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Default Worcester Bosch 28CDi combi - what's a normal system pressure risefrom cold to hot?

Boiler is in daughter's house. System has 7 single panel radiators.

With the cold pressure on 1.2 bar, it rises pretty quickly to about 2.5 bar when hot.

I found my neighbour has exactly the same boiler and that barely moves - a couple of tenths of a bar.

Is the pressure rise anything to be concerned about, and is there a logical reason why they would they be so different?
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Default Worcester Bosch 28CDi combi - what's a normal system pressure rise from cold to hot?

wrote:
Boiler is in daughter's house. System has 7 single panel radiators.

With the cold pressure on 1.2 bar, it rises pretty quickly to about 2.5 bar when hot.

I found my neighbour has exactly the same boiler and that barely moves -
a couple of tenths of a bar.

Is the pressure rise anything to be concerned about, and is there a
logical reason why they would they be so different?


Most likely a failing (or failed) expansion vessel. If it is built into
the boiler it's usually a lot cheaper and easier to add a new external
expansion vessel than to replace the original.

The installation manual for your boiler should contain details on how to
check the pressure of the expansion vessel. If water comes out of the
schrader valve it's a sign that the diaphragm has failed.

Tim
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Default Worcester Bosch 28CDi combi - what's a normal system pressurerise from cold to hot?

On 18/02/2014 12:36, Tim+ wrote:
wrote:
Boiler is in daughter's house. System has 7 single panel radiators.

With the cold pressure on 1.2 bar, it rises pretty quickly to about 2.5 bar when hot.

I found my neighbour has exactly the same boiler and that barely moves -
a couple of tenths of a bar.

Is the pressure rise anything to be concerned about, and is there a
logical reason why they would they be so different?


Most likely a failing (or failed) expansion vessel. If it is built into
the boiler it's usually a lot cheaper and easier to add a new external
expansion vessel than to replace the original.


The service booklet for this boiler is available online.
http://www.worcester-bosch.co.uk/cac...ntinued-07.pdf


It says (for expansion vessel replacement):

Drain the central heating circuit as described in Section 16.3, a.
Hinge the facia panel into the servicing position as described in
Section 15.3, c.
Remove the air pressure switch, gas to water heat exchanger,
water to water heat exchanger and pump as described in 16.4, 2, 5,
14 and 20 and remove the vessel.
Fit the replacement vessel in the reverse order.


As you say, fit an external one if it's not too unsightly.







The installation manual for your boiler should contain details on how to
check the pressure of the expansion vessel. If water comes out of the
schrader valve it's a sign that the diaphragm has failed.

Tim



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Default Worcester Bosch 28CDi combi - what's a normal system pressurerise from cold to hot?

On 18/02/2014 12:15, wrote:
Boiler is in daughter's house. System has 7 single panel radiators.

With the cold pressure on 1.2 bar, it rises pretty quickly to about
2.5 bar when hot.

I found my neighbour has exactly the same boiler and that barely
moves - a couple of tenths of a bar.

Is the pressure rise anything to be concerned about, and is there a
logical reason why they would they be so different?



Well 2.5 bar is not too high and is within the allowable range. However
its quite a large rise for a system that does not sound like it has a
huge water content. Hence as others have suggested, it may indicate a
lack of charge in the pressure vessel.

Assuming it has not actually ruptured, it may just need a top up. To do
this you need to turn off the boiler and let the system cool, then bleed
a couple of litres of water out of the system somewhere (from a rad
connection say) to relieve the pressure in the system.

Then (if you can get to it) there should be a shrader valve (like on a
car tyre) on the expansion vessel. Test that with a pressure gauge, and
if its low (i.e. less than say 2 bar) pump it up with a normal tyre
pump, to whatever is recommended in the manual (probably over 2 bar)

If when you test the valve pressure, waster comes out, then you know the
vessel is knackered.



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Cheers,

John.

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Default Worcester Bosch 28CDi combi - what's a normal system pressurerise from cold to hot?

Thanks - I thought that if the vessel had failed then the pressure would shoot up over 3 bar and water would be ejected? I'm pretty sure there no rad full of air to absorb the increase either.

I checked the pressure vessel with the water pressure out of the system and it's 9psi - the manual says it's set at 0.5 bar initially although there's a table (9) in the manual that doesn't makes sense to me (I can't tell which is the pressure vessel setting and which is the water presure setting).


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Default Worcester Bosch 28CDi combi - what's a normal system pressurerise from cold to hot?

On 18/02/2014 13:03, John Rumm wrote:
On 18/02/2014 12:15, wrote:
Boiler is in daughter's house. System has 7 single panel radiators.

With the cold pressure on 1.2 bar, it rises pretty quickly to about
2.5 bar when hot.

I found my neighbour has exactly the same boiler and that barely
moves - a couple of tenths of a bar.

Is the pressure rise anything to be concerned about, and is there a
logical reason why they would they be so different?



Well 2.5 bar is not too high and is within the allowable range. However
its quite a large rise for a system that does not sound like it has a
huge water content. Hence as others have suggested, it may indicate a
lack of charge in the pressure vessel.

Assuming it has not actually ruptured, it may just need a top up. To do
this you need to turn off the boiler and let the system cool, then bleed
a couple of litres of water out of the system somewhere (from a rad
connection say) to relieve the pressure in the system.

Then (if you can get to it) there should be a shrader valve (like on a
car tyre) on the expansion vessel. Test that with a pressure gauge, and
if its low (i.e. less than say 2 bar) pump it up with a normal tyre
pump, to whatever is recommended in the manual (probably over 2 bar)


Are you *sure*? If you charge it to 2 bar or more, it won't do anything
until the system pressure exceeds that. I always reckon to charge it to
about 10psi (about 0.7 bar). Then, if the cold water pressure is set to
(say) 1 bar, the expansion vessel provides a bit of resilience, and
allows you to bleed the radiators.

[Are you sure you're not thinking of the vessel for a non-vented HW system?]
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Roger
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Default Worcester Bosch 28CDi combi - what's a normal system pressurerise from cold to hot?

On 18/02/2014 13:34, Roger Mills wrote:
On 18/02/2014 13:03, John Rumm wrote:
On 18/02/2014 12:15, wrote:
Boiler is in daughter's house. System has 7 single panel radiators.

With the cold pressure on 1.2 bar, it rises pretty quickly to about
2.5 bar when hot.

I found my neighbour has exactly the same boiler and that barely
moves - a couple of tenths of a bar.

Is the pressure rise anything to be concerned about, and is there a
logical reason why they would they be so different?



Well 2.5 bar is not too high and is within the allowable range. However
its quite a large rise for a system that does not sound like it has a
huge water content. Hence as others have suggested, it may indicate a
lack of charge in the pressure vessel.

Assuming it has not actually ruptured, it may just need a top up. To do
this you need to turn off the boiler and let the system cool, then bleed
a couple of litres of water out of the system somewhere (from a rad
connection say) to relieve the pressure in the system.

Then (if you can get to it) there should be a shrader valve (like on a
car tyre) on the expansion vessel. Test that with a pressure gauge, and
if its low (i.e. less than say 2 bar) pump it up with a normal tyre
pump, to whatever is recommended in the manual (probably over 2 bar)


Are you *sure*? If you charge it to 2 bar or more, it won't do anything


In fact I might be thinking of my unvented DHW one rather than the
boiler... however go by what the manual suggests.

until the system pressure exceeds that. I always reckon to charge it to
about 10psi (about 0.7 bar). Then, if the cold water pressure is set to
(say) 1 bar, the expansion vessel provides a bit of resilience, and
allows you to bleed the radiators.

[Are you sure you're not thinking of the vessel for a non-vented HW
system?]


Yes, quite possibly ;-)




--
Cheers,

John.

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Default Worcester Bosch 28CDi combi - what's a normal system pressurerise from cold to hot?

On 18/02/2014 13:21, wrote:
Thanks - I thought that if the vessel had failed then the pressure
would shoot up over 3 bar and water would be ejected? I'm pretty
sure there no rad full of air to absorb the increase either.


If it had failed completely, then yes. If however its just lost
pressure, then the normal "cold" pressure of the system may be taking up
most of the expansion room in the vessel.

(note see my reply to Roger, I think he is correct - I over stated the
"relaxed" pressure)

I checked the pressure vessel with the water pressure out of the
system and it's 9psi - the manual says it's set at 0.5 bar initially


Yup that seems reasonable.

WRT the neighbour's system, they may have a rad full of air. Also are
they running lower flow temperatures than you?

although there's a table (9) in the manual that doesn't makes sense
to me (I can't tell which is the pressure vessel setting and which
is the water presure setting).


To be fair, the default setting is normally fine unless you have a
particularly large water content system.


--
Cheers,

John.

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Default Worcester Bosch 28CDi combi - what's a normal system pressurerise from cold to hot?


WRT the neighbour's system, they may have a rad full of air. Also are

they running lower flow temperatures than you?


It's obviously a possibility that there's air in one or more of my neighbour's rads. I did notice the CH was set a bit lower on the boiler too, although the house was comfortably warm.
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