Tom’s Tips About Water Heater Pressure Relief Valves

By Tom Neale, 11/16/2006

  1. We’ve found that these valves are much more likely to open on boats than the ones in houses. This can be caused by the fact that many boats have water heaters that operate not only by electricity like those in homes, but also with a heat exchanger through which the hot water in your engine passes to heat the water in the hot water tank.

  2. There’s less temperature control with this system than with an electric thermostat controlled heating element, so there’s more of a likelihood of higher pressure occurring from time to time.

  3. Also, with varying sources of water going into your boat’s tank, there’s more likely to be grit or other impurities in the water. Once these valves open just a tiny bit, to let off a tiny amount of excess pressure, they will often continue to leak. This is because of particles of grit that may get trapped in the valve seat. Also, the springs on the valves can weaken with use or simply from age.

  4. Better water heater makers such as Raritan have anodes within the units which should be replaced from time to time. This can help to minimize possible electrolysis that could harm the tank and impair the valve, which probably contains dissimilar metals as compared with the rest of the tank.

  5. Usually these valves will open only when the tank is being heated, as is normal when you’re plugged in, have a generator going, or are underway and have a tank that’s equipped with a heat exchanger.

  6. It’s unlikely that these valves will open with just the pressure from the boat’s water pump or connected dock water pressure, because this isn’t nearly enough pressure to overcome the spring, but sometimes the valve becomes weak or damaged enough for this to happen.

  7. Pressure Relief Valves are usually easy to replace although their manufacturers recommend having a qualified professional do it. If you do it yourself, read and follow carefully all warnings and instructions from involved manufacturers including that of the valve and hot water tank. Also follow all relevant codes and standards. Don’t get valves that exceed temperature and pressure ratings for your system. Mistakes can cause such things as electrocution and severe burning.

  8. Keep a spare aboard with temperature and pressure limits as recommended by tank manufacturer or other relevant authorities. Remember, you’d much rather have this awaken you in the night than suffer the results of a bursting hot water system.

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