Pressure rising in c/h system

[bob]

Hi, This is what you must do as the first part of the fault elimination process.

1. With the system cold, remove the filling loop

2, Drain down the system, not completly but untill the pressure gauge reads zero. 3, using a standard tyre pressure gauge, measure the pressure in the Expansion Vessel, it should be 15-20 psi. The expansion vessel is shaped like a big red victorian Bed Warmer my bet is the gauge will read zero, if this is the case use a car foot pump to increase the pressure. If the EV is faulty leave it where it is and fit a new one using a conex tee at a point somwhere on the ground floor if the EV has 15-20 psi in it let me know and we will go onto step 2 (the English book of Common Prayer) regards Bob

[NickW]

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

[news.mistral.net]

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

[Andy Hall]

OK.

We should try to find out whether the pressure vessel is part of the boiler primary circuit or the hot water system. Likewise I would like to know where the header tank, which sounds to be the size of a CH header tank, fits into all of this.

If you could find a way to take photos and get them onto a web site or even ftp server somewhere it would be a real help.

Make and model would definitely be a help.

Generally what you describe would suggest an indirect tank with cold feed in at the bottom, one coil connection at the bottom, the other half way up and then the outlet from the top.

However, the various pieces that you have don't completely add up for me at the moment.

The pressure vessel could quite reasonably be part of the boiler primary or the hot water side. Can you identify which?

The role of the header tank is also unclear. If the system is sealed on both sides, I can't see a reason for it being there. Again, knowing where it connects to would be useful.

It sounds as though you might either have some kind of a heatbank/thermal store system or perhaps a much older system that has been converted to sealed operation and some of the pieces left in place.

.andy

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[NickW]

I am 99% sure it's connected via a tortuous route to the pipe that goes into the h/w cylinder halfway up ie: the boiler primary circuit.

I hear the tank refilling when I run a bath but this could also be because the cold water tap is on at the same time! I'll check tonight.

I've a feeling we are going to find it's not related to the h/w cylinder because I was using h/w in the summer and the problem did not occur!

The only thing I can think of is that

a) The filling loop opens of it's own accord, maybe when the radiator pipe it's connected to gets hot. I disconnected it when cold and both the tap and non return valve both function (I disconnected it without turning the mains off or draining the c/h system).

b) There is another filling loop I haven't spotted...

[Andy Hall]

Right. Then it's likely to be for the boiler primary circuit.

However, according to the manufacturer's literature there are some Danesmoor models which are system boilers and they mention that for sealed systems these contain an expansion vessel. Often this is large enough for the complete system , but in a system with a large number of radiators an additional vessel may be required. If there is no obvious vessel inside the boiler when you take off the front panel then the one you have is it for the boiler primary.

If we could determine the exact function of this little tank it would help, then we can eliminate it. It would be a bit bizarre but not impossible if it were a small header tank for cold water for the bathroom, however it's a bit small.

Can you find out if it fills only when cold water is used, or when hot is. If it is either, then you don't have a pressurised hot water system and the cylinder is not where the problem is likely to be. Can you also check the pressure at the taps by holding your hand over the spout and turning on the tap. This will also tell you if the HW system is mains pressure. So far, there is nothing to confirm that it is.

I have also been thinking this, except for the possibility that there might be a small pinhole in the cylinder coil or somewhere which opens up with temperature. However, this does not completely explain the summer/winter effect. Is there just a simple motorised diverter valve to direct boiler water flow to cylinder or radiators or is there anything more complex that you switch over from summer to winter?

Try disconnecting it and leaving it disconnected. You should really, anyway,.

If we can eliminate the cylinder then yes. Either that or there is somewhere where perhaps the plumbing was modified at some time in the past or when the system was changed to sealed that is in some other way connected to a source of higher pressure.

As a matter of interest, where is the pressure gauge that you are reading and what are the values when you take water out and afterwards?

.andy

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[Ed Sirett]

Yes it would, I realised that after I had posted. I think it is time for the OP to give all the details of the syetem and preferably some pics.

[Ed Sirett]

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?

[Andy Hall]

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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[Lobster]

Unless I've missed it somewhere in this thread, you haven't yet checked whether disconnecting the filling loop cures the problem? Needs ruling out, especially given that (as somebody else has pointed out) it should not be left connected anyway, by rights.

David

[Ed Sirett]

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.

[Andy Hall]

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.

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[NickW]

Hi everyone (thanks for your help so far BTW)

OK, at the weekend, I ran the hot tap in isolation and after a short pause, the header tank started to refill itself. So it is obviously filling the water cylinder which therefore cannot be mains pressurised. The pressure vessel is definately connected to boiler primary circuit.

The pressure gauge is located on the safety relief valve which passes through the outside wall next to the boiler (Danesmore Worcester 20/25 room sealed). I have seen the system rise to about 2.2 bar before I got worried and took some water off.

I haven't spotted any other filling loops.It's a bit difficult to tell what's happing in the airing cupboard, there are so many pipes and lots of stopcocks too. It could be one of those (but none of them are flexi hoses).

I would like to disconnect the filling loop, trouble is that it's not completely watertight when hot. I unscrew the flexihose from the non-return valve (which is on the c/h side), there are no leaks when cold. However, when the pipes are hot, you can see a little bit of water sitting in the top apeture of the non return valve fizzing and dripping constantly.

Can anyone recommend a female fitting to cap this off? I asked at my local plumb center and they had no idea what I was talking about! I measured the thread with verniers and it's about 19mm diameter. I could do with a male one for the open end of the flexi hose too.

Nick

[Roger Mills]

It's probably 1/2" BSP - so would need a 1/2" BSP female endstop.

As a way of verifying whether it is 1/2" BSP, see whether a standard shower hose screws onto it. If it does, it *is* 1/2" BSP.

Roger

[Ed Sirett]

Or the end of any 15mm compression fitting.

[NickW]

I had a look, I can't find one with a closed end. I looked at Jewsons, BES online and Screwfix online. Any ideas?

I could bodge on up with an ordinary compression nut, a coin and some araldite or something...

Nick

[Andy Hall]

Nick, by the time you've mucked around, it would be a lot easier to just change the valve......

.andy

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