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Default worcester bosch boiler pressure

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.

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.

The boiler is ten years old. Can it be fixed? Should I be looking at a
new boiler? Apart from the pressure dial, the boiler is fine.
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Default worcester bosch boiler pressure

On 17/11/2020 21:08, AJH wrote:
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.

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.

The boiler is ten years old. Can it be fixed? Should I be looking at a
new boiler? Apart from the pressure dial, the boiler is fine.



It's probably a failed expansion vessel


That would be my guess too, or perhaps it just needs topping up? Ever
since I had an old Vaillant getting on for 30 years ago where the vessel
was totally inaccessible, I have had an external one on my system.
Modern stuff seems to be much better designed for servicing.

If you are half handy with DIY, it may not be difficult to remove the
outer casing, then it is just a question of unscrewing the valve cap (if
water comes out, the diaphragm has failed), attaching a bicycle pump and
pumping it up to 20 psi or so (you will need to look that up). You
should be able to do all that without doing anything like disturbing gas
or opening the combustion chamber.

You imply that you never need to top it up. Also maybe worth taping a
clear polythene bag over the external overflow and checking after a few
days whether any water has come out.
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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?


Probably

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:

http://wiki.diyfaq.org.uk/index.php/...rIntakeOff.jpg

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.


--
Cheers,

John.

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Default worcester bosch boiler pressure

In message , AJH
writes
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.
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.
The boiler is ten years old. Can it be fixed? Should I be looking at

new boiler? Apart from the pressure dial, the boiler is fine.



It's probably a failed expansion vessel


Certainly old enough.

Pressure vessels/system pressure has been exercising my mind lately
(never mind the just discovered manifold drip).

While the system was out of use, I checked the expansion vessel
pressure. Around 7.5psi but with a slight dribble of water. Pumped up to
around 1 bar.

Room temp to full system heat gives a pressure rise of around 0.8 bar so
within reasonable limits. However the slight drip has made me aware how
sensitive this set up is to system water volume changes.

My inclination is to reduce the expansion vessel pressure to say 0.8bar
(static head in a chalet bungalow not an issue) and the room temp.
system pressure to say 1.2 bar. This would use up some of the available
expansion volume but make the system more stable.

As has been said elsewhere before, I should really fit an additional
expansion vessel!

--
Tim Lamb


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Default worcester bosch boiler pressure

On 17/11/2020 23:33, John Rumm wrote:
that causes the emergency pressure reduction valve to dump some water
out of the blow off pipe outside


IME once one of these cracks open at over pressure they do not reseal
well and continually leak slightly thereafter.
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Default worcester bosch boiler pressure

In message , AJH
writes
On 17/11/2020 23:33, John Rumm wrote:
that causes the emergency pressure reduction valve to dump some water
out of the blow off pipe outside


IME once one of these cracks open at over pressure they do not reseal
well and continually leak slightly thereafter.


Oh! That's a bit worrying. I suspect mine has been opening due to water
hammer when the changeover valve operated. I have put in a relief valve
set at around 0.8 bar shunting the boiler output through the DHW coil
and then back to the boiler.

--
Tim Lamb
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Default worcester bosch boiler pressure

On 18/11/2020 10:23, AJH wrote:
On 17/11/2020 23:33, John Rumm wrote:
that causes the emergency pressure reduction valve to dump some water
out of the blow off pipe outside


IME once one of these cracks open at over pressure they do not reseal
well and continually leak slightly thereafter.


Yup can happen - although probably more likely if it does it a few times
and can get a chance to scale up. (and if its happing often that also
suggests more top up water, less inhibitor and more scale in the first
place just to compound things).


--
Cheers,

John.

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Default worcester bosch boiler pressure

On 18/11/2020 10:23, AJH wrote:
On 17/11/2020 23:33, John Rumm wrote:
that causes the emergency pressure reduction valve to dump some water
out of the blow off pipe outside


IME once one of these cracks open at over pressure they do not reseal
well and continually leak slightly thereafter.


That's been my experience too

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On Sun, 22 Nov 2020 15:49:25 +0000, AJH
wrote:

It's probably a failed expansion vessel


When the British Gas engineer visited recently, he said something
about "capillary".

Basically the pressure dial drops well into the red zone, and when you
tap it, it goes up into the green zone.

Would that be a problem with the dial itself, "capillary" or whatever,
or would it be a faulty expansion vessel?


Sounds like a sticky needle in the dial gauge. If it were the faulty
expansion vessel the pressure would drop below the point at which a
pressure switch would prevent the boiler from firing.


How would it be fixed? Can you replace the dial gauge, would that
solve it?
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On Tue, 17 Nov 2020 23:33:02 +0000, John Rumm
wrote:

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?


Probably

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.


We had a British Gas engineer a couple of days ago, and he said the
loss of pressure over a period of two or three months is natural, that
the boiler needs to be topped up every two or three months, and not to
worry about it.

Is he right? I'm not into this stuff, should I worry?
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On 05/06/2021 20:58, wrote:
On Tue, 17 Nov 2020 23:33:02 +0000, John Rumm
wrote:

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?


Probably

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.


We had a British Gas engineer a couple of days ago, and he said the
loss of pressure over a period of two or three months is natural, that
the boiler needs to be topped up every two or three months, and not to
worry about it.

Is he right? I'm not into this stuff, should I worry?


If it has been recently drained and refilled (or its a completely new
system) then there will be a fair amount of air dissolved in the water.
This will tend to come out of suspension and accumulate in a radiator
somewhere. When you bleed that rad, you will drop the pressure a bit and
may need to top up.

However once a system has been running a while and the the water is
basically stagnant, then it ought not change much or at all.

It is possible to have tiny leaks in a system - so slow the water
evaporates before you ever see it. This is generally not a problem if
you only need top up once or twice a year. Although you should
periodically add some extra corrosion inhibitor since it will get
diluted over time, and the fresh oxygen introduced each time you top up
needs to be "mopped up" by the oxygen scavenger in the inhibitor which
will get used up by the fresh water being introduced.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd -
http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
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