combi boiler leak

[Mark]

Hi

I have a 3 year old Boxer C24 combi boiler. the problem with it is it is loosing pressure from the central heating. I have to top it up every day now.When its running water is coming out of the overpressure pipe outside the house. DHW and CH work fine when I repressurise the system, but the next morning the pressure gauge is on 0 again. Any ideas?

[ARWadsworth]

Pressure vessel failure.

or

Adam

[Gilbert]

Rather than the pressure vessel, the pressure relief safety valve may have gone. Top up the system and test the safety valve by (usually) twisting the knob - check to see if it opens/closes the flow in the external overflow pipe. If there is continuous flow then it is knackered.

Cheers Gilbert

[Mark]

Ok, checked that. if I open the valve it releases all the pressure, and water out of the overflow. Is it possible its not seating correctly? Is it possible to take the valve apart?

Why would a pressure vessel failure make the system loose pressure?

thanks Mark

[Andrew Gabriel]

If pressure climbs to 3 bar when hot (and discharging) and drops to zero when it cools, that's pretty certain to be the pressure vessel not doing its job. Normally, the difference between hot and cold pressures should be no more than 1.5 bar.

[Andrew Gabriel]

The pressure vessel is half full of air under slight pressure, with a rubber membrane separating it from the water so it can't escape into the heating system and get lost. When the water in the system is heated, it expands, and the pressure vessel absorbs this expansion, increasing the air pressure in it. When it cools and contracts, the pressure vessel maintains the system at the cold pressure (typically 1 bar).

If the air all leaks out of the pressure vessel, there's nowhere for the expanding water to go, so the pressure in the system climbs very much more than it would. At 3 bar, the pressure relief valve will let the expanding water out of the system. Now when the system cools and the water contracts, there isn't enough in the system anymore and the pressure drops to zero.

A pressure vessel can typically fail in 2 ways. It has a tyre valve on the air side, and this might leak, letting out the air. The other thing that might happen is the membrane inside breaks, and the air is lost into the heating system and either dissolves in the water or is bled out of the system. If you press the tyre valve release and water comes out, then the membrane has broken and you need a new vessel. If nothing comes out then you might get away with pumping the vessel back up with a tyre pump (to around 8 lb/sq-inch when the heating system is at zero pressure), although it might leak out again eventually.

If you do fix this, bear in mind that pressure relief valves often don't seal off very well once they've operated, due to debris getting caught on the valve seat. Also, your system will have probably lost its inhibitor, and that will need replenishing to avoid rusting through the iron components such as radiators.

[Mark]

ok, thanks for the reply.

Checked the pressure vessel and air comes out of the valve. Haven't checked the pressure gauge to see if its getting too high yet, but I will tomorrow.

Thanks again.

Mark

[Mark]

Ok, checked the gauge when it started up. Was just under 2 bar cold, increased to just over 2 bar hot. Water was coming out of the overflow as soon as the water got hot.