Removed radiator - now boiler will not fire

[Ric2013]

Sorry for lack of detail and inability to think, as very tired. But...

Cylinder almost certainly indirect (google 'indirect cylinder diagram') so the fact that you are drawing water does not prove there isn't air in the coil.

And I don't think a plumber will fix it in 10 minutes if it is a blockage.

 

[Rodders]

So the 'hot return from the boiler' is red hot, so I'm happy the boiler is working. This pipe is hot right up to the point of entering the cylinder.

Is it possible that air in the coil will prevent hot water entering the cylinder (and therefore returning to the boiler), and the hot water is backing up and running down the bypass valve which goes back to the boiler. So all the boiler is doing is circulating the same piping hot water (and eventually shutting off).

So final question; how the heck do you get air out of the coil? Was thinking of unscrewing the hot water feed into the cylinder, to hopefully let out air and no doubt a little bit of water?

 

[Rodders]

So I just fired up the HW. The hot return from boiler got piping hot instantly, right up to entering the cylinder. The bypass going back to the boiler got instantly hot, suggesting hot water was going back to the boiler and the pipe exiting the cylinder (going back to the boiler) got pretty hot pretty quick.

Surely that must mean HW is flowing through the coil?

 

[Ric2013]

Assuming the hot water going to the coil is pumped uphill, there could be some air in the coil which would tend to settle to the top, but because the system is pumped, water could still be entering the coil and going down by gravity, fighting the air running back up on the way. Loosening the nut at the entry to the coil as you have suggested might just work. I'd be surprised if the coil itself were blocked.

Sometimes there is a 'balance' gate valve on the pipework to or from the coil that may be mostly shut, but could now be blocked with gunk that shifted during the draindown? If so, may be worth opening fully and letting some flow through before putting back to how it was.