How do I fill my CH system

[JK]

I've moved into a house with a Vaillant Thermo-Compact combi boiler. The pressure guage has been reading 1bar which is OK according to the installation manual I've found. However, one of the radiators had a large amount of air in it, and after bleeding it, the pressure has gone down to about 0.5bar. So I looked for a filling loop, and discovered it isn't meant to have one, it's meant to be a sealed system with a mechanism for venting air built into the boiler. There are diagrams in the install manual showing that it fill when commissioned by connecting the CH return to the mains temporarily. So I looked around for a connection to do this and found that in fact there is a pipe coming from what looks like the mains into the CH return. This has been permanently plumbed in with two stopcocks right next to each other. So I presumed this would be the way it fills. But, when I open the valves, nothing happens. The pressure does not go up, there's no sound of wather flowing.

At this point I admit defeat. The heating still works, but I'm worried. Any advice?

Thanks

John

[Christian McArdle]

That does sound like your filling loop, although it is supposed to also incorporate a non-return valve and a detachable hose. Assuming you opened both valves up fully, it sounds like either there is a 3rd valve hidden away somewhere, the mains supply has been cut off, or one of the valves is faulty.

Christian.

[JK]

The valves seem pretty good, they move easily, and one of them starts dripping slightly when it opens, suggesting there is pressure there. I don't think I'm missing a valve, I can see all the pipework, though it's all behind cupboards so I guess I could be wrong. Don't think so though, there's a hole cut in the cupboards to allow access to the valves and nothing else like that in the house.

I wondered if I need to open the radiator bleed screws to aallow filling, but if the pressure dropped as a result of bleeding the middle level rad, that water would be replaced by the level dropping one the top floor creating a degree of vacuum which should get filled when the valves open.

[Christian McArdle]

Actually, what happens is that you have a pressure vessel half full of water. When you bleed the rad, the water comes out of here until the pressure drops to zero. When you refill, the additional water compresses into the vessel and the pressure rises.

There has to be something preventing the water getting through. The valve may still be damaged, even if the handle turns. Alternatively, if you have really low mains pressure, you may not be able to refill. (Indeed, if you have a missing check valve, then you could be filling the mains with your filthy boiler water!)

Christian.

[JK]

I did wonder about this last point and ran the taps for a while! I guess the way to find out if I have sufficient pressure would be to take the garden hose up to the highest point in the system.

[JK]

I presume that means 34' above the level of the boiler? Could I check this using the hose on a pole?

Thing is, the valves are almost certainly there for filling the system, which is a bit mysterious. Thanks for the FAQ, I'll read it now.

J

[John Rumm]

Alas not in this case. For a conventional system with a header tank, then yes the only requirement on the mains presure it that it can get water as high as the header.

In a sealed CH system such as yours, the mains pressure needs to be at least as high as you are going to try pressurise the system with (typically 1 bar, or the equivilent to a 34' head of water). Having said that, most mains supplies ought to be able to manage that much.

More info in the sealed CH FAQ:

[JK]

OK, thanks for the replies, I've sorted it. It was one of the valves. I filled it to 1.5bar as suggested by the installation manual.

John