hi, i have a glow worm swift flow 80 combi boiler. the pressure gauge keeps dropping to just above 0. i can keep topping it back up but it slowly over
3/4 hours goes back down. when the heating is on the gauge can go up to 3 or
Sounds like a problem with the expansion vessel, causing the pressure to rise excessivly during operation. This then causes the pressure relief valve to open, venting the excess pressure out. ( look for the vent pipe somewhere outside, usually on an outside wall near the boiler. ) Then when the system cools down, you see the very low pressure you indicate.
Agreed, split diaphragm on expansion vessel. Easy to replace (the vessel).
I agree that it's a problem with the expansion vessel - but it may not have failed completely, and may just have lost its charge pressure.
The expansion vessel may be a bomb-like device located somewhere in the system - maybe close to the boiler or perhaps in the airing cupboard (if you have one) - or it may be a pancake-like device inside the boiler casing. It will have a schrader valve (like a car tyre valve) on it for checking/adjusting the air pressure.
With the water system unpressurised, check the air pressure in the expansion vessel using a car tyre pressure gauge. Use a car tyre pump to pump it up to about 0.7 bar (about 10 PSI). The use the filling loop to pressurise the water system (cold) to about 1 bar.
This may fix the problem. If it doesn't, the pressure vessel will have to be replaced.
If you can get water to come out of the tyre valve, then diaphragm has split. If not, then it just wants pumping back up (and might need a new tyre valve if that's leaking air out).
claire armstrong formulated the question :
That sounds likely to be the expansion vessel lacking air, or a split diaphragm. As the water in the system heats up, it expands -the expansion vessel takes care of the expansion without loosing any water.
It is just a small tank shaped like a bomb often painted white or blue, water in the bottom, a diaphragm separating the water from air above it. As it heats up the air can be compressed. If there is no air, or the diaphragm has burst, the pressure rises as you describe. The first step is to try pumping air into the vessel with an ordinary car tyre pump, via a tyre type valve on top of the vessel. If that doesn't work the entire vessel needs to be replaced.
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