Boiler pressure issues

[inkey$]

So my Worcester Greenstar 24i has decided to play up again. Pressure dropping to 0 randomly; sometimes it will hold for a day, sometimes not and refill required 2-3 times a day.

Picture below of fill tap under boiler position which looks a permanent loop (thought they didn't need to be).

Can only bleed 2 rads as key too large for others that are slightly rounded off at bleed screw.

No visible sign of leaks anywhere and overflow not getting rid if water excessively (at all), so I'm mighty confused!

Any ideas?

 

[GreyZed]

Expansion vessel?

All combis have an integral expansion vessel to buffer the pressure change in the closed CH system due to heat rise (water is not compressible and expands when heated). So you have a little metal tank with a rubber membrane in with water on one side and air on the other. The air side is usually pre-charged (via a Schrader valve) to anywhere between 1 and 1.5bar. Air is compressible so as your CH circuit heats up water enters the vessel and compresses the air thus keeping your boiler pressure reasonably static (around 1.5 bar plus or minus 0.5 bar).

If your pressure vessel loses air pressure it takes in more water and your pressure drops. If the membrane is ruptured then water passes through into the air side and your pressure drops.

 

[inkey$]

Expansion vessel?

All combis have an integral expansion vessel to buffer the pressure change in the closed CH system due to heat rise (water is not compressible and expands when heated). So you have a little metal tank with a rubber membrane in with water on one side and air on the other. The air side is usually pre-charged (via a Schrader valve) to anywhere between 1 and 1.5bar. Air is compressible so as your CH circuit heats up water enters the vessel and compresses the air thus keeping your boiler pressure reasonably static (around 1.5 bar plus or minus 0.5 bar).

If your pressure vessel loses air pressure it takes in more water and your pressure drops. If the membrane is ruptured then water passes through into the air side and your pressure drops.

Cheers GreyZed. So sounds as the the expansion vessel membrane may be buggered then. Time to call out my friendly neighbourhood plumber. The arse of this is that we want to have a new kitchen this summer which would include new boiler and maybe repositioned. Need this one to last just a few months more!

 

[GreyZed]

Sometimes they just need recharging which you can do with a bicycle pump. Find the tyre valve and see if water comes out of it as this will tell you if the butyl membrane has gone or not.

If the membrane has gone then just add an aftermarket pressure vessel to your CH return pipework. Don't bother with replacing the integral vessel as it will be far more costly and may even require the boiler to be dismounted from the wall depending on the design.

Screwfix sell 8L pressure vessels for under £30 IIRC. 10 minute job to fit.

Most plumbers unfortunately may tell you they need to drain down your entire system and take hours to do the job (I wonder why????).

 

[GreyZed]

If your CH pressure rises by over 1.5 bar when running (from cold start to hot) then it indicates expansion vessel.

If you are continuously losing pressure from the supposedly sealed CH system and it isn't coming out of your blow off valve then you either have a leak somewhere on your radiator pipework or you are passing through the one way check valve on your fill loop or maybe your plate heat exchanger has pinholed (corrosion) and is passing water into your DHW circuit.

The trick is to positively identify the root cause rather than let a plumber loose to randomly change components at your cost until he gets lucky.

 

[GreyZed]

I had a similar problem not long ago. My symptoms were pressure dropped back to nearly zero, topped up but then noticed pressure was climbing really high (above 2.5bar). On my boiler the expansion vessel is behind the main combustion chamber so would be an off the wall and total strip down scenario to replace with OEM.

I bought an aftermarket vessel from screwfix for around £25. lowered the boiler pressure to zero using the blow off then closed the CH out and return valves and did a super quick speedfit tee in to the CH return pipework near the fill loop. Hardly any water came out (eggcup or 2?).
Fill loop before...


Fill loop after...


Expansion vessel connected via flexi...


The expansion vessel came pre-charged at 1.5 bar. I lowered the air pressure slightly using a tyre gauge to about 1.2 bar before fitting. After fitting I topped up the CH boiler pressure to 1.2 bar and that was that.