Pressure rising in c/h system

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We have a pressurised central heating system.
I have to drain off about 3 pints a week when the central heating has been running because the pressure increases steadily. Oddly, this doesn't seem to happen in the summer when only the domestic hot water is being used.
As far as I can tell, there is only one filling loop. It consists of a flexi hose coming off a mains water pipe with a little tap at one end and a non-return valve at the other end.
I have unscrewed the hose from the non-return valve and it doesn't leak. Could it leak under other conditions? Like when the pipes get hot?
Very puzzled
{Nick dot walczak at azuresolutions dot com}
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There will always be some increase in pressure when the water gets hot. It expands and has nowhere to go, so the pressure goes up. If it stays under the red line, there is not a problem.

Under the water regulations, that flexible hose should not be connected, except when you are using it to top up the system.
Colin Bignell
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On Wed, 26 Nov 2003 08:38:05 +0000, nightjar wrote:

Yeah, dead right.
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Ed Sirett - Property maintainer and registered gas fitter.
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It can't be gas buildup because I regularly check all the radiators. There is inhibitor in the water too. The amount of water I keep taking out can only come from the mains.
I think the water tank idea is a goer - I haven't looked to see if there is a make/model yet but the pipe entering the top looks like it comes from the mains. I was confused initially because there is a header tank nearby but I think it's just to supply the shower.
Nick
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On 27 Nov 2003 01:01:23 -0800, snipped-for-privacy@bt.com (NickW) wrote:

If it's a conventional and not a combi boiler
- and the filling loop is not connected - and there is no other heat exchanger with mains water behind it - and there is no gas built up if you bleed the radiators - and you are taking out say a litre of water per week
then by elimination it almost has to be something to do with the HW cylinder or system.
The header tank that you talk about.... Is it some kind of open tank
e.g. like www.bes.ltd.uk item 11448 (how big if so)
or is it a pressure vessel like 11350?
You will have a pressure vessel as part of the heating circuit anyway, although that may well be inside the boiler itself (can you provide boiler type).
It is quite common for sealed, pressurised hot water cylinders to have a pressure vessel on the top or close by.
http://www.discountedheating.co.uk/shop/acatalog/Pressurised_Hot_Water_Cylinders__Unvented_.html
has some examples. Sometimes the vessel is fitted to the top of the cylinder, sometimes nearby.
Is there any way that you can take some photos and post to a web site?
We need to try to identify what you have, but it is sounding like it is whatever is exchanging the heat between the boiler and the hot water has a pin-hole leak.
.andy
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On 25 Nov 2003 23:56:51 -0800, snipped-for-privacy@bt.com (NickW) wrote:

Do you have a mains pressure hot water cylinder, or other form of mains pressure water heating?
If so, then there is a fair chance that there is a leak (for example in the coil of the cylinder) which is allowing mains pressure water to enter the heating system.
Can you provide more info. about the water heating and the boiler?
.andy
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Do you mean the pressure rises when the heating is on? If so, then the system is working as designed. There should be a pressure vessel on the system, this absorbs the water when it expands as it is heated. The vessel is sized to ensure that the pressure rise is kept within safe limits. Usually this means a cold pressure of 1 bar and a hot pressure no greater than 2 bar (often much less). If the pressure goes above 2 bar, then the pressure vessel is either undersized or broken.
Alternatively, if you mean it rises and keeps on rising, you have one of two situations. You have a filling loop that someone has failed to disconnect that leaks, or you have another leak to mains, usually either the coil in a pressurised hot water cylinder, or a combi heat exchanger.
Christian.
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Yes, it is a permenant rise in pressure. I have to bleed 3 pints of water off once a week, every week. There is a red pressure vessel in the airing cupboard which I am told is adequate.
It's a brand new Danesmore Worcester oil boiler (just replaced a propane boiler). By the way, this used to happen with the propane boiler too.
I don't know if the water cylinder is at mains pressure or not.... it looks like the pipe going into the top is from the mains. However - as I said this pressure rise did not occur during the summer when the c/h was not in use (but the h/w was).
Cheers
Nick.
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It has possibly only just popped, then. How old is the cylinder? Are there any markings on it, such as make and model? Can you identify all the pipes going into and out of it and their locations?
Christian.
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One thing which doesn't quite make sense is
<Quote> It's a brand new Danesmore Worcester oil boiler (just replaced a propane boiler). By the way, this used to happen with the propane boiler too. </Quote>
I think that means that there must be some link in the main CH and the problem. Otherwise it is really strange that the problem should go away in the summer. Sorry I know that this isn't very helpful in indicating what! As an aside, isn't it a bit strange for the feed pipe of a tank to go into the top? I thought that they always fed the bottom so that the new cold water (heavier) would rest under the top hot water and wouldn't mix to make the whole tank tepid when you took say, half a tank off for a bath.
Stephen

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Mains pressure cylinders are frequently quite complicated inside, with separate components all installed within a cylinderical outer casing. Even if the mains water connection is near the top, it doesn't mean it isn't internally routed to the bottom of the actual water storage unit.

We don't know conclusively that it went away in the summer, only that it started in the winter. It would certainly be odder if we knew that the problem occured last winter as well as this one.
Christian.
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Hi Christian,

But Nick says ' By the way, this used to happen with the propane boiler too.'
which I took as meaning that it happened the previous winter?
Stephen
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I read it that he had just had the boiler installed. Certainly, the plot thickens if the events occured last winter.
Christian.
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Ah yes, I see what you mean. I re-read his post and I agree. I am sure you are right. Fairly bizzar!

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I don't know if it's the input or the output but it did look like the pipe going into the top is a mains water pipe. I shall double check this.

That's exactly what happened! Throughout last winter, I had to bleed water off, then during the summer, the pressure settled at around 1 bar and didn't rise anymore until we started using the central heating again.
(in response to other questions)
Bob - the pressure would increase when the system is hot but return to normal when cold if there was a fault with the pressure vessel wouldn't it?
Andy - there is a pressure vessel near the hotwater cylinder AND there is a header tank (about 70cm wide by about 50cm tall) higher up. Difficult to see where the pipes come from, I'll try and get a clearer picture this weekend.
I know that the hotwater tank is indirect because there are four pipes going in. Two at the bottom, one halfway up and one at the top. I can get make/model if this would help.
Nick
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Hi Nick,
Rather than trying to trace the pipes, try a simple test.
Make sure no one is doing anything else in the flat/house i.e. no washing machine/dishwasher/loo/bath Turn on a hot tap. See if the header tank starts to fill. If so, it is probably feeding the hot water system (sorry for stating the obvious)
One thing is certain. If the pressure is rising and no gremlins are systematically crushing your radiators/pipes, then the volume of the contents of the system is increasing. There are really only two causes:
Gas. Which would rapidly make all the radiators stop working as the water was displaced, and thereforecan be discounted. And Water. This means some point of contact with water at a higher pressure than the system and so (unless you live in a block of flats!) contack with the mains. That leaves (I think) either the filling point or the hotwater cylinder (if it is unvented). The easiest of the two to test is the filling loop which I guess you have disconnected already and if not, should do so for regulatory reasons posted above.
So... If there are STILL problems, however odd about the summer/winter difference, they all point to the hot water cylinder.
Stephen

Even
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On Fri, 28 Nov 2003 08:47:12 +0000, news.mistral.net wrote:

I have some other questions. I still hold to some problem with the expansion vessel ( I don't think your HW is unvented as unvented sylinders usually have 5/6 holes. (2 indirect, inlet, outlet and 1/2 relief valve holes.)
What pressure is the system rising to? What happens when you don't remove the excess water? Is there only the one filling point or was there one before and also now one inside the boiler?
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Ed Sirett - Property maintainer and registered gas fitter.
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On Fri, 28 Nov 2003 18:47:27 +0000, "Ed Sirett"

But what are the symptoms of an expansion vessel that has a problem?
As I figure it, there are three scenarios:-
a) Membrane OK, air/nitrogen pressure behind too high. Not sure what happens here.
b) Membrane OK, sir/nitrogen pressure behind too low. AIUI, if the system pressure is adjusted correctly when cold then on heating it will rapidly rise and blow the relief valve
c) Membrane ruptured. If air has dissolved or otherwise disappeared, outcome same as (b), AFAICS
If the expansion space runs out, the result is a *loss* of water through the relief valve or the pressure will climb as indicated here.
OK, so let's say the OP sees the pressure rise and takes water out. The pressure when cool will now be below the normal range. At about a litre a go, it won't be too long before it's outside spec.
So water has to be coming from somewhere........
The expansion vessel misbehaving doesn't explain the summer/winter effect either......
I guess it may be part of the story, but it seems that there must be more.. .andy
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On Fri, 28 Nov 2003 19:30:49 +0000, Andy Hall wrote:

Effectively the same as having no expansion vessel - pressure shoots up and then releif valve opens.

As above but not a severe since the diapragm will probably still have some movement.

Hence my asking if there was an additional filling point anywhere else.

The difference in winter is that the bulk of the primary water is heated and thus expands.

Indeed this is one of the most bizzare postings we have had for a while.
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On Sat, 29 Nov 2003 15:31:23 +0000, "Ed Sirett"

I see what you mean. Makes sense - just less expansion room again..

Of course...... it could be 100 litres or more being heated in the CH, perhaps as little as 10 with DHW only.
However, this would explain the pressure peaks in the winter.
If the water is being acquired through a filling loop or some other means from the mains, then that would be happening all the year round I would have thought.

.andy
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