Our 371 has a 20ltr Isotemp hot water calorifier installed under the bed in the aft port cabin. The calorifier has a 220V heating coil and is also connected to the engine cooling water. 

As is necessary, the calorifier is fitted with an overpressure safety valve. This spits out a little bit of water while the contents of the calorifier heats up (and expands). But…the water is just relased into the underlying locker which then overflows into the engine bay bilges. With regular use I’d say about 1 to 2 litres a week is released. Not a massive amount, but as this is now the only source “wetting” the bilges I would prefer to eliminate it. 

Had anybody been successful in rerouting this drainvalve to somewhere? An overboard connection maybe? As thisnis all happening in the bottom of the boat, a gravity feed to elsewhere is not going to work. 

For now I have placed an empty yoghurt tub under the valve and empty it every few days…

Not the most professional system but very effective.

Bob

Normally I have only 1- 2 ltrs in it.

I am switching off the water pressure system, when we are not on the boat.

The 220 V supply to the water heater however stays on.

Mine seems to produce more overflow from the electric heater than the engine.

If you fit a pressure accumulator the expansion of the heated water will not excede the blow off pressure of the pressure relief valve.

Having said that the PRV fitted to my old Isotherm Calorifier was very low quality similar can be bought from China. I have sourced a better quality domestic PRV from local plumber shop, but you have to fit an additional one way valve [our plumbers call them "check Valves]. As the Iso therm PRV has an inbuilt one way valve.

I recently bought a replacement for the failed [corrosion]  Isotherm calorifier and an accumulator from UK company [Surejust]. same price as Isotherm, but made of quality materials with good valves.

Regards

Perry

2. When the water is heated by the engine, then the thermostat is bypassed. To prevent overheating and the overpressure to open we alway switch off the pressure pump and open the hot water tap. Then there is some water coming out of the tap, after a few minutes it stops. Then the water has expanded enough.

The thermal expansion coefficient for water is approx 210 ppm/deg C

To heat your calorifier by electric or engine to same temperature will have same expansion.

water heated from 20-60 deg C will expand 40* 210 e-6 *20000 ccm = approx 170ccm

Ignoring the expansion of the tank volume  approx <1ccm

In some yachts the numerous rubber hoses in the hot water system will accomodate this amount of expansion. Some calorifiers are plain steel [NOT:- Copper, stainless, acier inox, rostfreier Stahl] pressure vessels with vitreous enamel liner. heat and pressure cycling may limit their life as the vitreous enamel coat detaches.

Yes,  one solution is to turn pump off, open tap to de-pressurise system every time you heat the calorifier.

We forget this  and so have fitted the pressure accumulator vessel which also benefits by reducing the pump cycles.

Bottom line is a good system should be built to work with thermal expansion of water, not just vent off to protect the pressure vessel.

Regards

Perry

What size accumulator did you fit?

2 lires seems fine. On Bloto today, and my wife says the pump runs less and doesnt sound as harsh. Result!   The treasurer is Happy!

Perry

PS I see your in Belgium, My mother in law was borm Antwerp [Folkstraat], still got familt there.

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I blanked the valve off in 2005 & just remember not to overtighten the hot taps in the heads & the sink ( there is no need to force them anyway)when starting the engine or shore power if the water is cold. The tap sometimes drips a small amount into the sink, but that is no big deal. If I am using a lot of water & it is being heated by shore power, the tap is being turned on & off , thus releasing pressure anyway.

At the end of the year I drain off the water by disconnecting a hose so do not have to use the safety valve to release water

Pressure vessel with no pressure release valve?

If your immersion heater over temperature cut-out fails you will boil the water to steam and the calorifier looks like a bomb. Steam explosions were the problem in early steam engine boilers, and so the PRV was invented.

Sounds pretty dangerous to ignor safety history? typically pressure vessel safety regulations also prevent pipe connections to PRV outlet, this is in order that the blow off [preventing explosion]  cannot be restricted.

You are relying on the rubber hoses to be your accumulator, and you havent had a problem, 'so far so good'.

I think over pressurising the calorifier is just not a good idea, please see my previous comment on Isotemp mild steel pressure vessels with vitreous enamel interior coating for corrosion resistance.

Regards

Perry

I do not think there would be a problem. It is nothing like a pressure vessel in a steam engine. That would have a void for large quantities of steam in the vessel. This has steam that is compressed & can release pressure extremely quickly. Water does not do that. These calorifiers are virtually all filled with water. When testing steam boilers the method includes filling the vessel with water & pressurising the tank. If it fails, a bit of water seeps out of a seam. There is no violent bang. In my installation it would be more liable to blow a plastic pipe fitting long before it got to danger state.

 I do not let the water run dry. If it does the tap would be open or the shore power would be turned off. The thermostat would trip. It is not set to scald. Engine temp will not boil the water, unless there was a major engine fault & the engine alarm would go off long before that.