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

 

[JK]

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.

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.

 

[JK]

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!)

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]

Can you loosen off the nuts on the pipework of these stop cock(s) a bit, on
the side(s) where the stop cocks are adjacent to each other? By opening and
closing the valves you should be able to work out whether you've got mains
water pressure behind them, or if they are faulty. You could also turn off
your main incoming stopcock, and that shuts off the water to your apparent
'filling loop', that would confirm it.

Good idea. I'll see if they're compression joints. I think these valves
must be intended to fill the system because they connect the mains
supply to the CH return, exactly as the manual suggests a temporary
circuit should do.

If you've any air in there, then yes you need to open the bleed valves to
lose it, but there's no point in doing it to allow filling - if you're down
to 0.5 bar then you ought to be able to hear it filling from the mains
(unless you've got incredibly low water pressure genaerally - do you?).

Is it actually risky running the heating at this pressure?

 

[JK]

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.

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]

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.

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:

http://www.makewrite.demon.co.uk/SealedCH.html

 

[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