Constant c/h pressure drop with no sign of leaks

[MitchieLad]

Hi folk
For some time, my central heating system (boiler is a Worchester Bosch Greenstar 24i Juniorwhich is serviced annually) has been losing about 1/2 bar of pressure a week. I tried leak sealer (2 bottle Fernox and leaving the system running with all valves opened for several hours) without success. As the house is two story any leak upstairs would have shown long ago.
Before going away for 5 days I switched off the power supply to the boiler. On return the pressure was again down about 1/2 bar. A check of all rads and exposed pipework before the system was switched back on showed no trace of dampness and all carpets are, and always have been dry.
Yesterday and today I lifted flooring in each downstairs room and still can not find any sign of leakage – the concrete sub-floors have a thick covering of dust, etc so any long term continuing drip should have been easily spotted.
The vent pipe from the pressure release valve is just outside the bathroom window and, despite constant checking, there has never been any sign of a drip and the concrete path below it is dry. I even climbed up a ladder to check with some kitchen roll just to make sure after the system was running for some hours.
The expansion vessel has been checked and declared OK.
I am now wondering if there may be a tiny leak in the heat exchange unit? Although no staining has shown in the bath, basin or sink I can’t think of any other possibility. The last thing I want to do is have the h/e unit and air pressure valve – the advice of my c/h engineer as he could not come up with any other idea - changed at a cost of some £300 plus and find the problem is still there.
All advice will be gratefully received.
Mitch

[Someone-Else]

Put a plastic bag over the pressure relief valve and leave it for a few days (Don't make it too tight)

[MitchieLad]

Hi someone-else
Thanks for reply. The engineer showed me the pressure release valve which sits at the back of the boiler against the wall and said that unlike most other boilers, to access either it or the h/exchange unit the boiler had to be swung out and away from the wall something I would not even think about at present.

[Razor]

It's a shame you didn't post before going away!

You need to find out whether it's a boiler problem or a system one. The best way would be to turn the system off and when it's cold pressurise the system to 2 bar and turn the isolation valves off at the boiler - this does mean no heating or hot water for a day or so though!

If the pressure has dropped when you come back then it's the boiler but if the pressure is still more or less 2 bar but drops when you open the iso valves the leak is on the system

[gas4you]

Does the condensate pipe drip all the time, even with the boiler off?

[MitchieLad]

Many thanks to all, especially Razor. Will do as you advised next week.
Mitch

[MitchieLad]

Hi again
Went to use Razor's elimination idea - ‘to find out whether it's a boiler problem or a system one, turn the system off and when it's cold pressurise the system to 2 bar and turn the isolation valves off at the boiler’, only to discover there are no isolation valves in the cupboard! Or anywhere else I look.
Am I right in thinking I have no choice but to drain down the system enough to cut the pipes and install valves. And, of course the pipes are clipped tight against the wall.

[PLB]

Hi again
Went to use Razor's elimination idea - ‘to find out whether it's a boiler problem or a system one, turn the system off and when it's cold pressurise the system to 2 bar and turn the isolation valves off at the boiler’, only to discover there are no isolation valves in the cupboard! Or anywhere else I look.
Am I right in thinking I have no choice but to drain down the system enough to cut the pipes and install valves. And, of course the pipes are clipped tight against the wall.

The others guys maybe busy at the moment but could you check again and have a good look under the boiler.
I think the isolation valves on that model are screwdriver slot type valves as opposed to butterfly type valves and they should be directly underneath the boiler.

[gas4you]

The isolation valves will be on/under the boiler itself where the 22mm heating flow and return pipes join the boiler.

[PLB]

Thanks for the confirmation Dave.
I was a bit concerned that the op would start cutting pipes before anyone got back to him.

[MitchieLad]

Many thanks to PLB and gas4you for your quick replies. You have made my day! Will look when I up.

[MitchieLad]

After a long search I have found a small leak at the filling point assembly which evaporated as quickly as it appeared in the heat of the boiler cupboard. So, after weeks of lifting floors it was a 10 minute easy fix. Sods law at work again. Many thanks again to all who advised me.

[PLB]

After a long search I have found a small leak at the filling point assembly which evaporated as quickly as it appeared in the heat of the boiler cupboard. So, after weeks of lifting floors it was a 10 minute easy fix. Sods law at work again. Many thanks again to all who advised me.

Thanks for letting us all know.
Glad to hear it's all sorted.