While repairing his hot water boat calorifier, Chris Mardon found it had a spring and one-way plunger. But what purpose do they serve? Stu Davies has the answer

The hot water calorifier on my yacht is made by the Italian company Quick and is a 20lt size from their BX range of water boilers.

A minor long-standing leak between the immersion heater boss and the casing revealed itself when it became more serious and was leaking gallons of water into the bilges.

Time to take it out and have it expertly welded for £50; much cheaper than the £500 replacement cost.

Reinstallation went well until I came to fit the pressure relief valve (PRV) onto the calorifier’s lowest, ½in cold water inlet threaded stub.

Among the parts I’d taken off the calorifier before it was welded were the spring and plunger you can see in the photo.

This is not the pressure release mechanism which is still intact inside the PRV. So what is it? What purpose does it serve? And how should it be fitted?

There’s no shoulder or washer for the spring to bear against and it’s smaller than the inside of the calorifier’s ½in threaded inlet stub.

My search of the Quick installation instructions shows an exploded diagram including the PRV but no plunger/spring is shown or mentioned.

I eventually reinstalled my boat calorifier without the mystery one-way plunger and its spring as I couldn’t see where it was supposed to go nor what purpose it served. I’m pleased to say that it’s all been working well since.

Can anyone help resolve this mystery?

Chris Mardon

Stu Davies replies:

The lever on the PRV is to test whether the PRV is working and functions by pulling at the shaft of the little PRV valve.

I would expect the spring and plunger to be fitted under the lever and beneath the cap inside the PRV.

The shaft would be connected to the lever to allow this.

It pulls the rubber seal off its seat against a spring and the water under pressure can then escape from the smaller port on the PRV.

Continues below…

I’d expect the ‘shoulder’ to be under the lever cap, and am guessing that trying to put the shaft and spring in the inlet where the PRV screws on to the calorifier is the wrong place for it to be.

Check that there is something under the lever cap and that the lever has spring pressure against it.

If it doesn’t then that could be the issue.

But if, as you say, the PRV is fully intact and works properly already then perhaps the plunger and spring should never have been in your boat calorifier in the first place.

Enjoy reading Should my boat calorifier have a spring and plunger?

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