Pressure drop on my gas combi boiler

2 Nov 2003
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United Kingdom
My combi gas boiler was working fine whilst just heating up water - central heating switched off - during the summer.

I have now switch it to both hot water and central heating and it worked ok for a couple of days - but yesterday the pressure dropped and the boiler stopped working.

I aired all the radiators and topped up the pressure using the feed loop up to 1 Bar. and left the boiler in the hot water and central heating option but did not turn on the central heating (switched the timer to off) to see if the pressure would stay. after about a day the pressure had dropped to 2/3 of a bar.

I looked at the Pressure release overflow pipe and it was letting water out (very low flow of about a couple of drops every ten minutes) and the ground below it was wet.

Can you please help me with the following questions:

- Why is the overflow dripping if the pressure is only at 1 bar - could it be that I have a faulty PRV or a faulty expansion vessel?
- In the past I have had a leaking central heating pipe that was repaired so I do not think that it is the Central heating pipes as it was ok after the repair during last winter- If there was a leak in the central heating pipes would the water also escape from the overflow as a symptom?

More info:

The boiler is an ICON 23T about two and half years old.

Thanx in advance for your help.

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Open the pressure release valve (PRV) and reduce the system pressure to zero and check the expansion vessel. Lots of posts are on the forum about this. If you have a 2 storey house 0.5 bar air pressure is enough, and set the system pressure to 0.7 bar when cold.

There may be dirt on the valve seat, try opening PRV fully and quickly then let it shut quickly. You may need to do it a few times, so make sure the system has some pressure in it to make water come out of the discharge pipe.

The valve may need removing and cleaning, or replacing. If it was dirt on the seat, where did it comefrom? it indicates your system needs cleaning.

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