hot water tank..new vs old

Your water heater cannot create a system pressure rise unless your water system is a "closed" system.

Do you have a check valve or back-flow preventer somewhere in the system? If so you need a system pressure relief valve or an expansion tank.

Run some water out of a hot water valve in the bathroom or kitchen (about gallon or so) and see if the pressure drops.

My best guess is that the expansion of the water in the water heater from cold to hot is pressurizing a closed system.

cheers Bob

Sounds like you have a check valve in the supply line to your hw heater. You should install an expansion tank on the line at your hw heater. They make small ones that can be supported by copper pipe without additional bracing. If you have some other piping you may need to support it. Lowes and home Depot both usually carry them. Typically around a $100. If you are on a well then your normal storage tank should handle the expansion. Look for a chack valve somewhere between the hw heater and the well tank.

jimmy wrote: ...

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I'd venture the existing PRV has failed and you weren't aware of it before...

Any water system is a "closed system" when not running; otherwise there's no holding it in.

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Some water heaters come with a connector nipple on the cold side (inlet) that have a plastic ball in them that acts as a check valve.

No, few suburban household water systems are "closed". The service entrance means it's not closed. For many years hw expansion pressure was absorbed by the service line coming in. Rural might satisfy the definition of closed but well systems almost always have storage tank that doubles as an expansion tank.

Back flow preventers have been in the code for years. Most services are not "open" if you live in a fairly new neighborhood. The pressure rise should go away immediately as soon as he opens a hot water valve.

jamesgang has the correct answer. your new heater has a check in the inlet. Take it back apart and throw the somebeach in the round file. THEN put an expansion tank on, (anywhere in the entire system is fine, they don't have to be right next to the heater).

there's no holding it in.

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Water hammer minimization suggests limiting supply pressure to 65 psi max.

cheers Bob

I think it's safe to say that well in excess of 90% of suburban homes still have nothng between them and the city water supply. The op found out he does have one. But I can tell you my neighborhood of