high pressure on unvented cylinder

6 Mar 2008
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United Kingdom
hi guys , first post on here.

anyway a friend of mine has a system boiler and an unvented cylinder. he noticed the pressure gauge was reading close to 4 bar. i told him to drain some water of the central heating system. which he did. the pressure dropped down to 1.5 bar but within 1 minute went back up to 4 bar. this was when the heating was off by the way.
so my question is this
im thinking it could be a a leak in the heat exchanger thereby the central heating loop is becoming mains pressure. although i thought pressure relief valves were set around 3 bar!

or somehow a faulty expansion vessel although it wouldnt explain the fast rise back to 4 bar in pressure(would it?)
any other ideas what it could be as my friend has turned his heating system off now :confused: :confused:

he hasnt said what boiler/unvented cylinder he has sorry
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Which gauge, the boiler gauge or the gauge on the cylinder.
We are in the trade and would not give advice to a customer in any detail if we didn't know what they had.

However, it sounds here like the filling loop is not staying fully shut. Even a slight let by will do as you describe.

And as you rightly say, the unvented has a combi valve which is set to 3-3.5bar (although some models go higher). Bear in mind that the boiler pressure gauge may not be entirely accurate and could be reading 4 when it is 3.

If you have a filling loop or unvented let by problem the problem will not go away when you turn the boiler off.
thanks guys.

simond im in the trade myself and am looking for other ideas outside of what ive suggested im at a loss as to what it could be.

whats the likelyhood of a faulty pressure gauge??
ive not come across this problem before. so i can only think of obvious answers. so if anyone has come across this before is there anything i havent though of to check!!

thanks for your thoughts on this

i will be going over to have a look later today so hopefully it will be an obvious problem.
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Which gauge, the boiler gauge or the gauge on the cylinder.

As an aside (still learning) why are there two guages (noticed this on a mate's new unvented cylinder)? The guage on the cylinder measures pressure in the primary heating system so what does the guage on the boiler measure?
If you're in the trade chrissyp, :oops: :oops:

The primary water which is in the boiler and the radiators is an sealed system.

You have two options and it ain't rocket science.

The coil in the un-vented has gone toes up or the filler loop is letting by. :LOL:
Disconnect the filling loop and if you are sprayed with water then it is a fair bet that the valve is letting by...:cool: Failing that the only other place where mains water comes in to contact with the systems water is the heat exchanger
went around and had a look ... drained it down and the pressure went up straight away.. turned mains stop tap off and drained it and it helps it pressure. therefore must have a crack in the heat exchanger.. phoned ariston who confirmed the problem ... my mate is gutted!!!!

thanks for the help guys
What is the model and age of the cylinder.

Some have an 5 year tank guarantee but most have an 10 year tank guarantee.
yeah thanks doitall. the ariston has a five year guarantee. but it was fitted 6 years ago.

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