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.
Pressure vessel failure.
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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.
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?
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
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.
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.
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.
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