Hi there,
I recently moved into a house with an open fire and a grant back burner.
I have noticed a problem and have managed to pinpoint this down.
When I light a fire the boiler does it job and it heats the cylinder and then in turn the pump come comes on and dumps the water to the rads however ive noticed that when the fire is burning a long time (say 4hrs) that I start to hear the cylinder making banging noises (might be pump) and i note the pump stops pumping and the whole things gets hot, this is ok but then this same pump wont work when you switch on the oil burner. So in a cold night then nothing is heating the rads. generally when the hot water in the tank cools off or is dumped i find the oil boiler starts to work again and the pump works as normal. Now if i dont light a fire the pump and the oil central heating works perfectly without problem so I am assuming that the pump is not liking the heat of the water produced by the back burner at the fire, the stat on the cylinder (that controls the pump) is set to 45d and ive been told that seems fine, the stat must be working ok if the back boiler works fine for the first few hours etc.
Its an odd one and im thinking the open fire/ back burner while nice is too much hassle.
It wouldnt be so bad if it didnt stop the oil boiler using that same pump thus leaving me without heating for 1/2 day.
Any help would be appreciated...

What he said, get it checked. Steam explosions are very dangerous - not just because the explosion is violent but because when steam condenses it gives up a lot of heat without cooling down (google latent heat of condensation). Makes it much more dangerous than water at 100C.

I believe systems like this are normally designed to fail safe in the event of a power cut so I think they are usually gravity fed rather than pumped. Also common to have a heat dump rad to dissipate excess heat when the tank is upto temperature to stop it boiling. Sounds like that's not working right and could be pretty dangerous.

The cylinder stat may be cutting the boiler/pump off wrongly.

It really needs checking over properly before you use the back boiler.

I'd still get it checked.  Not obvious why the back boiler needs a pipe stat.

Are you allowed to pump a soild fuel boiler? I thought they had to be gravity. I'm probably wrong or I've missunderstood his system.