Pressure gauge goes to zero.

9 Jun 2014
Reaction score
United Kingdom
Vaillant ecotec pro 24
A bathroom radiator was only getting 1/3rd hot. I bled radiator fully. Noticed pressure gauge drop to zero. Filled from inlet feed. Pressure went back up. Turned system on , radiator 100% hot. Came back 3hrs later.....gauge went back to zero. Did i do something wrong?
I filled again, pressure back up. Radiator 100%. Afraid that when system is turned off pressure will drop again. Pls help
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Expansion vessel problem possibly. See if pressure relief valve pipework outside has had water running out of it.
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Is central heating currently on ,if so what does pressure gauge read. And how long has it been on for ? Prv won't normally open until pressure is 3 bar ( ish )
If pressure drops again after system is switched off how safe is it to keep filling up or will system just stop working after a while.
When water heats it expands ,and system pressure increases. The expansion vessel ( EV) accommodates this and when system cools pressure drops back to what it is when all is cold. If the EV is not functioning ,and you have a radiator that's full of air ,the rad acts like an EV ,by the air being compressed in the rad and accommodates the expansion. By bleeding the air out ,this now cannot take place. So pressure builds up ,the PRV opens allows water out ,reduces pressure. When boiler goes off ,water cools ,pressure drops to very notice and top up again ,the cycle then starts all over again ,and again ...
Possibly ,also possible it just needs to be recharged. The EV is basically a tank ,with a diaphragm inside .one side has the CH water the other side is charged with compressed air. If the diaphragm splits you replace EV. If its just lost its charge it can be repressurised ,bit like a car tyre . If you turn off heating tonight ,and tomorrow morning its back to nowt ,tie a plastic bag to prv pipe outside. Bring pressure back to 1 .25 bar ,put c/ heating back on. After a few hours check bag for water. I think you will find the prv has lifted and water in bag.
Tnx for advice terry. After filling for the second time radiator is fully hot and boiler has held the pressure. After system is off for 8 hours the pressure still holds. When system is off gauge drops a fraction and when system is on gauge goes up a fraction. Does this mean i still have air in rads and they need bleeding more?
When COLD pressure should be 1 bar. When heating is on its normal for it to increase , and return back when cooled. Its best to add water /top up pressure when system is cold. I think you bled your rad when c/ heating was on ,then topped up pressure .to 1 bar .so when you turned off ,it cools and dropped .so you then topped up the second time , when cold ( I presume ) so now its all good.

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