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.
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.
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
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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