I have a gravity fed hot water system and foolishly let the header tank
run dry.  I think I have air in the system and this is stopping the
boiler from heating the water.  I can't find a way to bleed the air
out, can anyone help me?

Fill the system from the bottom.

In an earlier contribution to this discussion,
Iceman   wrote:


> I have a gravity fed hot water system and foolishly let the header
> tank run dry.  I think I have air in the system and this is stopping
> the boiler from heating the water.  I can't find a way to bleed the
> air out, can anyone help me?


the usual trick is to fill it from the bottom upwards. Assuming you have a
drain cock at the lowest point, attach a hose to this - with the other end
connected to a mains cold tap. Open the drain cock and then open the mains
tap (gently). This should fill the pipes and expel all the air out of the
vent pipe.
-- 
Cheers,
Set Square
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Thanks for that, gave it a try but as soon as the weight of water built
up it started backing out of the drain cock valve.  This was before it
had reached the height of the feeder tank. Any ideas?

In an earlier contribution to this discussion,
Iceman   wrote:


> Thanks for that, gave it a try but as soon as the weight of water
> built up it started backing out of the drain cock valve.  This was
> before it had reached the height of the feeder tank. Any ideas?


There are drain cocks and drain cocks - some of which have a good seal round
the threads of the piston bit, and some which don't. I presume that your
hose is a good seal on the 'outlet' pipe of the drain valve, and that water
is coming out round the square shaft which you turned to open the valve? You
may be able to get a good enough seal by pressing some bungy rubber onto the
end of the valve, whilst getting an assistant to operate the mains tap.
Otherwise, you could change the drain cock for a better one. Does it have
compression fittings, so as to be easily removeable?

If my understanding is correct, what you need is a Type A (glanded) valve.
-- 
Cheers,
Set Square
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Thanks again, managed to get enough in there to get it working and it
seems to have sorted it self out.  The outlet was soldered!  Cheers.