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Jim T
 
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Default What's wrong with my central heating system?

Something seems odd about the pressure behaviour of the central
heating system in my 3-bed house, powered by a Britony 80 combi
boiler. Over the past 3 days, with the boiler running, I've been
carefully recording the water pressure levels (according to the
boiler's pressure guage) at various intervals. Before doing this, I
rigged a pipe going from the pressure relief valve into a bowl, so I
can see what gets expelled from the valve. Here is my record:

Wednesday
4:30pm: I top up the pressure to 1.75-bar. The bowl is empty.
5.30pm: The guage reads 2-bar. Bowl is still empty.
10.45pm: Guage reads 1-bar. Bowl now contains 1/4 pint of water.

Thursday
6.00am: I empty the bowl and top up the pressure to 1.75-bar.
7.20am: guage reads 2.65-bar and there is 1/4 pint in the bowl.
8.42am: guage reads 1.4-bar. Another 1/8 pint has gone into the bowl.
8:56am: guage reads 2.1 bar. No further expellationinto the bowl.
10.00am: guage reads 1.3-bar. No further expellation into the bowl.
10.30am: guage reads 1.8 bar. No further expellation into the bowl.
2.00pm: guage reads 0.8 bar. No further expellation into the bowl.
6.20pm guage reads 1.5-bar. No further expellation into the bowl.
11.20pm guage reads 1.0 bar. No further expellation into the bowl.

Friday
10.04am, guage reads only 0.2bar! No further expellation into the
bowl.

Anyone tell me what's going on? Expansion vessel problem perhaps?
If so, what? What's the procedure to cure it? The whole system was
installed about 4 or 5 years ago.

Thank you for any help.

Dave L

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Set Square
 
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In an earlier contribution to this discussion,
Jim T wrote:

Something seems odd about the pressure behaviour of the central
heating system in my 3-bed house, powered by a Britony 80 combi
boiler. Over the past 3 days, with the boiler running, I've been
carefully recording the water pressure levels (according to the
boiler's pressure guage) at various intervals. Before doing this, I
rigged a pipe going from the pressure relief valve into a bowl, so I
can see what gets expelled from the valve. Here is my record:

Wednesday
4:30pm: I top up the pressure to 1.75-bar. The bowl is empty.
5.30pm: The guage reads 2-bar. Bowl is still empty.
10.45pm: Guage reads 1-bar. Bowl now contains 1/4 pint of water.

Thursday
6.00am: I empty the bowl and top up the pressure to 1.75-bar.
7.20am: guage reads 2.65-bar and there is 1/4 pint in the bowl.
8.42am: guage reads 1.4-bar. Another 1/8 pint has gone into the bowl.
8:56am: guage reads 2.1 bar. No further expellationinto the bowl.
10.00am: guage reads 1.3-bar. No further expellation into the bowl.
10.30am: guage reads 1.8 bar. No further expellation into the bowl.
2.00pm: guage reads 0.8 bar. No further expellation into the bowl.
6.20pm guage reads 1.5-bar. No further expellation into the bowl.
11.20pm guage reads 1.0 bar. No further expellation into the bowl.

Friday
10.04am, guage reads only 0.2bar! No further expellation into the
bowl.

Anyone tell me what's going on? Expansion vessel problem perhaps?
If so, what? What's the procedure to cure it? The whole system was
installed about 4 or 5 years ago.

Thank you for any help.

Dave L


Unfortunately, your carefully logged details are missing one vital piece of
information - namely what the heating system was doing at the time in terms
of being switched on and off - either by a programmer or room stat.

Nevertheless, from what you say, you have a problem either with with your
pressure vessel or with your pressure refief valve.

When everything is working normally, the pressure should be about 1 bar when
cold and 2 bar when hot. [A cold charge pressure of 1.75 bar sounds a bit
high to me - have you always used this pressure?]

When the system heats up, and the water expands, the pressure vessel is
supposed to be able to absorb the expansion with only a modest rise in
pressure. If the pressure rises above 3 bar, the pressure relief valve is
supposed to open - to let some water out and reduce the pressure.

First check your pressure vessel. Let some water out of the system to reduce
the pressure to well under 1 bar. Measure the air pressure in the pressure
vessel with a car-type pressure guage applied to the Schrader valve. If
water comes out of the valve when the pin is pressed in, the diaphragm is
split, and the pressure vessel is shot. If no water comes out but the
pressure is low (say, less than about 1.3 bar) pump some air in to increase
the pressure. This might cure the problem.

If you still get water coming out of the pressure relief valve, you need to
determine at what pressure this is opening. If possible, stand by the system
at a time when water is coming out and observe the reading on the pressure
gauge. If this is substantially less than 3 bar, the relief valve is shot.

If the pressure vessel and relief valve are both ok, try running at a lower
charge pressure. If you still have problems, it would suggest that the
pressure vessel isn't big enough, and that you need to install a bigger one.
This sounds unlikely if the system has worked ok for 4 years - unless it has
recently been extended by adding additional radiators.
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.


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Ed Sirett
 
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On Fri, 22 Oct 2004 10:05:13 +0000, Jim T wrote:

Something seems odd about the pressure behaviour of the central
heating system in my 3-bed house, powered by a Britony 80 combi
boiler. Over the past 3 days, with the boiler running, I've been
carefully recording the water pressure levels (according to the
boiler's pressure guage) at various intervals. Before doing this, I
rigged a pipe going from the pressure relief valve into a bowl, so I
can see what gets expelled from the valve. Here is my record:

Wednesday
4:30pm: I top up the pressure to 1.75-bar. The bowl is empty.
5.30pm: The guage reads 2-bar. Bowl is still empty.
10.45pm: Guage reads 1-bar. Bowl now contains 1/4 pint of water.

Thursday
6.00am: I empty the bowl and top up the pressure to 1.75-bar.
7.20am: guage reads 2.65-bar and there is 1/4 pint in the bowl.
8.42am: guage reads 1.4-bar. Another 1/8 pint has gone into the bowl.
8:56am: guage reads 2.1 bar. No further expellationinto the bowl.
10.00am: guage reads 1.3-bar. No further expellation into the bowl.
10.30am: guage reads 1.8 bar. No further expellation into the bowl.
2.00pm: guage reads 0.8 bar. No further expellation into the bowl.
6.20pm guage reads 1.5-bar. No further expellation into the bowl.
11.20pm guage reads 1.0 bar. No further expellation into the bowl.

Friday
10.04am, guage reads only 0.2bar! No further expellation into the
bowl.

Anyone tell me what's going on? Expansion vessel problem perhaps?
If so, what? What's the procedure to cure it? The whole system was
installed about 4 or 5 years ago.

See the FAQ.
Given that most combi boilers have an internal vessel capable of
coping with the size of most typical systems[1]. I would say that the
vessel has failed.

As a work around you can drain a radiator (or 2)in a less used (or less
important to heat) room and then refill without bleeding that radiator.
This will likely provide enough cushion to enable you to have heating
until you can replace or install a good vessel.

Unless the manufacturers state otherwise, precharge the vessel to 0.75 bar,
and fill the system initially to 1.0.


[1] Retro style radiators or larger systems nearing the 24kW heating
output, may need additional vessels to limit the presure rises.
Whilst in a domestic setting the large system is unlikely to be connected
to a combi (because HW usage would demand a different installation), you
might find that say a modest village hall might be heated with a 24kW combi,
where the HW demand is only for wash hand basins and a kitchen sink but the full
power on the heating side is needed for the radiators.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html


  #4   Report Post  
Jim T
 
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Default

On Fri, 22 Oct 2004 12:36:48 +0100, "Set Square"
wrote:


Unfortunately, your carefully logged details are missing one vital piece of
information - namely what the heating system was doing at the time in terms
of being switched on and off - either by a programmer or room stat.

Nevertheless, from what you say, you have a problem either with with your
pressure vessel or with your pressure refief valve.


Thanks for your input on this. I think the pressure relief valve must
be OK because it's brand new. I replaced it a few days ago, because
the old one was my chief suspect, but the new one behaves exactly the
same as the old one.

When everything is working normally, the pressure should be about 1 bar when
cold and 2 bar when hot. [A cold charge pressure of 1.75 bar sounds a bit
high to me - have you always used this pressure?]


Well, to be very honest, since I instaklled the system, I've been
topping up the sytem to 3-bar! I thought mistakenly that the 3-bar on
the guage was 2-bar. It was only in the past could of weeks that a
Brittony boiler expert made me aware that I shoul only top it up to
1.5 or 1.75 bar. Nevertheless, I would imagine that the pressure
relief valve has been sparing the system from any damage resulting
from my over-topping-up of the presure....

When the system heats up, and the water expands, the pressure vessel is
supposed to be able to absorb the expansion with only a modest rise in
pressure. If the pressure rises above 3 bar, the pressure relief valve is
supposed to open - to let some water out and reduce the pressure.


Yes, that certainly happens. In fact, water starts trickling out at
about 2-bar (on both the old and the new pressure relief valves). Both
of them tend to blow when the pressure guage is not much past 2-bar.
But I reckon it's unlikely that they are both faulty in exactly the
same way. They don;t even look the same. The replacement I just put in
is a new design. (Designed to be better than the older design, I
guess).

First check your pressure vessel. Let some water out of the system to reduce
the pressure to well under 1 bar. Measure the air pressure in the pressure
vessel with a car-type pressure guage applied to the Schrader valve.


Schrader valve??? Never delved into this area of my boiler before, but
I am willing to venture into the unknown.. Looks like my time has come
to be a hero.... ;-)

If water comes out of the valve when the pin is pressed in, the diaphragm is
split, and the pressure vessel is shot. If no water comes out but the
pressure is low (say, less than about 1.3 bar) pump some air in to increase
the pressure. This might cure the problem.

If you still get water coming out of the pressure relief valve, you need to
determine at what pressure this is opening. If possible, stand by the system
at a time when water is coming out and observe the reading on the pressure
gauge. If this is substantially less than 3 bar, the relief valve is shot.


See my above comments on the valve...

If the pressure vessel and relief valve are both ok, try running at a lower
charge pressure. If you still have problems, it would suggest that the
pressure vessel isn't big enough, and that you need to install a bigger one.
This sounds unlikely if the system has worked ok for 4 years - unless it has
recently been extended by adding additional radiators.


No, it hasn't been extended. There is, of course the possibility that
the pressure guage is incorrect - but I suspect not... because I've
noticed the boiler starts making a moaning noise (or whining noise)
when the pressure drops to around 1-bar. That symptom is the same as
it always as.

So I guess the chief suspect now is the expansion vessel.....

Many thanks for your help.

Dave L

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Jim T
 
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On Fri, 22 Oct 2004 14:37:10 +0100, "Christian McArdle"
wrote:

Anyone tell me what's going on? Expansion vessel problem perhaps?
If so, what? What's the procedure to cure it? The whole system was
installed about 4 or 5 years ago.


Possibly. However, you're filling up to quite a high pressure and you don't
have much headroom before the pressure relief activates, possibly dumping
too much water when it does so.

I would pressurise to 1 bar when cold. Then, if at any point the pressure
increases beyond 2 bar, you can assume that the expansion vessel is either
broken or undersized.


That sounds like a plan. I'll try it. Thank you for bearing with me on
this issue.....

Either fix it, or install a new one. Personally, I
would prefer a system to be designed to expand by only 0.5 bar. It means
that the pressure vessel has to be seriously deflated before you start to
get problems.

There are other explanations, such as dodgy pressure gauges and relief
valves.


Yes, I guess so.

Thanks again,

Dave



  #6   Report Post  
Jim T
 
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Default

On Fri, 22 Oct 2004 20:57:08 +0100, "Ed Sirett"
wrote:


See the FAQ.
Given that most combi boilers have an internal vessel capable of
coping with the size of most typical systems[1]. I would say that the
vessel has failed.


It does sound like it.... Anyone know how much a new expn vessel for
one of these might cost? Are they difficult to replace? As I mentioned
I've never delved into that part of the boiler before. I gather the
expansion vessell is situated at the very back of the boiler, behind
the main firebox, and I think it's integral with the boiler chassis

As a work around you can drain a radiator (or 2)in a less used (or less
important to heat) room and then refill without bleeding that radiator.
This will likely provide enough cushion to enable you to have heating
until you can replace or install a good vessel.


Ed, I had wondered about that! Nice to hear someone coroborate what my
logic had suggested... I could to that, and thus postpone this work
for as long as possible....

Unless the manufacturers state otherwise, precharge the vessel to 0.75 bar,
and fill the system initially to 1.0.


The 0.75 bar is about right. The boiler's manual says they pre-charge
them to 0.7 bar.

[1] Retro style radiators or larger systems nearing the 24kW heating
output, may need additional vessels to limit the presure rises.


This is a modern system with normal B&Q-style rads. It's a small 3-bed
house. The manual says the expansion vessel is suitable for systems up
to 145 litres capacity. I've never really measured the capacity - only
guesstimated it roughly, on the basis of the size of the house and the
number and size of the rads - and that was years ago when I designed
the system... the details of which have escaped my memory.

Many thanks..

Dave.

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Jim T
 
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On Fri, 22 Oct 2004 12:36:48 +0100, "Set Square"
wrote:


Unfortunately, your carefully logged details are missing one vital piece of
information - namely what the heating system was doing at the time in terms
of being switched on and off - either by a programmer or room stat.


Good point. Invariably the high pressure readings were while the
system was hot and the low readings were while the system was
elatively cooled-down.

Dave
  #8   Report Post  
Nick Read
 
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On Fri, 22 Oct 2004 22:29:52 GMT, (Jim T) wrote:

On Fri, 22 Oct 2004 20:57:08 +0100, "Ed Sirett"
wrote:


See the FAQ.
Given that most combi boilers have an internal vessel capable of
coping with the size of most typical systems[1]. I would say that the
vessel has failed.


It does sound like it.... Anyone know how much a new expn vessel for
one of these might cost? Are they difficult to replace? As I mentioned
I've never delved into that part of the boiler before. I gather the
expansion vessell is situated at the very back of the boiler, behind
the main firebox, and I think it's integral with the boiler chassis

As a work around you can drain a radiator (or 2)in a less used (or less
important to heat) room and then refill without bleeding that radiator.
This will likely provide enough cushion to enable you to have heating
until you can replace or install a good vessel.


Ed, I had wondered about that! Nice to hear someone coroborate what my
logic had suggested... I could to that, and thus postpone this work
for as long as possible....

Unless the manufacturers state otherwise, precharge the vessel to 0.75 bar,
and fill the system initially to 1.0.


The 0.75 bar is about right. The boiler's manual says they pre-charge
them to 0.7 bar.

[1] Retro style radiators or larger systems nearing the 24kW heating
output, may need additional vessels to limit the presure rises.


This is a modern system with normal B&Q-style rads. It's a small 3-bed
house. The manual says the expansion vessel is suitable for systems up
to 145 litres capacity. I've never really measured the capacity - only
guesstimated it roughly, on the basis of the size of the house and the
number and size of the rads - and that was years ago when I designed
the system... the details of which have escaped my memory.



Did you manage to sort this out?

The reason I ask is because I also have a Britony 80 combi boiler, and
have been having exactly the same problem with it over the past month
or so. I too replaced the pressure relief valve believing it to be
faulty, but it made no difference. Yesterday morning I depressurized
the system, checked the expansion vessel pressure & found it to have
dropped to below 6 psi when it should be around 10 psi (roughly 0.7
bar). I topped the pressure up with a footpump, and it seems much
improved, although there is still a very occasional drip from the
relief valve.

The pressure seems more stable now, there was previously nearly a 1bar
pressure difference between the system hot and cold. This seems to
have dropped to a difference of less than 0 .5 bar between hot and
cold.



Nick

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