I would like to pressure test a closed hydronic radiator system that has an expansion tank on it. I can detach the furnace, shut the water supply, and then mount a gauge with a nozzle on it that I can pump to something like 15-20 psi of compressed air and hope it keeps the pressure, meaning no leaks. I've done that with my gas pipes before successfully.

I am only concerned about the expansion tank. Should I remove it and put a plug in its system socket, or can it withstand that kind of pressure? I'm afraid of it exploding.

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Check the rating on the expansion tank, but I'm almost certain it would be able to withstand 15-20 PSI. But I'd still remove it and here's why. If you have a slow leak, the expansion tank would act as a buffer (add volume), to your system. This would make any loss of pressure much slower, perhaps to the point where you didn't realize you have a leak.

Is there a reason you don't want to use water to test? Water doesn't compress and any leaks would result in rapid pressure drop. You'd have to control for temperature variations but over a few days, you'd know if you had a leak.

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