[Solved] T&P valve leaking from thermal expansion

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Walkman

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New 50 gal AO Smith gas WH which is leaking small amounts of water out of the T&P valve.

- I'm getting normal pressure reading of 50 lbs, with a max pressure reading 160 lbs.

- I installed a new 2 gallon Everbuilt expansion tank which has 45 lbs of pressure.

Would a larger expansion tank or changing the pressure solve this problem? Could there be something wrong with the WH?

Solution: I finally noticed there is an inline check valve installed between the WH and the expansion tank. Looks like the plumber installed it when the recirculation pump was installed and the expansion tank has been a decoration ever since.
 
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Reach4

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Increasing the air precharge to 50 psi would help some, but you will probably also need to switch to a larger thermal expansion tank.
 

Walkman

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Increasing the pressure in the tank would help it absorb more thermal expansion?
 

Reach4

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It would let the tank absorb a little more volume of water. That expanded volume of water is caused by thermal expansion. The 5 psi increase is not going to cause a big increase in capacity. But that would max out the ability of your tank.

Question: do you turn off your WH at times, and then start it up before you plan to use hot water? That would call for a bigger tank than if you just kept it up to temperature all of the time.
 

Walkman

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We leave the WH on. We have a recirculation pump, but it didn't make any difference when I unplugged it and closed the valve to the return line.

I'll try a bigger expansion tank if extra PSI doesn't make a difference.
 

wwhitney

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This calculator says that with a starting water pressure of 50 psi and temperature of 90F (i.e. not a cold start, the tank is reheating), if you set a 50 gallon tank water heater to 140F, then a 2 gallon expansion tank with a 45 psi precharge should limit the final pressure to 86 psi.


Your experience is sufficiently far from this theoretical answer that I think there's something wrong. E.g. a bad expansion tank, a problem with the pre-charge (IIRC it needs to be done with 0 water pressure present at the tank), or an obstruction (like a closed valve) between the water heater and the expansion tank.

Cheers, Wayne
 

John Gayewski

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It's possible you've installed the expansion tank on the wrong side of a check valve. The formula Wayne is using is showing that your current tank isn't sufficient to keep your water at a reasonable pressure. That is to say 86 psi is too high. You need a larger expansion tank.
 

wwhitney

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The formula Wayne is using is showing that your current tank isn't sufficient to keep your water at a reasonable pressure. That is to say 86 psi is too high. You need a larger expansion tank.
I did assume 140F tank temperature, if the actual set temperature is lower then the peak pressure would be less.

What do you consider an acceptable peak pressure? Watts' only caution is that it should be no more than 85% of the T&P valve setting. So for a 150 psi T&P valve, that would be up to 127 psi.

Cheers, Wayne
 

John Gayewski

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I did assume 140F tank temperature, if the actual set temperature is lower then the peak pressure would be less.

What do you consider an acceptable peak pressure? Watts' only caution is that it should be no more than 85% of the T&P valve setting. So for a 150 psi T&P valve, that would be up to 127 psi.

Cheers, Wayne
Your plumbing system is meant to operate at 80 psi max. While things are tested higher and individual components are rated higher your system will be put under stress at pressures higher than 80.
 

Reach4

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What do you consider an acceptable peak pressure? Watts' only caution is that it should be no more than 85% of the T&P valve setting. So for a 150 psi T&P valve, that would be up to 127 psi.
The earlier version of the Watts calculator used something like 132 psi internally in its calculations.

It will be interesting to see if those who are adamant that 80 psi is a hard max by code, even for thermal expansion, will plug 80 into that Watts calculator, and find they need a 10 gallon thermal expansion tank. :)
 

John Gayewski

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The earlier version of the Watts calculator used something like 132 psi internally in its calculations.

It will be interesting to see if those who are adamant that 80 psi is a hard max by code, even for thermal expansion, will plug 80 into that Watts calculator, and find they need a 10 gallon thermal expansion tank. :)
Two of our best customers have high pressure above 80. They think the high pressure is nice, but always wonder why their cartridges are breaking and toilets don't work right for very long. Randomly cracked urinal flush valve parts ect.
 
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