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I had to drain my gas water heater to replace the broken gas control valve. When I refilled it, the pressure release valve (if that's what it's called) near the top of the heater was at first making a hissing noise for about 5-10 min (I guess until it bled the air bubble caused by draining) and then it started leaking maybe a single drop of water per minute. It is minimal but consistent. I put a glass underneath it overnight to measure and it collected maybe 3-4 oz. of water max.

Any idea what is causing this and what should be done to fix it?
 

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Is there an expansion tank connected. Make sure it's set to your incoming water pressure. Usually 55-65 psi.

If expansion tank is good, try opening the tpr valve and let some water out and closing it a few times. It may reseal. If not, tpr may need replacing.
 

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Is there an expansion tank connected. Make sure it's set to your incoming water pressure. Usually 55-65 psi.
There is an expansion tank but I didn't see a way to regulate the incoming water pressure.

If expansion tank is good, try opening the tpr valve and let some water out and closing it a few times. It may reseal. If not, tpr may need replacing.
What is a TPR? Is that the valve in the picture that is leaking ? When I pull the little lever, no water comes out, only air hissing.
 

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Yes, its what you have a picture of. Water should come out if held open. If not, replace it. That may be your problem. About 12 dollars at any big box store. You will need to turn off water to hwh and drain just below tpr valve to replace. Careful water will be hot.

Expansion tank has a schrader valve on bottom (like a tire valve) . Which can be checked with a tire pressure gauge. Should be set at incoming watrr pressure. (55-65psi) You add air there if needed with a pump or air compressor. If water comes out of the valve, its bad and needs to be replaced.
 

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I replaced mine recently. Easy task.

But as the others said, open it up and let water bleed out. If your only getting air, then you need to bleed that out. You should be able to open up one of your hot water faucets to bleed out the air.

One other thing....you're supposed to have a pipe on the output of the TPR valve that plumbs any water to a safe location....typically outside.
 

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There is an expansion tank but I didn't see a way to regulate the incoming water pressure.
To set the pre-charge air pressure in your expansion tank:

  1. Measure the water pressure for your home using a water pressure gauge attached to a hose bib. You should see about 60 psi if you have a water pressure reducing valve (also called a water pressure regulator) installed at the incoming water line from the city. If not 60 psi, fix this so it is about 60 psi. (Unless of course your city's water pressure is below 60 psi.)
  2. On the expansion tank, "momentarily" depress and release the pin in the tire like valve. If water comes out, the tank's internal bladder is leaking so it must be replaced. Doing this test first prevents water from going into your tire gauge if the bladder is leaking.
  3. Close the cold water inlet valve on top of your water heater.
  4. Open any hot water faucet to release existing water pressure.
  5. Measure the air pressure in the expansion tank using a tire pressure gauge. This pre-charge air pressure should equal the house water pressure in step-1 (about 60 psi).
  6. Pump air into the tank to equal the pressure in step-1.
  7. Open the cold water inlet valve that was closed in step-3.
  8. Close the hot water faucet that was opened in step-4.
Note that the tank's air pressure reading done in step-5 must be done with a hot water faucet open as in step-4. Otherwise if there is thermal expansion build up in the water lines, the pre-charge air pressure in the tank will be incorrect.

Also note that if air must be pumped into the expansion tank, it must be done with a hot water faucet open as in steps 3 & 4. Otherwise, the air being pumped into the tank will not be able to displace existing water in the lines resulting in an incorrect pre-charge pressure reading.

HRG
 
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