Pressure still rising with an expansion tank...is that normal.

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NJHomeowner

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Hello.

I live in NJ and just had a new Rheem 40 Gallon Gas WH installed. My plumber then tested the pipe pressure, I have 1/2 in pipes in the house, and it was a bit on the high side. It was just under 80psi in normal circumstances (sometimes got down to 72psi on cold nights) and would go as high as 98psi when the water was heated. So my plumber installed a Pressure Reducing Valve and gauge where my water meter is and a Therm-x-Trol ST-5 expansion tank on the WH cold water line right above the tank. I set my house pressure to 62psi and my plumber adjusted the expansion tank to be around 62psi as well.

Now I have noticed that when the WH kicks on the gauge will go up as high as 71psi but eventually come back to rest at 62psi. Is that normal? I thought the expansion tank was there to keep the system at 62psi at all times, even when the water gets heated. Is there a problem with the expansion tank or perhaps something else in the system? I just want to make sure everything is operating properly.

Thank you.
 

NJHomeowner

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Thank you for your reply.

My plumber is great and anyone living in Northeast New Jersey should use him. He always lets me ask questions and hang around when he works...I just never got an answer as to whether or not that rise in pressure is normal.

Originally the expansion tank was at the factory set 40psi. He came back yesterday and balanced it to 62psi so I figured the system would stay at that level. I thought the purpose of the expansion tank was to keep the system at the set pressure (62psi) all the time, even when the WH is heating. So what you seem to indicate is that there will always be some fluctuation but a 10-12psi rise isn't anything to get alarmed about as long as it eventually returns to the set pressure.
 
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Thermal expansion tanks reduce the amount of the pressure rise, they don't eliminate it. The tank is providing a volume for the hot water to expand into by compressing the air bladder. Of course compressing the air bladder increases the air pressure which is equal to the total system pressure when there is no flow. The amount of water that will enter the bladder is determined by the temp rise, size of the water heater, and the size of the expansion tank.

Also, before the expansion tank was installed it is likely that some portion of the system was "relieving" by bleeding off water somewhere. In mine I found out that source of relief was happening in the old toilet fill valves. When I replaced the toilets, I suddenly had a problem with the water heater T&P valve relieving a trickle of water during water heating cycles.
 

Hackney plumbing

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It's been noted by some water heater manufacturer that the thermal expansion tanks pressure should be set 10-15 psi higher than the service pressure. I read it but noit sure where. I've always set my tank pressures alittle high.
 

Ballvalve

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To receive more water, a bit underpressure seems correct, but here is the absolute BIBLE on water expansion. Quite a useful article. Probably should be a stickie. And perhaps his tank is undersized.

http://www.blueridgecompany.com/documents/Watts-ThermExpansion.pdf

Note the flow thru expansion tank - nice unit. Although not much more than a nice quality 50' RV garden hose in the utility room wouldnt do also.
 

Jadnashua

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There's an expansion tank size calculator on the watts site you might want to check out. If the tank is too small (and having the pressure low will make it function as if it was smaller), the pressure will rise more than if it is oversized. Having the air pressure in the tank low means that even without any expansion, most of the tank would be full of water - there'd be little room for expansion; and, would put more stress on the bladder making it fail sooner. It needs to be set to the static pressure, or maybe slightly higher. then, the full design volume of the tank is available to accept the expanding water.
 
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