DIY Doctor

Using hot water depressurises CH system

Postby starlinguk » Sat Dec 06, 2008 9:09 pm

Hello, y'all.

I've just moved into a rented property, and have found the CH and hot water are behaving oddly.

First of all, the pressure of the CH system is 3 bar. Far too high. The landlord says the plumber doesn't know why. There was also a "leak", apparently, and wonder if that was the pressure relief valve doing its job (oddly enough it doesn't get above 3 bar, even though the leak is fixed).

Secondly, when you use the hot water, the CH system depressurises until it's below 1 bar, and then it goes cold, UNLESS the CH system is on (ie it hasn't switched itself off because the radiators are hot enough).

And thirdly, I think we have a balance problem: you can only get the living room radiator to switch on when all the other radiators are off. Once you have that radiator hot-ish (they never get extremely hot), the one in the dining room won't get hot.

I've had a look at the boiler. It doesn't have an automatic refill valve, and since the system repressurises when you've used the hot water, the only thing I can conclude is that the refill valve/tap is always open, hence the high pressure. Thing is, if you switch it off, you lose pressure every time you use the hot water and it won't come back, so you have to repressurise the system!

In brief: there's something rather wrong with the CH/hot water, but what? I want to be able to tell my landlord what I think is wrong next time he comes 'round so he can't fob me off.
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Simply Build It

Postby plumbbob » Mon Dec 08, 2008 11:33 pm

The problem is caused by a faulty boiler. Specifically, there is an internal leak between the hot water and heating circuit.

The braided hose you mention should be turned off and only used to refill the system when the pressure falls. Normally it should be kept between 1.0 and 1.5 bar.

This fault needs to be fixed immediately as there could be at the very least, a risk of pipework failing causing a major flood.

There is no point in trying to fix any other problem until this one is fixed.
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