In an earlier contribution to this discussion,
wrote:

Set Square, thanks for your time and advice.



My system consists of a Baxi Bermuda located behind my gas fire
downstairs. The pump is under the downstairs floorboards. I think (but
I'm not sure, sorry) that the HW is circulated by gravity. I really
couldn't answer about the motorised valve. I haven't noticed any
around the pump or in the airing cupboard, but this doesn't mean
there aren't any (it's generally inconvenient getting to the pump)

It's the small one that is slightly warm that sometimes overflows (but
hasn't done for some time.) The ball valve is fine, and the water
level is normally well below the outlet.)

I used to be able to bleed the radiators with the pump running in the
past, but like I said, if I do that at the moment, the rads appear to
drain. Where could the water be going? I haven't spotted a leak
anywhere (and I did look under the floorboards too

OK, it's almost certainly gravity HW circulation (two 1" or 28mm pipes
connected to one side of the boiler and going to the indirect coil in the
hot water cylinder in the airing cupboard) and pumped CH (two 3/4" or 22mm
pipes connected to the other side of the boiler). Presumably there's no
cylinder stat, and the DHW gets hot whenever either HW or CH are switched
on, and sometimes gets *too* hot?

I would guess from your further information that you have no corrosion
inhibitor in your system - with the result that a lot of sludge has built
up, partially (or even completely!) blocking some of the radiators. I would
also guess that what you are bleeding out is hydrogen caused by the
corrosion rather than air. [You can test for this by holding a lighted taper
in the stream of gas coming out of the bleed valve. If it burns, it's
hydrogen!]

Sludged-up radiators would probably explain what happens when you try to
bleed with the pump running. The pump is probably installed in the boiler
return - so that it's in effect sucking the water through the radiators. If
the inlet of a radiator is blocked, and you suck on the outlet with the
bleed valve open, you're going to suck the water out. This will end up in
the header tank, with a resulting rise in level.

It seems to me that your system needs a really good flush out, and then
needs to have inhibitor added. By far the best way to flush it out -
particularly with summer coming on - is to remove the rads and take them
outside and give then a good flush out with a hosepipe. While they're off,
flush the flow and return pipes by keeping the header tank full and opening
each radiator valve in turn (with a bowl under it to catch the water!).
Incidentally, take great care not to spill any black sludge from the
radiators on your carpets - it makes a pretty indellible dye!

If you get a plumber in, he'll probably want to charge you an arm and a leg
for doing a power flush. However, the method I have described is far
cheaper - and probably more effective.
--
Cheers,
Set Square
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