Sudden pressure drop to zero

[Mr M]

I have a Main Combi 24 which was working fine up until a couple of days ago when I noticed that the pressure gauge reading had dropped to zero! I checked for leaks and there were none that I could see. I also checked the PRV outside and it was bone dry. I topped the system up to 1.5 bar via the filling loop (which didn’t seem to take that much water), bled the radiators which had no air in them, re-checked the pressure which was still at 1.5 bar, checked the hot water which was fine and turned the central heating on which was also fine and then turned the heating back off again.

I came back about 4 hours later and the pressure had dropped to 1 bar. I went through the same procedure described above again and left the boiler with the pressure gauge reading 1.5 bar and switched it off as I was out for the night.

I came back yesterday after being away for about 24 hours and the pressure gauge reading had dropped to zero once again. I went through the same procedure described above once again and set the boiler with a pressure gauge reading of 1.5 bar.

I did a bit of reading-up and one of the suggested causes for sudden pressure drop to zero maybe a faulty or uncharged expansion vessel or worse still, the diaphragm inside being ruptured. With the boiler still switched-off, I unscrewed the cap which covers the schrader valve. As soon as I released it, air hissed out without me pushing the valve in. No water came out of the valve. I screwed the cap back on to the schrader valve to stop any more air from being released and tightened it by hand. Once again, I went through the same procedure described above again and left the boiler with the pressure gauge reading 1.5 bar leaving it switched it off. I came back about 8 hours later and whilst the pressure had dropped to just above 1 bar, the drop did not appear to be as rapid as it was previously before I hand tightened the cap which covers the schrader valve.

Based on the above, what should my next course of action be? Does it sound like it's simply a faulty schrader valve and if it is, can this be easily removed and replaced with a new one? Or is possibly something else?

 

[Burnerman]

With the system completely depressurised, remove the schrader valve with the appropriate key and fit a new one......car type inserts are fine.
Pressurise the expansion vessel up to the recommended level (around 12psi usually), allowing water to escape as you do so.
Once you've checked for leaks at the valve, open the filling loop and repressurise. With this work done, you'll be able to note what happens next, if anything.
John

 

[Mr M]

Thanks for your reply. Just so i am sure what actually needs to be removed, is it a) just the little brass insert inside the circular valve housing ie the thing that you press in to let the air out or is it b) the whole unit that connects to the top of the vessel?

 

[Burnerman]

No, just the insert inside - identical to removing a car tyre valve inner. You'll need the proper tool though, sometimes there's one on the end of a metal dust cap.
John

 

[Madrab]

 

[Mr M]

Just ordered a valve removal tool and replacement valve off t'internet

 

[ianmcd]

Make sure you leave a drain point open while you re-pressurise your boiler

 

[Mr M]

Make sure you leave a drain point open while you re-pressurise your boiler

Can you clarify this. I am aware that I need to drop the pressure before I remove and replace the defective schrader valve. With the boiler pressure dropped (reading the pressure gauge on the bolier), once I have the new valve fitted, do i then recharge the pressure on the expansion vessel before i re-pressurize the central heating system back up to 1.5 bar via the filling loop? Or do you mean something else?

 

[ianmcd]

yes you re-pressurise the expansion vessel with air, but you MUST leave a drain point open somewhere while you pump the vessel up once the vessel is at the correct pressure then close the drain valve and pressurise the water side, if you dont leave a drain point open while you do it, then no point in doing it

 

[Harry Bloomfield]

Can you clarify this. I am aware that I need to drop the pressure before I remove and replace the defective schrader valve. With the boiler pressure dropped (reading the pressure gauge on the bolier), once I have the new valve fitted, do i then recharge the pressure on the expansion vessel before i re-pressurize the central heating system back up to 1.5 bar via the filling loop? Or do you mean something else?

Yes - release all of the pressure in the system and leave it open to atmosphere, so the water has to some where to escape to, as the expansion vessel is blown back up with air. You then replace the valve core, it just unscrews, or maybe there isn't one in there at all. Then blow it up with some sort of air pump - car foot pump, some bicycle tyre pumps fit, or one of those portable 12v pumps run from a car battery. Check the pressure with an ordinary car tyre pressure gauge.

Finally, you can repressurise via the water fill loop, as you have done before.