Plumbing Puzzle

[Old_Geezer]

Well it puzzles me anyway.

My woman's home heating system is driven from a combi boiler. Every now and again she sees that the pressure gauge is reading high, it's at the very top of the green. To stop it rising further to the red I bleed the kitchen radiator of a soupbowl's volume of water and then the pressure has reduced to the '2' mark.

We were wondering why it does this. No one changes any tap/valve/whatever to let more water in so where does it get it from ? And a secondary puzzle, how come a pressure relief valve isn't sorting it before it gets to that stage ?

Cheers.

[Atheist]

The PRV will be set to open at 3 bars. If the gauge shows less than 3 bars then probably not a problem.
How often does this happen?
Do you know how to add more water to the system? It usually involves opening a valve or two under the boiler. There was a time when there was a valve which would open itself to automatically top up the primary system (i.e. radiator water, not tap water), but I doubt if you've got one of them.
If you can give a bit more info, i.e. make and model of boiler, frequency of problem, if and when you ever top up the system, do you ever see water discharging from the PRV pipe (should be external so as not to wet your floor, but visible so that you realise there's a prob.)
There's better than me here who might be able to advise further.

[Old_Geezer]

Hi, thanks.

We don't check it every day but maybe every 2 to 3 weeks ?

'3' is well into the red on the gauge. She'd be having kittens if it was found that high !

Yes we can add water. In the past I've had to open the straight service valve in the pipe underneath, but that's not an issue at present. (Hasn't been for a long time.) It's now quite the reverse.

It's a Worcester, I can't see a model number on it. I could ask where the user manual is, if it's important.

Not aware of any water discharging,but if it did it'd be onto the kitchen extension roof and off into the guttering, so one might not spot it anyway.

[Gizmonster]

If the pressure is increasing, it would imply that water is getting in somewhere.
Are you sure that the valve to top up the system is fully closed off .... and if it is, are you sure it's not faulty and doesn't close off fully??

[Atheist]

Do you think it possible that the expansion vessel is faulty, or needs pumping up? The vessel is a metal 'bomb' whose inside space is divided by a flexible membrane. The space on one side of the membrane is connected to the primary water circuit, and when the primary water gets hot it expands and pushes against the membrane, which flexes and allows the expansion. The other side of the membrane contains air and also has a valve like a car tyre so that the air can be pumped up, and when everything is cool the membrane has water on one side and air on the other. As the primary water expands into the vessel it pushes the membrane.....
Now, if the air on the air side of the membrane is not pumped up from time to time (not often) or if the membrane is punctured or if the tyre valve is leaking, then the membrane will live permanently over towards the air valve side and the vessel will be full of water. When the water expands, there is nowhere for it to go, and so the pressure rises until the PRV releases some water. When the system cools down, the pressure drops and the PG (gauge) tells you that the system needs topping up - so you top it up and eventually the same problem arises. The mystery is solved - it's you who tops up the water.
The above scenario may not be applicable in your case, but I thought I'd try to explain a bit about what goes on in the system so that you could possibly get a better idea of what's up.
If the needle is in the red it needn't be frightening in itself, the PRV is there to keep the pressure below 3 bars.

[Old_Geezer]

It seems ok. If opened the pressure rises to a particular point then stops (I presume equal to mains pressure). Unsure how it can be tested without pulling everthing apart. I was wondering if a normal combi system had a secondary input somehow. (I'm not really familiar with them as I've always had the more traditional set-up.)

[Old_Geezer]

I have an expansion vessel in my system, but I'm unsure whether it applies to a combi, or maybe it's internal to the boiler.

I'll read through the post again and try to inwardly digest it.

[Atheist]

I'd expect a combi boiler to include an expansion vessel within the boiler casing. I think you need to get a Worcester-trained engineer to check it out.

[The Builder]

First thing I would check is the pressure in the 'expansion vessel'

https://www.bes.co.uk/expansion-vessel-central-heating-8-litre-11350/

It's a 'schrader valve' (bicycle pump). Pump it up to around 1 bar (check with the manual to be exact.)

[The Builder]

Sorry. Cross-posted with the others.

[Old_Geezer]

Thanks. I certainly have one of those in my own system, but I've not seen one in hers. I may recheck later today, but if it's inside the boiler, that is probably too risky when dealing with systems other than my own. Interesting though.

[bookbinder]

I had the same problem on our boiler: Ideal Logic C35, 14 months old. I called British Gas. A fitter came and reduced the pressure to what it should be: 1 to 1.5 bar when the boiler is at rest. Some time ago, the boiler packed up, because the pressure was too low. I increased the pressure by opening a pair of valves on the underside of the boiler. The fitter said that one of the valves was slightly open, so the problem was clearly my fault! Check the adjusting valves on the underside of your boiler, in case one of them is slightly open.

[Old_Geezer]

Interesting. I've not gone looking for any expansion vessel yet, but I had a quick look underneath the boiler. It's different to how I recall; but we've had a couple of plumbers around for different jobs since I last looked. I recall one valve, the type needing a screwdriver to turn it, but seems there are two, one each end of the flexible pipe. The one I recall looks left permanently open to me, but I've no idea how to check; but the other has a plastic lever which is at it's end stop, right angles to the pipe, so presumably properly closed.

[Gizmonster]

OG you are correct - on each end of the filling loop there will be a valve. If one of them is fully open like you say, then the other one may also be slightly open, causing the water to trickle through and the pressure will build up over time. You can close both valves fully, as these valves only need opening when topping the boiler up.
The plastic lever needs to be at 90 degrees to the pipe for the valve to be closed. If the lever runs parallel to the pipe, the valve is fully open.

[Old_Geezer]

The plastic lever one is definitely at 90°. I'll try closing both and see if that makes a difference then. Cheers.

[New Judge]

There are only two reasons why the pressure in your system can go up (assuming you examine the pressure when the system is at the same temperature each time). Either additional water is getting into the system or the expansion vessel is faulty.

You can actually disconnect the filling loop (the flexible connection between the boiler and the incoming mains). If you do that you will certainly see whether there is any water seeping from the mains. The Water Regulations state that unless it incorporates an approved backflow prevention device it must be removed anyway (though few people I know disconnect it):

https://www.installeronline.co.uk/all-you-need-to-know-about-filling-loops/#:~:text=In%20short%2C%20no%2C%20the%20filling,until%20it%20is%20used%20again.

"Where a fill point connection incorporates a “flexible connection”, when not in use it is good practice for the hoses on these connections to be completely disconnected and removed, however a partial disconnection, that is to say only detaching one end of the hose, is equally acceptable providing that the disconnection is made between the hose and the backflow prevention device on the supply/distribution pipe."

But my guess is that the expansion vessel is faulty.

[Old_Geezer]

Cheers.

As mentioned above, I think any expansion vessel must be inside, and I'm unsure I want to take her boiler apart then find I get into difficulties.

I did try to close the open valve below. That was a surprise. Turned the screw and immediately water started spouting out from the area of the turned screw. Shocked, I tightened it back again. Really wasn't expecting that ! Unsure I wish to touch that again.

[New Judge]

//As mentioned above, I think any expansion vessel must be inside, and I'm unsure I want to take her boiler apart then find I get into difficulties.//

Yes it is inside. It simply looks like a small container (the shape and size varies with boiler make/model). Inside they consist of two parts divided by a flexible diaphragm. One part is filled with water from the heating system and the other usually with nitrogen. The diaphragm allows expansion in the system as it heats.

They are not repairable and replacement is not a DIY job. Any fitters doing the work should be GasSafe registered.

But....

//I did try to close the open valve below. That was a surprise. Turned the screw and immediately water started spouting out from the area of the turned screw. Shocked, I tightened it back again. Really wasn't expecting that ! Unsure I wish to touch that again.//

That shouldn't happen and it sounds as if the valve is faulty. Have you tried disconnecting the flexible connecting loop to see if the valve is letting by? There should still not be a problem because there are two valves - one on the mains feed and one on the boiler top up point (i.e.one at each end of the filling loop). They would both have to be faulty for water to get through to the pressurised heating circuit. But it may help you determine where the problem lies, and in any case the ball valve that is leaking from the screw slot definitely needs looking at.

[Old_Geezer]

Am now at my place. Will have to check further sometime next week. Thanks.