where is the pressure " stored" in a condensing combi boiler central heating system?

[Duncan Di Saudelli]

Hello

Our new house has a 2007 installed boiler system which includes a removable flexible hose: you connect this hose between the cold water mains riser and the central heatng pipe work, turn on a pair of taps and flood water from the mains into the radiator system. A guage on the boiler tells you what pressure you have driven the radiator system up to - in our case somewhere about 1.5-1.6 bar is recommended.

Water isn't compressible and, seeing as it takes a good ten seconds to squirt thew water in and change the pressure from 1 to 1.5 bar, we must be doing work on something. I can't imagine that the pressure is due to the radiators and pipework "ballooning" a bit - we must be introducing a good few pints of water in that ten seconds - so is there some kind of air bellows system? The water forces the bellows back in a tube, compresing some trapped air in doing so - and that acts as the "spring" to maintain pressure?

Either that or there is a LOT of bleeding for me to do.

I ask because I think we have small leak. Approx. 1.5-1.6 bar fluctuates from 1.2 to 1.8 bar as the radiators swicth on and off (TRVs) and as the boiler fires up and drops into standby, and as it switches from central heating to domestic hot water duty. After a week or so, the trend is downards, towards 1 bar.

So ... where would these "bellows" be if they exist (not been in the loft yet) and might that be where the pressure leak is? Have I got it completely wrong and it *is* pure hydrostatic pressure responsible for the workings of our central heating boiler system?

DDS

[David Hansen]

On Sun, 28 Feb 2010 20:45:37 -0000 someone who may be "Duncan Di Saudelli" wrote this:-

Yes, one of these

It may be filled with nitrogen above the diaphragm, rather than air.

Most likely to be at the coolest part of the system, beside or inside the boiler.

Unlikely. Far more likely to be in the pipework.

[Duncan Di Saudelli]

Dave - perfect. Thanks for these pointers; I shall start hunting for leaks (seeing as I am dropping half a bar in a week, presumably fairly sizeable and hence easy to spot!).

Thanks for the information and feedback so swiftly.

DDS

[The Medway Handyman]

There are people here much more knowledgeable than I on this subject, but combi boilers have a thing called an 'accumulator' or pressure vessel. This is a cylindrical vessel with a rubber diaphragm across the middle. One side contains pressurised air, the other water.

The air is compressed by the water pressure and therefore maintains it. The flexible hose is known as a 'filling loop' and is normally left connected, but with the isolating valves turned off.

As to where it is, usually near the boiler or sometimes integral to it.

If you post the make & model of your boiler someone here will no doubt know all the details.

HTH

[Bruce]

I had a combi system with a leak which produced about a thimbleful of water in a week, if that, and that caused a pressure drop of half a bar. So don't search in the expectation of finding a major leak - it doesn't take much water to cause such an apparently large pressure drop.

[Usenet Nutter]

The presssure should be checked when the system is cold .When it heats up the pressure will show a rise ,dropping when it cools back down . Are you comparing the pressure loss over a period ,each time when the system is cold ..

[Mike Clarke]

It might be a leak or it could be just that the gas in the expansion vessel needs re-pressurising. Over time they sometimes loose gas pressure and are unable to absorb the expansion of the water with the result that the water pressure rises as heat is applied and some is vented out via the pressure relief valve. When the system subsequently cools down the pressure drops to a lower level than before. You should see a valve on the pressure vessel just like the one on your car tyres where you can check the pressure and top it up if needed - it's usually a case of pumping it up to about 0.5 to

1 bar while the system is de-pressurised and open. There's information about this in the sealed heating system section of the FAQ

See the

[Duncan Di Saudelli]

Aha - right ... hadn't appreciated that. In which case I think I know where the leak might be - I've noticed a small drip near a lockshield on one of the downstairs radiators. A useful tip - thanks.

DDS

[Duncan Di Saudelli]

No - I haven't compared like with like. Schoolboy error! Thanks for the pointer; I shall re-read what's written here and the FAQ at

and will revisit the problem.

Thanks to all who posted

DDS

[<me9]

If a thimbleful produces a half bar drop, then the pressure vessel (normally somewhere near 10 litres capacity) is shot. I'd expect a litre or two, if the system is hot most will evaporate and you won't see it. Best to check when cold, or look for stains of lime or inhibiter around likely places.

[John Rumm]

One needs to bear in mind that much of a slow loss from a hot system will be evaporated away before you even see it. Most of the loss will occur when the system pressure is at its highest (i.e. hottest)

If the pressure vessel was knackered then you would expect a dramatic rise in pressure as it heats up - often enough to lift the over pressure blow off valve (which should be piped out through the wall to a safe discharge point outside). The usual result there is insufficient pressure for the boiler to fire again after it has cooled. (un-bled radiators may however mask a failed pressure vessel)

[RobertL]

or it might be, as in my case, that an over keen plumber had pumped up the pressure vessel to hard. If it is pumped up to a bladder pressure of 1.5 bar (as in my case) when the water side is unpresureised then the balloon is filling the whole of the space in the vessel unless the system is filled so the pressure is above 1.5 Bar.

Robert