Water heater overpressuring water system?

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Are you on city water, with a check valve? A well pressure tank could handle the hot water expansion, if the water heater has no check valve.

Water's density at temp T (F) is about 62.46-0.01(T-68) lb/ft^3. Copper pipes can expand a little. A larger house with more pipes would have a smaller pressure increase.

If so, put it on the cold water side. Hot water may ruin it.
Nick
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Theo wrote:

Well, there are already plenty of replies, but Internet bits are cheap. I'll try to make this sound educated.
If there is an inlet supplying pressure at 30psi, the pressure rise due to expansion of heated water will just push a tiny amount of water back up the pipe, and the pressure will stay at 30 psi. There seem to be just two things that might cause pressure to exceed 150psi (the setpoint of your relief valve) -- boiling and water hammer.
Water hammer in a home system happens when a valve or faucet is closed suddenly, causing a shock wave to travel backward through the pipe until it hits something, like an elbow. Just possibly, if your tank is actually filled solid with no trapped air, the pressure wave might exceed 150psi at the relief valve, causing it to open very briefly. The cure for this problem is an air chamber installed, usually, near the valve. The air will allow the shock wave to dissipate into the trapped air, eliminating it.
Far more likely is the possibility that the water is overheating, and occasionally exceeding the boiling temperature of water (at 30psi, that is about 250F). I don't know just how the temperature is sensed and controlled, but that is where I would look for the problem. That or just turn the thermostat down to 120F. That should be hot enough for normal purposes, dishwashing included. Oh by the way, boiling in the water heater could be very disastrous if the relief valve should stick shut, of if the boiling is rapid enough to overwhelm the relief. There is an impressive video somewhere on the Internet of a water heater launching itself a good distance into the air, powered by steam.
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With a density 62.46-0.01(T-68) lb/ft^3 at T (F), 40 gallons at 55 F weighs 62.59 pounds per ft^3, ie 62.59x40/7.48 = 334.7 pounds. Heating it to 140 F raises the volume from 5.348 to 5.421 ft^3, an increase of 0.073 ft^3, or 0.55 gallons. In a large house with a working check valve on a city water supply, those 2 quarts might expand the pipes elastically with no damage at say, 60 psi, but that seems unlikely, since copper doesn't stretch much at that pressure.

"Tiny" as in 2 quarts :-) But we can't push water back through a check valve, which is often a safety requirement with city water supplies...
This could be a non-problem if the water heater were a more elastic $60 1"x300' 13-gallon black plastic HDPE pipe coil in 140 F solar-heated water in a 4'x8'x3' deep plywood box tank with a folded 10'x14' EPDM rubber liner.
Nick
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If I can't convince you I may as well join you.
Get a threading browser or get ignored.

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Hey Solar Fart, you just figured dow out? Most of the rest of us have been ignoring him for months, now.....
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I can't read your post. Must be my defective killfilter.
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Or as the next poster points our eventually, there is a check valve that prevents water expansion from going back into the water supply system. The expanding water has nowhere to expand to. Usually that is handled by adding a small air tank similar to the bladder types that well users have. I doubt it's water hammer, though. Yeah! I've seen that water heater video! It's impressive! Shot right through the roof of the enclosure they build over it! Forget how far it went, but it was impressive!
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