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headroom | 18:31 Sun 31st Jan 2010 | How it Works
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I have an Ideal Isar boiler (which I now know is rubbish) the system keeps losing pressure which as I understand indicates a leak on the system. I have to top it up about every 4 days, my question is is it possible to determine whether there may be a leak on the expansion vessel? I suppose there could be a leaky joint somewhere, but how would I find that out? Thanks.


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I'm not a Gas Safe registered heating engineer, but I have the same boiler, and mine did the same. It was leaking inside the boiler from an automatic bleed valve - the leaked water was falling onto a hot bit and evaporating, so no sign of a leak. The heating engineer simply raised the pressure a bit more - to about 1 bar, I used to top it up to about .7 bar - problem solved. He told me that the bleed valve is a floating ball pushed against a seal, if it gets covered in cack (technical term) it doesn't seal properly, but a bit more pressure squashes the cack against the seal. If that hadn't worked, he was going to replace the auto bleed valve (cost about 10 pounds + half hour labour).
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Hi Vagrant, is the automatic bleed valve the same as 'safety relief valve'?
Don't think so, but as I said, I'm not an expert plumber, just have the same boiler - as far as I know, the bleed valve is to let air out when you fill up with water - the safety relief lets water out to the outside if the pressure goes too high - if that's leaking, you'll see drips coming out of the pipe that sticks through the wall to outside.
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Got it, the valve you are talking about is automatic air vent, so when yours was releasing air it was also allowing water to escape too because the floating ball wasn't sealing the vent off properly, so basically it could be that OR the safety relief valve, Hmmm! how to tell which, I have placed a bag over the external pipe so that might give me a clue! Thanks again.
fill your boiler to exactly 1 bar, then turn on the central heating with the thermostat set to maximum and let it run, keep an eye on the pressure guage and see if it rises towards 3 bar as the rads get hot. if it does and the boiler lets water out the back via the pressure relief valve then there is a problem with your expansion vessel.
another answer is your prv may have opened up and then not shut off properly, your bag over the pipe trick will tell you that.
the only other thing you can try is to turn off the heating flow and return valves, if the pressure then does not drop you know the leak is on the heating system, if it does drop you know its an internal boiler problem, only problem with this is it means you cant use your heating for a few days
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Hi Vagrant & Gucciman, problem sorted! It was a leaking joint! we had a radiator moved about 12 months ago, one of the joints was dripping big style!......Thanks for all your help.

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