Boiler overpressure with Potterton Precision

[sutty600]

Dear All

New member here, coming looking for help. A few weeks ago I had work carried out by a British Gas Homecare service technician. It required a drain and refill of the system, and as he left, he advised my I would need to bleed all the radiators, at some point, given a few days, when everything had settled down. I followed this advice, but got no gurgling or spitting from any of them, and just nice clean water coming out, so I felt happy about that.

Since the work was carried out though, I have been having awful problems with overpressure. Not that my boiler tells me that easily, no light codes, as far as I can see, but it regularly fails to start, and if I catch it trying, it just clicks a few times, and then gives up. When I check the pressure gauge, which is in the loft, I see significant overpressure, such that I need to bleed around a gallon and a half of water out of the system to bring it back down again, which I do at an easy to access radiator.

Where the gauge is there is a red coloured pressure vessel, and leading to that is what I take to be a mains water feed, with two taps in a line, I assume for double security. Both of these taps are definitely in the off position, because if I turn them both on, I hear water flowing, and the gauge goes up, which is what I would normally do to top the system off, if ever it's needed, which on occasion, it has been in the past.

As it stands though, that is never going to be required again, because since the technician was here, in order to keep my hot water and heating system going, I have had to drain gallons and gallons of water out of the system, around a gallon and a half every few days.

My thought is there is another feed location, somewhere nearer the boiler, which he utilised to recharge the system, and he has left that in the on position, or partially on. Before I remove all the woodwork surrounding and covering the pipes near the boiler, is this a possibility, and should it be easy to identify. It's quite the task to get all this off, and I don't want to waste my time if there is no such alternate feed.

I'm a complete novice when it comes to boilers and heating, but I'm not helpless in terms of general mechanical ability, and DIY, so I'm hopeful that if my thoughts are correct, I will be able to spot another valve or similar, and crank it down.

If it helps the boiler is Potterton Precision, and has been operating for around 10 years, with regular service and periodic repairs from British Gas Homecare. They tell me that my system is an S plan heating system, whatever that means, again if that helps.

If you need more information, so that I can be helped, do please let me know, and I will try to find out.

I'll not go into detail as to why I can't just log a recall with Homecare, but please take my word for it, that at this moment, I cannot. If you think that's definitely what I should do, I will explain why.

Thanks in advance for any help you can offer.

Cheers

Sutty

 

[Chuck]

What type of hot water cylinder do you have? (Is it a pressurised 'unvented' type or a low pressure 'vented' type fed from a tank in the roof.

 

[sutty600]

As I said, I'm a beginner, so I'm not certain, however, I could guess that it's sealed, because our hot water pressure is dependent on the cold water pressure. If the cold water has low pressure, due to some network fault, or similar, the hot water is equally affected.

It is the pressure in the radiators though that the gauge shows, and I can correct it for a couple of days or so, by bleeding a lot of water from the radiators, and then my boiler will start. Not sure how that relates to the hot water, but hopefully what I've described will point to a sealed hot water tank, if it does help. Let me know if it doesn't, and maybe I could take photos?

 

[Chuck]

There should be a flexible section of pipe (hose with a metal braided covering) between the two isolation valves you use to fill the system. With the isolation valves 'off' this should be disconnected during normal use because the valves only need to pass a very small amount of water to create the problems you are describing. One end of the hose will have lugs so you don't need tools to disconnect it. If one of the valves is passing you will probably be able to see water slowly dripping out.

Another possibility, but much less likely, is the heat exchanger in your hot water tank leaking so domestic hot water is getting into the heating circuit. You might be able to prove this for yourself as follows. Wait until the CH pressure has risen. Take a photo of the pressure gauge. Turn off the valve that supplies cold water to to the cylinder. Open the lowest hot tap in the house and let it run until it stops. (You'll have to waste a couple of quid's worth of hot water, sorry) Now monitor the CH pressure over several hours to see if it starts falling.

A professional (G3 qualified) plumber might isolate and then pressure test the heat exchanger or put fluorescent dye in the CH water to see if it emerged in the domestic hot water but this these are not tests you should attempt yourself.

 

[sutty600]

There should be a flexible section of pipe (hose with a metal braided covering) between the two isolation valves you use to fill the system. With the isolation valves 'off' this should be disconnected during normal use because the valves only need to pass a very small amount of water to create the problems you are describing. One end of the hose will have lugs so you don't need tools to disconnect it. If one of the valves is passing you will probably be able to see water slowly dripping out.

Another possibility, but much less likely, is the heat exchanger in your hot water tank leaking so domestic hot water is getting into the heating circuit. You might be able to prove this for yourself as follows. Wait until the CH pressure has risen. Take a photo of the pressure gauge. Turn off the valve that supplies cold water to to the cylinder. Open the lowest hot tap in the house and let it run until it stops. (You'll have to waste a couple of quid's worth of hot water, sorry) Now monitor the CH pressure over several hours to see if it starts falling.

A professional (G3 qualified) plumber might isolate and then pressure test the heat exchanger or put fluorescent dye in the CH water to see if it emerged in the domestic hot water but this these are not tests you should attempt yourself.

Okay, from what you describe, it would seem there is no other place it can be being filled, without a fault, so no point looking. I can perform the tests you describe, and will do so. There is a braided hose, as you described.

The technician described something to me, whilst here, but I don't recall it properly, along the lines of, you have a such and such system, and I love working on systems like this, because they are so easy to refill, and I suppose that's probably what got me to thinking along the lines of an alternate fill option.

I'll try to get more details for tomorrow, but I need to get some sleep now. Thank you for the advice so far.

Regards

Sutty

 

[sutty600]

It's already significantly over pressure, after only 16 hours, having bled it down at around 7PM yesterday, and with it being 11AM when I checked it a short while ago. The boiler is still running, but it's close to where it will stop, judging by the position of the gauge.

Not clear how it can get to this state, if only one isolation valve is faulty. Wouldn't both have to be? Anyway, I'll take a look after lunch, to see if there is a leak there, by disconnecting, as instructed.

If not, I will restore that, and then look to drain the system down, until empty. Will this present any difficulty refilling, with it being sealed? How does that work if the low tap is now off again, and the tank is now full of air, when I turn back on the feed to the tank? I won't do that until I hear from someone that this will go okay.

 

[Chuck]

Not clear how it can get to this state, if only one isolation valve is faulty. Wouldn't both have to be? Anyway, I'll take a look after lunch, to see if there is a leak there, by disconnecting, as instructed.

After a bit of use, those valves may well 'pass' to some (small) extent but that can be all that's needed to cause over-pressuring. That's why you a have a piece of flexible hose that you can disconnect at one end. If you read the instruction book for your system it will tell you to disconnect it after topping up.

If not, I will restore that, and then look to drain the system down, until empty.

Whoa there cowboy! I did not say 'drain the system down' I said open the lowest hot tap and let it run dry with the inlet to the hot cylinder isolated, which might require a couple of gallons of run off. (On reflection, my 'couple of quid' of hot water was an overstatement.) This will leave the cylinder full of water but at a pressure below that of the heating circuit. When you've finished make sure the hot tap is closed and gently open the valve supplying cold to the cylinder again. Expect a bit of sputtering when you first draw hot water after doing this so open the tap carefully.

If you don't understand what you are doing and why, please call a plumber.

 

[sutty600]

Those valves quite often 'pass' to some (small) extent but that can be all that's needed to cause overpressuring. That's why you a have a piece of flexible hose that you can disconnect at one end. If you read the instruction book for your system it will tell you to disconnect it after topping up.



Whoa there cowboy! I did not say 'drain the system down' I said open the lowest hot tap and let it run dry with the inlet to the hot cylinder isolated, which might require a couple of gallons of run off. (On reflection, my 'couple of quid' of hot water was an overstatement.) This will leave the cylinder full of water but at a pressure below that of the heating circuit. When you've finished make sure the hot tap is closed and gently open the valve supplying cold to the cylinder again.

If you don't understand what you are doing and why, please call a plumber.

Lol, yes, my assumption was that if the HW fill valve was closed off, and the low tap opened, and it was £2 of hot water, that would be emptying the whole tank. As for calling a plumber, yes, that would be best, but, for the record, the reason I'm loathed to do so is that my mother just passed away, and this was her account with BG Homecare. I'm in the process of opening an account for me, and even when done, I have no doubt that a recall across accounts, will prove to be exceedingly difficult to arrange, but it should be a recall, because it has never worked since they left. Even if I can't arrange the recall, I don't want to be paying for Homecare insurance, and then not use it, by involving a different plumber. This, why I'd like to solve this myself, just for now, and just for this one issue. If it proves to be beyond me, so be it.

As for not knowing what I'm doing, that's true, but I can follow instructions, and have mainly understood what you are suggesting, and why, so far, and I'm grateful for the guidance. A small misunderstanding about relieving the HW tank pressure, as opposed to the tank being emptied, given my lack of starting knowledge, is perhaps understandable. Now you've explained, I'm with it.

Thanks again.

 

[sutty600]

Update on the link. As you described, lugged, and not overtightened connections. Undid one end, and collected the water. Undid the other end, and removed it, so I could more easily dry and observe any water build up in the socket, rather than in the length of pipe. Zero change, or build up of water occurred, and certainly no drips, at either side. Looked for the HW tank fill isolation valve, and saw none at the bottom of the tank, from where I presume it fills, keeping the water hot at the top, but none to be seen near the tank. Will have to look round tomorrow for that, because the issue is not the valves either side of the link.

 

[Chuck]

Update on the link. As you described, lugged, and not overtightened connections. Undid one end, and collected the water. Undid the other end, and removed it, so I could more easily dry and observe any water build up in the socket, rather than in the length of pipe. Zero change, or build up of water occurred, and certainly no drips, at either side.

What had 'zero change'? Has removing the link stopped the pressure buildup in the heating system or not? (I suspect not but I try to avoid assumptions.)

Looked for the HW tank fill isolation valve, and saw none at the bottom of the tank, from where I presume it fills, keeping the water hot at the top, but none to be seen near the tank.

Can you post a photo of the tank and nearby pipework? (Just want to check we really do have an unvented cylinder.) Also, what make and model of boiler do you have?

The inlet to the cylinder will be at the bottom. So will be the flow and return to the heat exchanger. There is usually an expansion vessel, maybe 20 litres in volume and painted white between the isolation valve and the cylinder.