3 out of 12 radiators not working - ideas?

Always been like this? Check layout of pipes.

Have gone like this fairly suddenly? Check water in header tank (not like to be a problem since the water squirts out). Possible blockage in common return or flow pipe.

HTH

If you turn off all other rads and these still don't get hot with the pump running, then the water isn't circulating. Most likely an airlock in the pipes - water coming out of a bleed valve on a cold rad doesn't mean there isn't one somewhere else in the pipes.

If an open system, have you checked the header tank has water in it?

I was like that a few years ago. Now I probably understand my system better than a plumber :-)

Was the pump running when you did this? If so, you can not bleed the system. switch of the pump and try again.

Shut all the hot radiators down, don't use the lockshield valves. Make sure that the pump is not running. Bleed each cold radiator until you get a fair amount of water out of them (about a pint or so). Put the pump back on at its fastest setting, after closing 2 of the 3 rads that are not getting warm. This should pump hot water through the one radiator that is turned on. If this one gets warm, then turn it off and try one of the others, until you get them all warm. In my system, this can take a few tries to get it all up and running again, so don't give up on the first try.

HTH as I am no expert on central heating :-) Just my own.

Dave

Do any of these rads have thermostatic valves on?

If so it is not uncommon for the valve bodies to "stick" in the off position... often you can tell by removing the thermostatic head with it's thumbscrew, and looking at the pin that is pushed by it. It should notmally be in the raised position and should be possible to push it into the valve by hand. It should spring back when released. If you find the pin is stuck down, you may be able to free it with a tap to the side of the valve with a (small!) hammer. In still no joy then replace the valve.

I have exactly this problem if I drain my system.

I used the above technique plus the fastest pump speed I could get to clear the air.

I'll give it a go and let you know how many attempts!

Thanks

-- nico62