Mystery central heating pressure issue - advice please!

[MHW]

My first post here, and I could do with some advice re. the central heating system in the house I bought a few months ago. Basically, what seems to be happening is that the central heating is losing pressure when turned off, but when I let a bit more water in and run it, it goes over maximum pressure.

Before I go into full details, a few bits of background info that might be useful. The house is a Victorian two-up-two-down, but with a loft conversion, so effectively three floors with radiators on all of them. The central heating is a sealed system, powered by a Sime Format C combi boiler. I had it serviced last week by a reputable local firm, who gave it a clean bill of health.

Anyway, what's happened in full is this. The system pressure should be about 1 bar when it's cold - so the engineer informed me - and so it was, but when I bled the radiators yesterday (and the one at the top of the house badly needed doing) it knocked the pressure right down to 0. So I opened the valve and let enough water in to bring it back to 1 bar.

So far so good, but when I turned the heating on again it went up over the maximum 3 bar mark, and there was a drip from what I assume to be the pressure-relief valve, on the outside wall behind the boiler. Realising that I'd over-pressurised the system I switched it off and let a little water out via a radiator bleed valve, until the pressure in the still-hot system went back below 3.

Again, so far so good, but when the system cooled right down the pressure gauge went back to 0 bar. Obviously it isn't a good idea to run the system without pressure so I let water in and bring the system back to 1 before starting it again. That seemed to be fine, and it ran comfortably at 2.5-3 bar for a few hours, but again, when I turned the system off it went back to 0. It did this again whilst switched off overnight, after running well yesterday evening.

Wondering if it might be a gauge fault of some kind I switched the boiler on experimentally this morning with the gauge showing 0, but I could hear the pump trying to work, so turned it off again and gave it a bit more water. Again, it ran absolutely fine, albeit slightly over the 3 bar mark, and again with a small drip from the relief valve. Since then I've let it cool down part way and then turned it on again with the pressure at about 1 bar, and it's currently running comfortably at 2.8ish with no drip from the relief valve.

Excuse my ignorance here, but I'm a bit stumped. The boiler's evidently working fine, the pressure gauge seems to be accurate, and there's no sign of a leak anywhere. All of the radiators are fully bled and there are no noises suggesting air in the system. Obviously the likely candidate is the relief valve, but I'm not sure it is actually that since it's not dripped except on the two occasions when the pressure's been over 3.

I've an engineer coming again this week to replace the timer clock so will mention it to him, but before that, any suggestions gratefully received.

 

[denso13]

Pressure vessel needs looked at http://www.diynot.com/wiki/Plumbing:faq7

 

[jackthom]

When the water in the system heats up it expands and the pressure is certain to rise, as you have found out.

That's why an expansion vessel is added which is part air filled and can provide some room for the water to expand into, without a dramatic increase in pressure.

I'm wondering what the engineer did during his service that may have caused this to suddenly stop working. Did the problem begin as soon as he left?

 

[MHW]

I'm wondering what the engineer did during his service that may have caused this to suddenly stop working. Did the problem begin as soon as he left?

No, the problem started when I bled a radiator yesterday. I now wish I hadn't!

 

[jackthom]

It's possible that the expansion vessel hasn't been working properly for a while in that case. Radiators containing some air would act as additional expansion vessels and you wouldn't have any problems until they were bled.

The link provided above by denso13 contains some useful info.

 

[MHW]

It's possible that the expansion vessel hasn't been working properly for a while in that case. Radiators containing some air would act as additional expansion vessels and you wouldn't have any problems until they were bled.

That makes sense. That said, would I be right in thinking the engineer would have checked the expansion vessel when he serviced the boiler?

 

[jackthom]

I would say he probably should have, but it looks like he didn't.

 

[MHW]

I would say he probably should have, but it looks like he didn't.

Starting to sound that way, isn't it?

I've ten thumbs and am not confident with DIY work but I'll try and check the expansion vessel myself and see if it's working. Either way, I'll phone the firm who serviced it in the morning and request that the engineer takes a look when he does the timer clock this week.

Just one question: is the boiler safe to use in the meantime, or do I risk damaging it?

 

[jackthom]

I'd be reluctant to keep on using it until you can get the expansion vessel repressurised. If you have a foot pump and can easily get to the schrader valve on the vessel it's not that difficult. Just follow all the instructions in the FAQ.

It isn't really a question of safety, just that once the pressure release valve has operated a few times you may find it will continue to leak even at lower pressures and will then need replacing.

 

[Rampage77]

It's not a danger to use it, but it won't do it any good.
You should be ok to use it for hot water but if you do use the heating then run it at a lower temperature, that should reduce the amount of expansion. I would only do this if you really need it until you get the engineer back.

 

[MHW]

Thanks very much both posters above for useful advice. I can't see the schrader valve so am assuming it's behind the front panel, and I don't think I'm competent to go removing that so will contact the engineer a.s.a.p..

Meanwhile, I don't think I can avoid using the system altogether as it's going to be cold tonight (although I could just go to the pub, I suppose!), but will bear in mind what you say about lowering the temperature.

 

[jackthom]

If you can find a system drain valve you could let off some water to get the pressure right down to zero and then continue draining a little more while also allowing some air back into the top of one of the radiators via it's bleed valve to act as a temporary expansion vessel.

Then repressurise the boiler and see how it goes.

 

[MHW]

If you can find a system drain valve you could let off some water to get the pressure right down to zero and then continue draining a little more while also allowing some air back into the top of one of the radiators via it's bleed valve to act as a temporary expansion vessel.

Then repressurise the boiler and see how it goes.

That's an idea, especially since I reckon you might have a point about the top radiator acting as an expansion vessel until I bled it. I'll go looking for the system drain valve this afternoon.

 

[Rampage77]

I'm still in favour for the pub idea.

 

[Agile]

I think that the air fill valve is at the top of the expansion vessel as accessed from the rear top above the boiler.

But you really need to ensure you let pressure out of the system as you add air as it must be kept at virtually zero as its repressurised.

In fact all you ever needed to know is on the FAQ on this site!

Your boiler is quite old now and rather overdue for replacement.

The engineer should really have checked the EXV is absorbing pressure or alternatively repressurised it. I would guess that hardly 10% actually do that!

Tony Glazier