Rad works with stove but not when run on oil

[pricelesspete]

Age old problem of one cold rad in a house again. Would be grateful if I could pick the brains of you experts out there.

Some background. I have an oil fired boiler and a stanley stove as back up. Recently done some work done at home on oil fired heating as it had been sucking in air. The pump had been sending water up the cold feed to F&E tank causing the tank to overflow and the radiators to start emptying. so the cold feed was reposition before to the pump which is on the return just prior to boiler. My heating all now appears to be working properly but I cannot for the life of me get heating to one radiator upstairs. I have tried closing all other radiators but this did not seem to make any difference. I drained down and changed the valves as I suspected that could be the fault but to no avail. There was an issue when it came to draining down in that the F&E tank failed to empty so clearly there was some kind of airlock. To rectify this I backfilled the system through the mains and all radiators filled this way.
I used the stove last night and the radiator in question
heated up fine but when using the boiler again this morning this rad was again stone cold. I turned the pump up to 3 hoping that this would clear the problem but all this did was cause the system to vent into the f&e tank.
Could somebody please explain how I can get this one radiator to work using my oil fired boiler

 

[JohnD]

you have a solid fuel stove?

I don't know much about them but they often have an "always on" rad as a heat sink to take the excess heat if the stove gets too hot

I wonder if this rad is connected to the stove but not on the usual heating loop.

Or, if it is in the bathroom, it might be plumbed to heat up on the hot water circuit (this is often done so it warms the towels even when the heating is off)

You might have to look under the floor to see what the pipes do

 

[JohnD]

There was an issue when it came to draining down in that the F&E tank failed to empty so clearly there was some kind of airlock.

or, more likely, a sediment blockage. has the system been cleaned recently? what inhibitor do you use?

 

[pricelesspete]

The system has not been cleared ever to the best of my knowledge but has now been drained and refilled a number of times. A fernox inhibitor was added the last time it was filled The rad used to work but since the changes it has now stopped so the always on rad is not the answer