Larry's post regarding use of a check valve on the water supply side is right on. A related issue is that marine pressure release valves usually have much lower release pressures than home units...maybe because the systems are much smaller....anyway, if you replace a pressure release valve aboard it's safest to get a low pressure one.. I just checked the one in my basement its 150 PSI at 210 degrees...boy that seems awfully high..?? I replaced it several years ago and I now wonder if I got the right pressure....anyway that's way too high aboard... Rob Brueckner Hatteeras YF New Rochelle, NY ------------------------------------------------------------------------ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now.

Robin Brueckner wrote: > Larry's post regarding use of a check valve on the water supply side is > right on. > > A related issue is that marine pressure release valves usually have much > lower release pressures than home units...maybe because the systems are > much smaller....anyway, if you replace a pressure release valve aboard > it's safest to get a low pressure one.. > > I just checked the one in my basement its 150 PSI at 210 degrees...boy > that seems awfully high..?? I replaced it several years ago and I now > wonder if I got the right pressure....anyway that's way too high > aboard... > > Rob Brueckner > A T(emperature) & P(ressure) valve is what separates a water heater from a bomb. Your T&P valve is rated to open at either the working pressure of the tank, usually 150 psi. and just under the boiling point or 210 deg. Its function is to prevent the water in the tank from reaching critical temperature, pressure or combination thereof that would lead to the contents of the tank flashing into steam. They are an often overlooked piece of safety equipment that play a very important role in our everyday safety. Remember, that when water flashes into steam, it expands to 1520 times its original volume (8,000 cu. ft. for a 40 gal. tank) in well, a flash. Without the T&P valve, that 40 gal water heater in your basement could easily become a steam powered rocket. There are documented cases of water heaters lifting off from their spot in the basement and shooting up through 3 stories of structure and coming down blocks away. There are videos of water heaters that have been intentionally rigged to explode, it is a dramatic event. Google Watts. If you think that because it's on a boat it can't develop enough pressure to cause a problem, think again. Although the pressure at which your piping will fail is likely lower than that of a land residence, keep in mind that as the pressure increases, the boiling point lowers (think... pressure cooker). So it is possible to have water flash off into steam at a temperature lower than its atmospheric boiling point. If a vessel of water is superheated (temp higher than the boiling point, pressure higher than atmospheric) and the pressure suddenly drops, the superheated water will flash off into steam. The likelihood of such an occurrence is certainly slim, but not impossible. So when working with safety relief devices of any kind, keep in mind that they're there for good reason. A seemingly innocuous change could negate the function of the device. Remember a few years back when an elementary school cafeteria down south was leveled because the janitor changed a leaking T&P valve? The new one was too short to reach into the tank, so he simply added a fitting to extend it. In the process he broke the thermal element, and although the pressure side still functioned, it didn't open when the temperature went up into the 300 degree range. Water at 150 psi can be heated to that degree and remain in the liquid state provided the pressure remains at that level. However, once the pressure drops below the flash point, it all turns to steam.... Bang. The fellows who investigate these incidents refer to it as a "boiler room disturbance". The ratings are determined by the working pressure of the tank, and the amount of heat the burner or heat source can provide. Always match the original in rating and configuration. Never install anything between the device and the tank. NEVER EVER plug or restrict the discharge of a relief valve. If you encounter a runaway condition with any pressure vessel, remove the heat source, and walk away. Never add cold water, open the relief valve, or try to drain it; any action could lower the pressure and cause the contents to flash with explosive results. BTW, when you "exercise" your T&P valve, have a new one on hand with the same rating as the one you're about to replace. If you open an old valve, you *will* be replacing it <G>. steve sipe solo 4303 "Maerin"