Water Heater Overfilling

[botanybay]

Ah yes, of British clorifiers. The pressure setting on the one on my Oyster 55 was at 2.5 Bar (~37psi) and the working pressure for the pump was set for 30 PSI shut off. Due to the pressure setting of the pressure relief valve only being about 7psi above the system working pressure I would find that as the spring aged it would start leaking a bit of water into the bilge.

The water for the marina is also rather hard so due to the slow leak there would be build up in the valve and the end result would be a leak which would get worse and worse. Taking the valve out, setting it in the open position and placing in vinegar would remove the deposits and make it work again for a time.

The primary time when I would get water out of the valve under normal conditions was starting with a completely cold condition and applying power, as there was a check valve on the cold water inlet to the hot water heater the water has no where to go and the system pressure rises on the hot side.

It turned out that there is both a cold water side and hot water side accumulator. The cold water side accumulator is set the normal way, the hot side should be set to have minimal hot water in the accumulator except when the hot water heater expansion drives water into it. This keeps the system pressure in the desired range.

At any rate, the copper hot water heater eventually sprung a leak at a seam (it was 30 years old) and I replaced it with a modern isotherm unit with dual heat exchanger coils.

Tried running the system without the check valve and found that opening the cold water tap in the galley would result in a slug of backfed water so installed the check valve again. Serviced both accumulators and now running the system at 3.7 bar (~50 psi) which is well within the design spec for the isotherm.

So, long story short, pressure and temperature valves are expected to blow off some water in the case of a check valve being used to prevent backflow and not having an accumulator on the hot water side. If the system pressure is close to the pressure relief setting the valves will tend to need more care and feeding to keep them from leaking.