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jmon 12-16-2012 12:54 PM

psi setting for expansion for hot water heater
What should the psi on the expansion tank be set to on a hot water heater?

Guy at home depot says just put it on like it is. It comes preset at 20 psi - I checked it with a digital psi gauge.

When I googled the question, it says psi on hot water expansion tank should equal whatever your water pressure coming into the house is. My water pressure coming into the house is 80psi.

The max psi setting on this expansion tank is 80psi.

Should I set expansion tank to 80psi? or maybe something in between 50 and 80 psi?

80 psi seems very high. just want to check with a master plumber or someone experience in this field.

thanks in advance.

TheEplumber 12-16-2012 06:07 PM

199 Attachment(s)
80 psi is the max for water pressure per the Uniform Plumbing Code.
Most people will say 60psi is best and if you're above that then you need a pressure regulator.
Your tank should set at the house pressure.

joecaption 12-16-2012 06:12 PM

Having your main water pressure that high can cause all kinds of issues.
Leaks, water hammer, toilet valves are more likly to leak, can even get cross over. (hot water going out of the cold water lines)

jmon 12-16-2012 06:30 PM

thanks all. house Water pressure is at 70 psi. I will set tank at 65 and i'll go from there.

thanks to all who responded.

May everyone have a happy and safe holiday season.


AllanJ 12-17-2012 07:41 AM

The expansion tank will work quite well when preset to a pressure between 60% and 90% of the water system pressure with all faucets turned off. (It'll probably work okay between 40% and 95%.)

If you need to recalibrate the pressure tank after it was installed or in use, turn off the main water shutoff and open a faucet until the system pressure at the expansion tank is completely relieved (dissipated; reduced to zero) and the air cushion expands and pushes all of the water out of the pressure tank. Because you don't know how much of the pressure tank is occupied by water, measuring or calibrating the pressure tank with the water turned on gives unpredictable results.

After the expansion tank is installed and the water turned on, water enters the expansion tank until the air cushion is compressed to the point its pressure equals the system pressure. (Pressure in PSI varies inversely with the space a given quantity of air is permitted to occupy in the sense that PV equals a temperature dependent constant and you can ignore the temperature part over the typical range of indoor temperatures) If the starting pressure was too low, so much water enters the tank before equilibrium is reached that there is not much more room for water when the next water heater tankful of water expands when heated and the water heater relief valve still trips. If you set the starting pressure too high you can damage the pressure tank bladder; it herniates through the connection pipe.

jmon 12-17-2012 10:26 AM

thanks allenj for responding,

Just for your information, I set the tank for 67psi unhooked from the system. According to instruction sheet, the max psi aloud by code and for this tank is 80psi. The water pressure coming into house averages between 68-72 psi. I know that's kind of on the high side, average house has around 50-55psi coming in from main. It's been like that forever, haven't blown any pipes apart yet.

There is a backflow preventer installed on cold water feed coming into boiler and hot water heater side. So the fresh water coming in from main is seperated so you don't get cross contamination or siphon effect from boiler and hot water heater side.

It seems to be working, all drips from pressure relief valve have stopped. That was my main concerned. Both hot and cold water coming out of faucets has excellent pressure.

I took readings in basement, first floor, and second floor. Basement was highest at around 72psi (closes to water main). first floor around 68psi second floor about the same. The water pressure coming into house varied between 68-72 psi depending on time of day and where in the house you took it.

Additionally, there is a pressure reducer valve installed on boiler, preset at 15psi, which is what is reccommended for boilers here according to annual hvac tech inspection. He said anywhere between 15-20 was good. Nothing higher because relief valve is set to blow at 30psi on boiler. That is the only place where a water pressure reducer is installed in this house. And the expansion tank on boiler is set to 15psi as well, last time hvac tech checked at begining of cold season to clean and service boiler. He said those two pressures should be set about the same for this house which is 2 stories high.

Your detailed explanation helps explain a lot.


jagans 12-17-2012 01:55 PM

That seems really high for system pressure. I thought most homes were 40-50? I guess you get a great shower though huh? :thumbup:

jmon 12-17-2012 05:27 PM

your right about that. It is nice to get all the soap off fast.

checked several times by different gauges. comes up about the same.

jaydevries 12-17-2012 06:06 PM

you might like this i had a person complain that they would get random water leaks in the middle of the night in 2 bathrooms so hooked a gauge up over night and the next day i checked it and the highest it got was 110 psi and they said it did not leak that night so pressure regulator fixed it but i always wondered how high it got to leak

jmon 12-17-2012 08:06 PM

yea that's good to know. Those are good rules to live by.

Have a happy and safe hoilday season.

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