Radiators not heating up after the "summer" - hot

[MikeWW]

I have a Danesmoor 20/25 Oil-fired boiler. After the so-called summer (ie, when we have been using it only for hot water) and we re-engage the central heating, the radiators don't heat up. It happened this year, and has happened in the past as well. The header tank is ok and bleeding the radiators doesn't help. There are no odd sounds and I am sure the thermostat is fine too. Last time we called in a plumber and he mumbled something about sediment, got it working and didn't overcharge us, but I wasn't around to watch what he did. I undid the large screw on the front of the pump to let any air out of there, but nothing has changed.

Can someone suggest what I might need to do, and also how to prevent it happening next year?

Thanks in advance.

 

[namsag]

If its a grunfoss pump undo that big air screw again and stick an electrician screw driver into the slot in there and spin the shaft to free it

 

[MikeWW]

That did the trick - many thanks for your help.