07:47PM | 06/09/06
Member Since: 06/08/06
1 lifetime posts
I noticed a small puddle of water in my crawlspace which was coming from the pipe that leaves the pressure relief valve on the hot water heater. A plumber came out and installed a new relief valve. It still leaked. He came back and ran a test saying there was probably too much pressure on the lines causing the valve to release water. The valve is designed to trip at 175 lbs psi. He put a gauge on the cold water line and let the hot water heater come on and heat up. The pressure went to 175 lbs. So, he was right. But as soon as you open a cold water line the pressure drops back to about 65 psi. But WHY is the pressure that high after running hot water? Other than this overflow into the crawlspace I am having no problems. But 175 lbs is really high. The plumber said there must be a "check valve" (whateever that is)somewhere between the water meter out by the road and the house which as about 110 feet. But he had no idea where. He said this could be corrected by simply turning the cold water on after the hot water heater turns on thus releasing the pressure. I turned down the thermostat and this reduced the high end pressure to about 125, but thats still too high. Plumber also said he could install an expansion tank to correct this for about $250. I see no need to do that when the pressure shouldnt be that high in the first place. Can someone help? I need some "real" answers. You can email me direct at


08:35PM | 06/09/06
Member Since: 04/25/05
1916 lifetime posts
Water is not compressable and when heated it expands.

In the "past" that was not a problem. The expanding water just backed up into the water main.

Now days cities are often installing backflow preventers or check valves at the meters. The purpose is to prevent any contanination of the city water by problems in your house.

Those make the system closed system and when the water expands as it is heated it has no place to go. But it increase the pressure until the pressure relief valve opens.

Adding an expansion tank is the proper solution.


03:57PM | 03/04/07
Member Since: 03/03/07
1 lifetime posts
I had the same problem. I went out and bought a expansion take for 28 dollars. if your hot water heat is smaller than 50gal it is not that expensive. I read a few web sites on how to sweat pipe and did it in 30 min.


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