Calorifier pressure relief valve dribbling

[wonkywinch]

This calorifier produces hot water from the Incoloy electric heating element. Also featuring a safety / non-return valve that allows discharging of the water heater in case of disuse, set at 6 bar (±1). (Taken from Quick Calorifier 22 litre manual). So it might actually be doing it on purpose because the boat has been idle for some time.

The drain function isn't automatic, only the overpressure. To drain the tank you need to push the lever down, again from the manual "During the cold season, if the water heater is not in use, it is advisable to empty it using the valve lever. This is to
avoid damage caused by frost".

See my, and others posts about the routine maintenance (operating the lever) needed to prevent leaks.

 

[jwilson]

When you buy a new PRV of the correct spec (important) to fit to your calorifier tank, it will probably come with instructions to rotate the valve knob once a month to make sure the outlet is clear from limescale. Each time you do this a half-cupfull-ish of water will come out. Maybe 95%+ of boat owners have never had this done and it all still works.

When it doesn't work properly the water goes into the bilge, either intermittently or constantly as long as the pump is giving pressure and/or the engine or immersion heater is heating the water and making it expand.

If replacing the PRV does not fix this you need an expansion tank in the circuit: as someone else has said some builders only provide a tiny expansion tank on the assumption that the plastic pipework gives enough expansion to cope with heating water. In that case you need a bigger expansion tank or a second one. New plastic pipework is more flexible than old, so this is a problem that may creep up on you as things age.

The spec of the PRV should be a bit higher than the pressure your water pressure pump will produce: often about 25-40 PSI, typically paired with a 3 bar (44 PSI) PRV. You need to look at your water pump to check.

 

[PeterWright]

Hi Little Sister,

The prv is a safety device to prevent the calorifier being burst by the water expanding as it is heated up. If you have not found one on previous calorifiers, that's probably because you didn't notice it. A Calorifier on a pressurised water system would be in breach of the Pressurised System Safety Regulations and anybody selling one would be liable to prosecution.

For safety, the prv should have a set pressure (the pressure at which it opens) less than the safe wrking pressure of the calorifier vessel, which should be clearly marked on the vessel, but most naufacturers do this by sticking a paper label on the vessel, which soon decays and falls off. The set pressure should also be greater than the pressure at which the domestic water pump turns off, otherwise the pump will empty your tanks into the bilge.

The prv should not leak when the pump is switched on but you are not heating the water. However, earlier yachts, including Moodys, we're built with no expansion tank in the hot water system, so that as the water is heated up and expands, it lifts the prv to release the pressure to a value below the prv set pressure. This inevitably dribbles some water into the bilge, unless you route the discharge from the prv into an open topped container in the bilge which you empty periodically. To stop this "leakage", you can fit an expansion tank. David Longhurst wrote an excellent summary on the features of a modern domestic water system which you will find on here with a search, with the help of that summary, it's fairly easy to bring the older (pre about1992) systems up to modern standards.

Peter.