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the Thermal Expansion Control Valve was not just dripping put trickling steadily, do I need replace it, or there is something wrong with the HWT, the HWT just 3 years old, thx
 

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Are you talking about the T&P(temp and overpressure) valve on the hot water heater??? If yes, ifrst check the actual temp of the water at the closest faucet, it should be about 120deg. If a lot higher, find out why and correct it. Thermostat set too high, malfunction, etc.

If the temp is about right, there could be some grit in the valve. Into a bucket or something(careful, it is HOT), cycle the valve open/closed quickly. If it still leaks, repalce it. If the heater is older than 10 yrs, it might be time to consider a replacement.
 

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In 4 years I have only found 1 bad T&P valve, and it was on a commercial hot water booster.


That said, don't forget to check the expansion tank itself to see if there is water in the air side of the membrane. If there is, then your T&P is fine, and you need a new expansion tank.

Expect to pay 60-80 bucks for one at a hardware store.

Less than half if you can find a plumber who will sell one to you out the door.
 

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You should also check your water pressure because high water pressure, or occasional spikes in water pressure, can cause the T&P valve to open and once opened they frequently will not reseat completely--particularly if a few years old.

If the pressure and temperature check out ok and the expansion tank, if there is one, is ok then replace the valve.
 
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