radiator problem

[dwarfy]

Hi,
I have 2 radiators in the house that will not come on. I have recently drained the system down and the problem has only come about after this.
I can however get these two rads to work if I fix a hosepipe to the drain valve on the inlet side and open the valve. As soon as I close the drain valve, the radiator goes cold.
I have old thermostatic valves that regularly stick, and I have learned how to free them. This however feels different as the inlet and outlet pipes are both cold unless I open the drain valve, then everything heats up normallly...Do I have some kind of airlock??
Not sure what to do now so any help would be appreciated.
many thanks

 

[sonray ltd]

make sure the trv is open and shut all the hot rads off giving full flow to the cold rads.
then when cold rads are hot you can rebalance the off rads back into the system

 

[dwarfy]

Thanks for your help. It worked but as soon as I turned the rest of the rads on they have gone cold again

 

[sonray ltd]

i take you just opened them fully again.

 

[dwarfy]

Hi
yes opened the valves fully. There is a slight amount of warmth but that's it I'm afraid.
Thanks for your help.

 

[Kings]

You need to close down on the non trv valves, they are normally lockshield and mostly fitted to the return side of the rad.
Start with the ones closest to the boiler/pump, shut the valves down to 1 turn open, check that the dodgy rads are getting warm, if not then close down on the next rads until the furthest get hot.
When you do this leave all of the trvs fully open!
The last one I did required 80% of the rads to have the flow reduced, the nearest one to the pump was 1/2 a turn open.

Be warned you could end up with leaks if you "disturb" a very old valve, once you are happy then consider putting in scale inhibitor such as Fernox, especially if you have been draining parts of the system.

Good luck!