No hot water - please help!

I once had a similar problem which turned out to be the coil being blocked by the plumber using sh*t loads of LSX on the union with the coil.

Might be worth looking - you don't seem to be getting anywhere with the airlock theory.

thanks for the reply, I thought that the coil may be blocked (faulty cylinder or something) but have forced water (with a hose) round the coil and it flows just fine. Also checked all connections on the cylinder for blockages, nothing there...

just tried another idea somone gave me which was to force water down the cold feed from the header tank.

This water went around the system and back out of the breather pipe back into the header tank, so guessing no-air?

Suggestion from someone might be that the gravity system might not work because of the new cylinder having more coils inside. I think the new cylinder has 2 more coils than the old one.

Could this extra water in the system, plus the fact that the connections to the old cylinder was badly scaled up so ruducing the amount of water flow cause the problem?

Fit an electric immersion heater. Or join a gym.

Sorry - didn't realise there was a page 2 - had already typed the following so thought I'd leave it>>

then:
Remove a radiator, connect hoses to each valve, one to garden tap and one to drain. Temporarily cut &cap feed, and cap vent. Turn on garden tap. Watch colour of waste water.
I did more or less this today - the water ran solid black/brown for about a minute! (Old cylinder in this case too)

Or you could hire a power flusher and make a day of it doing the whole system.

Or you could fit a pump to the pipes to the cylinder - pump when cylinder cool and pipe hot (2 stats)