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John Rumm John Rumm is offline
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Default worcester bosch boiler pressure

On 17/11/2020 20:31, wrote:

I have a worcester bosch boiler, whose pressure dial is all over the
place. Sometimes it's at close to zero in the red zone, sometimes it's
just in the green zone.

And if its into the green when cold, does it go very high as the boiler
heats up?

It's been behaving this way for many months.

Is this a problem with worcester bosch boilers in particular, or
boilers in general? When we get a British Gas engineer to fix it, he
increases the pressure so it's in the green zone. After a few weeks
it's fallen again into the red zone.

It sounds like this is not a fault with the dial- its probably telling
you the truth. Its more likely that the system is losing pressure over
time for some reason.
The boiler is ten years old. Can it be fixed?


Should I be looking at a
new boiler?

Not if that is the only faulr

Apart from the pressure dial, the boiler is fine.

I expect the pressure dial is fine, and you either have a system leak,
or a non working expansion vessel.

With a leak, you would expect the pressure dial to act fairly normally -
rising a bit as the system warms up, and falling back to where it
started as it cools - however you may see a steady drop over a period.

If its an expansion vessel problem then the pressure change will be much
larger, swinging very high as the system heats, that causes the
emergency pressure reduction valve to dump some water out of the blow
off pipe outside. Then as the boiler cools it dropping to a much lower
pressure than when it started.

Expansion vessels are basically two sided chambers with a rubber sheet
separating the two sides. One side should be filled with air at
pressure, the other connected to the wet side of the CH circuit. As the
system heats and the water expands, it compresses the air. When the
pressure falls the air expands and pushes the water in the expansion
chamber back into the system.

The have two common failure modes; one is where the air side has just
lost some or all pressure. That can be fixed by pumping it up like a
bike or car tyre (same air valve as a car tyre). Alternatively the
rubber separator has perished, and the air bubble lost. If you find the
vessel and give a brief push on the valve centre you should get a puff
of air if working, or water if knackered.

If its knackered then you either need to replace it, or add another
expansion vessel elsewhere in the system. As a stop gap you can bleed
some water out of a rad and leave a pocket of air at the top - that will
act like an expansion vessel for a bit.

The expansion vessel is the silver thing on the RHS in this Vaillant:

You can see the shrader (car tyre) valve on the front in the middle.

On many boilers these tend to be positioned right at the back behind the
main heat exchanger and are a PITA to get at. (they are often painted
red as well). So these often get abandoned and another fitted somewhere
more accessible. On ones like in the photo, changing the built in one is
very easy.



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