Combi boiler issue

[Tim Hall]

Four year old system, boiler serviced annually. The system has been gurgling lately as it fires up so I wielded the bleeding key. One radiator had a fair amount of air. Pressure obviously dropped when I did that do I topped it up via the filling loop.

Now when it fires the pressure climbs much higher (2.3bar) then drops off as it cools. Trouble is it drops to below 0.5 bar and the boiler refuses to fire. Crack the tap on the filling loop to get the pressure back up and it fires.
Had the front off the boiler, pressed the tit on the expansion vessel, air came out so that seems to be ok.

Have not yet leant out of the window to see if the overpressure outlet has been running. The valve for that trips at 3 bar.
The radiator that had air is cold at the bottom, suggesting it's sludged up, further suggesting the corrosion inhibitor needs renewing. If it is low on inhibitor tbat might have been hydrogen that came out. Should have done a lighted natch test.
While I get round to that, anyone got any bright ideas what's going on? I'm out for a couple of hours so you've all got a bit of time to think.

[Feanor]

Expansion vessel probably needs pumping up.

That air and not water is coming out of is good, it has  not failed. But it has probably lost its air pressure.  The air in the system you bled off would, to some extent, have acted as expansion volume.

Whats happening is that when you fill it to say 1 bar when cold, as it heats up the pressure rises above 3 bar due to the lack of expansion volume. The relief valve opens and you loose some water. This means that when it cools, it drops down below the original 1 bar.

Its normally a schrader valve on the vessel. Use a bike pump to pump it up to around 1 bar when the system is cold.

[Diver300]

Its normally a schrader valve on the vessel. Use a bike pump to pump it up to around 1 bar when the system is cold.

I would suggest depressuring the system, the pumping up to 1 bar of air in the bit with the schrader valve, the repressurise with the filling loop.

[Feanor]

Yes, I'd depressurise before pumping it up too.

This ensures the air bladder is fully occupying all the internal volume of the expansion vessel before it begins to come up to the 1 bar of air pressure. So you get the proper volume of 1 bar air.

[Tim Hall]

Reporting back.

Using my Halfords digital tyre gauge, the expansion vessel reads 18psi (1.2bar) with the system depressurised.

I repressurised the system with heating disabled , so it wouldn't fire until I was ready, rather than trying as soon as it passed its minimum value. Still getting to 2.2 bar but the drop in pressure seems slower, although I've not timed it. Pressure on the expansion vessel didn't rise appreciably when the boiler was running.

I noticed that one radiator is cold all the time (in the kitchen so not really needed) even when the thermostatic valve is turned to max. Needs more investigation.

Years ago I had similar symptoms jn another house although that time the pressure climbed higher and quicker, tripping the release valve. That turned out to be the pipe connecting the wet side of the expansion vessel being blocked with crud