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Central Heating

Expansion vessel faulty or not?

Please help.
Can someone kindly explain what are the key indicators are that my combination boiler expansion vessel needs to be replaced?
If the vessel pressure is checked when the boiler is switched on and under pressure would it disperse water when a pump is attached and re-pressuring it with a bike pump?
What I have learned and understand so far is that there should be a release of air from the valve when activated when the boiler is switched on or off.
If the expansion vessel diaphragm is faulty when the air valve is engaged then water should/will come out?
However if the system is under pressure then my understanding is that of course water would come out of the air valve and the person doing the job is trying to con me into getting unnecessary work?
Answers to your various comments - The boiler was checked with pressure in the boiler also when given the combustion receipt I compared with a previous receipt and quickly noticed that the device that has been used is out of date of its calibration so gave of incorrect readings. I showed the couple the door and did not pay. I believe that the boiler pressure continues to drop though very slowly and sometimes has gone into the anti frost 5°C area, there is pressure in the boiler though it isn't been driven over 3 bar 2.5bar max which I understand this is good?
It's just the drop in beyond normal pressure which I do not understand, will place a bag over the pipe to see if any water is trickling outside.
Many thanks in advance for your help.

4 Answers from MyBuilder Heating Engineers

Best Answer

If you have a faulty pressure vessel then it won't be able to take the pressure rise when the boiler is on. The key indicator is that the pressure gauge will rise to 3 bar or above which will cause the pressure relief valve to open, you may notice water trickling from the pipe out side.


Answered 3rd Feb 2017

I Have Changed only about 5 Expansion Vessel on combi Boilers, (1000s of Repair Jobs Completed)
they are very rarely faulty . All i do is Service and Repair Boilers so i know what I'm talking about.
The Vessel needs to be checked with NO Water Pressure in the Boiler with a Foot Pump should read about 20 PSI on the Gauge.
Get a Proper Heating Engineer, Someone that Knows what they are Doing.
Water coming out of the Valve?.. Then change the Schrader Valve Core
Tell him to GO Away!


Answered 9th Feb 2017

If u get water from the air valve u need a new vessel. Diaphragm has failed


Answered 1st Aug 2017

Water will only exit the air admittance valve if the diaphragm is punctured, the diaphragm present in the expansion vessel is a means of keeping the water and air separate. And the charged air gap being a means of accommodating the expansion when the heatings at temperature and keeping the pressure in the system a more a less consistent pressure. It does indeed mean it needs replacing


Answered 3rd Feb 2017

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