Expansion Tanks & Tankless Heaters

Is there is requirement on a Tankless water heater to use an expansion tank or equivalent when the water pressure is greater than 80 PSI and or if the home has a pressure reducing valve.

I believe you only need an expansion tank in closed systems. You could have tankless hot water in a steam boiler set up and not need the expansion tank.

If there is more than 80psi, there should be a water pressure regulator added to the system regardless of what type of water heater you have.

With a tankless water heater, there is no heating until there is flow so there is no build up in pressure. Even when the flow is shut off, the minor amounts of water held in the heater are not enough to warrant an expansion tank. That being said, in my areas that require an expansion tank, I can find no exception for a tankless. It would be an AHJ determination.

Of more concern would be the capacity of the gas meter if there are any other gas appliances installed.

Absolutely yes, go on line to Rheem on demand water heater, as an example, and the piping diagrams all require an expansion tank, regardless of the application.

Looked at the first one. This is taken from their installation manual. Bold added.

Thermal Expansion
A thermal expansion tank will be required if the water heater is installed in a recirculation system. This prevents damage to the heater, related piping, and the relief valve.

Went to a second type of tankless from Rheem, it stated it is only needed for a closed loop system.

The vast majority of homes (exceeding 95% is my guess) do not have a recirculation system and pump or any type of closed loop system. No expansion tank needed in that case.

Actually the pressure will “drop” after tankless water heater use (non-loop type).

Peter - I hope you don’t think an expansion tank controls “pressure”, because it does not.
It controls volume (which effects pressure, both (+) & (-)).

You can control water force coming out of a valve that is mostly closed, but it does not change the pressure. Once the outlet is closed, it will stabilize at the head pressure of the system.