No such thing as a stupid question?

Alan (California Radiant) Forbes

If a customer has a check valve or pressure reducing valve on their incoming water main, we install expansion tanks on the domestic to allow thermal expansion without an increase in the water pressure, correct? That's in the case of a tank-type water heater. So, what if we install an "on-demand" water heater or we have a combi boiler that doesn't have a storage tank? Do you still install an expansion tank? Is one really necessary? I would guess not since most of the water that is heated is getting used right away; there is no reservoir of hot water to expand.

ChrisJ

I'm not a pro and honestly don't know what "the norm" is, but my guess would be you still want one.

It's not the tank only, it's anything with liquid in it. While the heater should only be heating with a faucet open what happens to the water in the heat exchanger when the faucet is closed?

Also, what happens as all of the water cools and shrinks?

JStar

I would still install one, but we certainly have tankless systems without one and don't have a problem. I bet the total expansion of the hot water doesn't add enough pressure to be a problem.

kcopp

I generally do not w/ tankless/combi water heaters. I have never had an issue. If it is very high btu input I do as I did have an issue one time w/ a 399k unit.

4Johnpipe

You will not need it for the purpose of thermal expansion. Some tankless units have optical sensors to detect water flow. A large expansion tank can play a not so funny game by allowing water to move back and forth causing a false water flow condition and a call for ignition. So as a wise man once said "it depends"...