hot water tank..new vs old

For the past 40 years I have installed 4- 40 gallon hot water tanks (natural gas) in my home. Today I picked up a new 30 gallon tank, installed it, fired up and could not believe the pressure rise as temp increased, went to 140# and still going at 120 deg temp My incoming pressure is usually 90psi, give or take 10 # I know I am going to need a press reducing valve, either for the entire system, or the hot water tank itself, or an expansion tank for the hot water tank. Question is why in the world am I having pressure rises like this on the new tank, and never had this problem before ? thanks
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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
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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.
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jimmy wrote: ...

...
I'd venture the existing PRV has failed and you weren't aware of it before...
90psi is about double what normal residential water pressure is. Amazing haven't blown toilet valves, icemaker, washing machine control valve, etc., etc., etc., ...
Any water system is a "closed system" when not running; otherwise there's no holding it in.
--
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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.
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On Mon, 29 Nov 2010 09:30:20 -0800 (PST), jamesgangnc

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.
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On Nov 29, 12:46pm, snipped-for-privacy@aol.com wrote:

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 1,200 homes was built within the last 20 years and none of them have a check valve, regulator, or backflow preventer between the city supply and their water system. Except for irrigation.
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there's no holding it in.<<<<<
actually not exactly......
When you use hot water, cold water replaces it in the water heater. When the cold water is heated to "hot water" temperature, it expands slightly.
How is this expansion accommodated? In the days before back flow preventers, the "extra volume" got pushed back into the municipal supply to have the pressure relieved by the 100's of incidental water usages.
Enter the check valve / back flow preventer....now the more nearly "closed system" needs pressure relief of some sort (valve or water usage) or an expansion tank.
cheers Bpb
Add a check valve or back flow o
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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.
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On 11/28/2010 9:52 PM, jimmy wrote:

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).
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Steve Barker
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Thanks everyone, found the problem This past summer had the main line replaced..all the way to the meter, a pressure regulating valve along with a back flow preventer was also installed making this a closed system. I forgot all about it until now. expansion tank will be installed way we have had 80lbs pressure for the 40 years lived here.recently went to 90, inlet press, I don't know if I could live with 40#...before we had main line replace pressure dropped to 20# when ever a valve was opened... I may consider dropping the inlet# tho thanks again everyone

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