What's wrong with my central heating system?

[Jim T]

Something seems odd about the pressure behaviour of the central heating system in my 3-bed house, powered by a Britony 80 combi boiler. Over the past 3 days, with the boiler running, I've been carefully recording the water pressure levels (according to the boiler's pressure guage) at various intervals. Before doing this, I rigged a pipe going from the pressure relief valve into a bowl, so I can see what gets expelled from the valve. Here is my record:

Wednesday

4:30pm: I top up the pressure to 1.75-bar. The bowl is empty. 5.30pm: The guage reads 2-bar. Bowl is still empty. 10.45pm: Guage reads 1-bar. Bowl now contains 1/4 pint of water.

Thursday

6.00am: I empty the bowl and top up the pressure to 1.75-bar. 7.20am: guage reads 2.65-bar and there is 1/4 pint in the bowl. 8.42am: guage reads 1.4-bar. Another 1/8 pint has gone into the bowl. 8:56am: guage reads 2.1 bar. No further expellationinto the bowl. 10.00am: guage reads 1.3-bar. No further expellation into the bowl. 10.30am: guage reads 1.8 bar. No further expellation into the bowl. 2.00pm: guage reads 0.8 bar. No further expellation into the bowl. 6.20pm guage reads 1.5-bar. No further expellation into the bowl. 11.20pm guage reads 1.0 bar. No further expellation into the bowl.

Friday

10.04am, guage reads only 0.2bar! No further expellation into the bowl.

Anyone tell me what's going on? Expansion vessel problem perhaps? If so, what? What's the procedure to cure it? The whole system was installed about 4 or 5 years ago.

Thank you for any help.

Dave L

[Set Square]

Unfortunately, your carefully logged details are missing one vital piece of information - namely what the heating system was doing at the time in terms of being switched on and off - either by a programmer or room stat.

Nevertheless, from what you say, you have a problem either with with your pressure vessel or with your pressure refief valve.

When everything is working normally, the pressure should be about 1 bar when cold and 2 bar when hot. [A cold charge pressure of 1.75 bar sounds a bit high to me - have you always used this pressure?]

When the system heats up, and the water expands, the pressure vessel is supposed to be able to absorb the expansion with only a modest rise in pressure. If the pressure rises above 3 bar, the pressure relief valve is supposed to open - to let some water out and reduce the pressure.

First check your pressure vessel. Let some water out of the system to reduce the pressure to well under 1 bar. Measure the air pressure in the pressure vessel with a car-type pressure guage applied to the Schrader valve. If water comes out of the valve when the pin is pressed in, the diaphragm is split, and the pressure vessel is shot. If no water comes out but the pressure is low (say, less than about 1.3 bar) pump some air in to increase the pressure. This might cure the problem.

If you still get water coming out of the pressure relief valve, you need to determine at what pressure this is opening. If possible, stand by the system at a time when water is coming out and observe the reading on the pressure gauge. If this is substantially less than 3 bar, the relief valve is shot.

If the pressure vessel and relief valve are both ok, try running at a lower charge pressure. If you still have problems, it would suggest that the pressure vessel isn't big enough, and that you need to install a bigger one. This sounds unlikely if the system has worked ok for 4 years - unless it has recently been extended by adding additional radiators.

[Christian McArdle]

Possibly. However, you're filling up to quite a high pressure and you don't have much headroom before the pressure relief activates, possibly dumping too much water when it does so.

I would pressurise to 1 bar when cold. Then, if at any point the pressure increases beyond 2 bar, you can assume that the expansion vessel is either broken or undersized. Either fix it, or install a new one. Personally, I would prefer a system to be designed to expand by only 0.5 bar. It means that the pressure vessel has to be seriously deflated before you start to get problems.

There are other explanations, such as dodgy pressure gauges and relief valves.

Christian.

[Ed Sirett]

See the FAQ. Given that most combi boilers have an internal vessel capable of coping with the size of most typical systems[1]. I would say that the vessel has failed.

As a work around you can drain a radiator (or 2)in a less used (or less important to heat) room and then refill without bleeding that radiator. This will likely provide enough cushion to enable you to have heating until you can replace or install a good vessel.

Unless the manufacturers state otherwise, precharge the vessel to 0.75 bar, and fill the system initially to 1.0.

[1] Retro style radiators or larger systems nearing the 24kW heating output, may need additional vessels to limit the presure rises. Whilst in a domestic setting the large system is unlikely to be connected to a combi (because HW usage would demand a different installation), you might find that say a modest village hall might be heated with a 24kW combi, where the HW demand is only for wash hand basins and a kitchen sink but the full power on the heating side is needed for the radiators.

[Jim T]

Thanks for your input on this. I think the pressure relief valve must be OK because it's brand new. I replaced it a few days ago, because the old one was my chief suspect, but the new one behaves exactly the same as the old one.

Well, to be very honest, since I instaklled the system, I've been topping up the sytem to 3-bar! I thought mistakenly that the 3-bar on the guage was 2-bar. It was only in the past could of weeks that a Brittony boiler expert made me aware that I shoul only top it up to

1.5 or 1.75 bar. Nevertheless, I would imagine that the pressure relief valve has been sparing the system from any damage resulting from my over-topping-up of the presure....

Yes, that certainly happens. In fact, water starts trickling out at about 2-bar (on both the old and the new pressure relief valves). Both of them tend to blow when the pressure guage is not much past 2-bar. But I reckon it's unlikely that they are both faulty in exactly the same way. They don;t even look the same. The replacement I just put in is a new design. (Designed to be better than the older design, I guess).

Schrader valve??? Never delved into this area of my boiler before, but I am willing to venture into the unknown.. Looks like my time has come to be a hero.... ;-)

See my above comments on the valve...

No, it hasn't been extended. There is, of course the possibility that the pressure guage is incorrect - but I suspect not... because I've noticed the boiler starts making a moaning noise (or whining noise) when the pressure drops to around 1-bar. That symptom is the same as it always as.

So I guess the chief suspect now is the expansion vessel.....

Many thanks for your help.

Dave L

[Jim T]

That sounds like a plan. I'll try it. Thank you for bearing with me on this issue.....

Yes, I guess so.

Thanks again,

Dave

[Jim T]

It does sound like it.... Anyone know how much a new expn vessel for one of these might cost? Are they difficult to replace? As I mentioned I've never delved into that part of the boiler before. I gather the expansion vessell is situated at the very back of the boiler, behind the main firebox, and I think it's integral with the boiler chassis

Ed, I had wondered about that! Nice to hear someone coroborate what my logic had suggested... I could to that, and thus postpone this work for as long as possible....

The 0.75 bar is about right. The boiler's manual says they pre-charge them to 0.7 bar.

This is a modern system with normal B&Q-style rads. It's a small 3-bed house. The manual says the expansion vessel is suitable for systems up to 145 litres capacity. I've never really measured the capacity - only guesstimated it roughly, on the basis of the size of the house and the number and size of the rads - and that was years ago when I designed the system... the details of which have escaped my memory.

Many thanks..

Dave.

[Jim T]

Good point. Invariably the high pressure readings were while the system was hot and the low readings were while the system was elatively cooled-down.

Dave

[Nick Read]

Did you manage to sort this out?

The reason I ask is because I also have a Britony 80 combi boiler, and have been having exactly the same problem with it over the past month or so. I too replaced the pressure relief valve believing it to be faulty, but it made no difference. Yesterday morning I depressurized the system, checked the expansion vessel pressure & found it to have dropped to below 6 psi when it should be around 10 psi (roughly 0.7 bar). I topped the pressure up with a footpump, and it seems much improved, although there is still a very occasional drip from the relief valve.

The pressure seems more stable now, there was previously nearly a 1bar pressure difference between the system hot and cold. This seems to have dropped to a difference of less than 0 .5 bar between hot and cold.

Nick