Since I am good at starting heated discussions

JUGHNE

Just so you don't get claim to the title.
I think you need an expansion tank if either your water heater tank or main water service has a backflow or check valve which would prevent the cold water from backing up out of the tank as it heated causing expansion. The hot and cold water connections actually are a common connection within the tank and usually the cold will back out of the tank and push water into the supply main. If there is enough water draw in the house you may get away with out an expansion tank if there is a backflow device. (a bad flush flapper has saved many people from realizing they might need the expansion tank)
We had backflow preventers added to water meters. Some relief valves would dribble, (the handiest homeowner would put a hose on the T&P valve to run to the floor drain, the dimmer bulb would put a plug in the valve) The elderly lady without a functioning relief had her 50 year old tank start to leak, so it was time for a change out.

icesailor

SWEI said:

icesailor said:

If you put a standard rated T&PY valve on a dishwasher booster heater, it will be blowing off regularly. More if no pressure shock (water hammer) device isn't installed.

That's why we have 125 and 150 PSI pressure-only relief valves.

@SWEI:

Its not the pressure that makes them leak, it's hot water too close to the maximum water temperature on the valve.

Or, why you need to use Watts #240 T&PR valves on booster heaters and not 100XL's. Hatco's all came with #240's. Find one dripping and someone might have been cheap and changed it to a 100XL.

SWEI

Had to go look that one up. It's a big brother to the 40XL. Looks like they all have the same 210°F temp setting, just like the 100XL?

A number of tankless manufacturers supply pressure-only relief valves, which I am told is to prevent false trips/drips from spot overheating.