There is something rather unwell with my heating system but it's never been
quite right since I bought this house. Current problem is the radiator
nearest the boiler and pump (which is upstairs in the back bedroom) is
continually filling with air. Bleed it and it goes hot instantly but will be
full of air in a couple of days. Air and bubbling can also be heard in and
around the boiler/pump. Additionally, there is a little overflow pipe
sticking out of the wall just above the boiler that is continually dripping.
I'm not sure how long it's been dripping for. It drips whether the central
heating is on or off.

Now some history:

The pump packed up a couple of years ago and the plumber who fixed it said
that whoever had installed the central heating got some pipe work wrong
around the pump area which meant it would be pulling air into the system.
Even if it was, does it explain why suddenly more air is getting in.

I recently drained the system twice in order to remove and decorate behind a
radiator in the bedroom. This radiator is working fine. However, some quirks
he firstly, the water only dribbled out of the release valve in the
cellar (the lowest point). Secondly, upon refilling it took an awfully long
time for the radiators to start getting hot again. The thermostat on the
boiler was set to very low - it's currently on medium. Could there be a
blockage somewhere?

There is a very slight weeping of one of the radiators downstairs - so
slight that the water evaporate before it drips. It's been sealed with
something I got from B&Q - a sort of silicon sealant stuff. Not sure how
effective it is though. Could a slight air leak like this cause a radiator
at the other end of the house to fill with air?

The radiator down in the cellar is damaged but as I don't particularly want
to heat the cellar, it's turned off at both ends.

Any ideas? I suspect I need a good plumber in here - whilst I understand the
principles of the thing, diagnosing something like this is a lot more
difficult.

Thanks, Rob.