Pressure rising in c/h system

[NickW]

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}

[nightjar]

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

[Andy Hall]

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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[Christian McArdle]

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.

[news.mistral.net]

Just a thought but,

Have you tried bleeding your radiators? It could be that you are getting a hydrogen build up somewhere.

That would certainly explain the symptoms i.e no build up in the summer (most chemical reactions are much slower as temp goes down, approx half for every 10 degrees C) The hydrogen will take up far more space at even 3bar than the equivalent number of moles of water. (I think the reaction goes something like 2 moles of H20 plus one mole of Iron gives 2 moles H2 (taking up about 22 litres at STP) plus 1 mole rust (FeO2)) Possibly, adding some inhibitor might help. If the above is the case, ironically you will probably also need to top up the system (once it has been bled)

I partly put this forward because it is exactly what happened in my system!

[Lobster]

Sounds like you have air in the ch system, which expands as it heats up causing the pressure rise. Try leaving the filling loop unattached while you use the c/h; my guess is that the pressure will still increase. Have you tried bleeding all your radiators / boiler?

David

[Andy Hall]

That thought crossed my mind as well, but quite a substantial amount of water is having to be taken from the system regularly, and if H2 build up were the main culprit, the radiators would empty and be cool at the top

Certainly inhibitor should be used, but my guess is that we are looking at some form of leak here - the amount of water and regularity is too much

.andy

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[NickW]

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.

[Christian McArdle]

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.

[Ed Sirett]

See FAQ. Possibly its corrosion of some sort?

[Ed Sirett]

Yeah, dead right.

[NickW]

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

[news.mistral.net]

One thing which doesn't quite make sense is

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 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

[Christian McArdle]

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.

[news.mistral.net]

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

[Christian McArdle]

I read it that he had just had the boiler installed. Certainly, the plot thickens if the events occured last winter.

Christian.

[Andy Hall]

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

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.

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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[news.mistral.net]

Ah yes, I see what you mean. I re-read his post and I agree. I am sure you are right. Fairly bizzar!

[Ed Sirett]

Is it possible that this a thermal store with boiler primary and DHW indirect coils?

[Andy Hall]

Wouldn't it have to be under mains pressure in the bulk of the water in the store in order that water is forced from the store to the boiler primary circuit?

.andy

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