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Roger Mills Roger Mills is offline
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Default Why's my boiler gone over-pressure?

In an earlier contribution to this discussion,
A.Lee wrote:

Alpha Combi boiler. Working fine, or so I thought.
A bit chilly this evening, so put the heating on for the first time
in 3 weeks or so.
Pressure was a little low, so fill up to the recommended 1 bar. Tap is
shut off.
Heating comes on fine. 20 minutes later, I'm outside and hear water
running - the pressure relief/overflow from the boiler is spewing out
a bit more than a trickle, but less than a hosepipe, of hot water.
Back inside, turn off heating. Notice pressure guage now at 4 bar,
hence the pressure relief.
First thought, air in system, though bleeding all the rads doesnt
produce much air - typical of what I'd expect after not being done
for 6 months.
Pressure now down to 2 bar when it is cool.

So, why has this happened?
The pressure relief has never opened, to my knowledge, in the 8 years
we've had the boiler.
All rads were warm, though not hot.
Thanks
Alan.


Chances are that the pressure vessel is shot [1]. That would certainly
explain the symptoms because (a) it would cause the pressure to drop
(prompting you to top it up) and then (b) it would have no resilience to
absorb the expansion volume when the system gets hot - causing the pressure
to build up to the point where it triggers the pressure relief valve.

[1] There are degrees of shottedness - some of which are fixable just by
topping up the air through the schrader valve, and some of which are not
without replacing the whole thing
--
Cheers,
Roger
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