspections. However, I do look to see that it’s installed correctly. Another thing that I check is the air pressure with a gauge. If no air comes out, or if water comes out, it’s defective.

Thanks for the feedback, @mwilles .

Agreed that the boiler and the indirect water are two separate systems, I just didn’t know if thermal expansion was still a necessity for this type of water heater. Now I believe that potable water does need a thermal expansion tank on the cold water supply line to the indirect water heater.

Thanks for the reply, @wstowell .

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