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Central Heating

Boiler losing pressure but no leaks from rads and no drips from pressure release outlet?

We have recently renovated our house and had a new boiler/central heating system installed and the boiler is always losing pressure. Once the boiler has been pressurised to around 1.5 bar it will run perfectly fine while it's on but once the thermostat switches it off it will lose pressure. We have had the boiler checked and there is no water in the boiler that shouldn't be there and no leaks from the pressure release outlet. This happens quite regularly and the boiler will often show 0 bar until re-pressurised!

I have checked all the rad valves and any pipe work I can see and there are no apparent leaks. There are no pipes under the ground floor and no leak patches on any ceilings. Any help would be much appreciated.

5 Answers from MyBuilder Heating Engineers

Best Answer

Seen this a few times, especially Worcester Bosch boilers.
Maybe the heat exchanger, it drains via the condense pipe.


Answered 3rd Feb 2015

Have an engineer pressure up the system to see if its the system or the boiler, isolate the boiler overnight on the flow and return pipes without a demand and see if the pressure drops, if yes its the boiler, if no its the system .... there a start for you but get a proper engineer not an installer.


Answered 23rd Feb 2015

Is the pressure rising quickly from 1.5bar to maximum when heating is switched on (after 10-30min)?
If yes, then expansion vessel need to be recharged.


Answered 3rd Feb 2015

The water is escaping somewhere. No 2 ways about it. Either leak you haven't found or a pinhole In the plate heat exchanger (if it's a combi) very unlikely if it's a new boiler.


Answered 3rd Feb 2015

Have you Cleaned out your condenser trap?
Have you gone round your radiators and bled them from air?
Where your boiler is situated there should be a valve to release air along with water, get a bucket/bowl and release the air slowly and you should see a change.


Answered 8th Feb 2019

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