CH Boiler Overheat.

Sounds like lack of water flow. I suspect either the boiler has an air pocket in it and rapidly overheats, or the pump is in an air pocket and not pumping anything. I would leave the pump running without boiler firing (which is probably the effect after the lockout), and go round bleeding everything. In addition to the radiators, you may have bleed points on high pipework, and inside the boiler. Don't try firing up the boiler too many times until this is resolved, as you might end up damaging the heat exchanger.

id guess that your system is air locked if it was functioning befor you drained it.......be careful not to burn out your pump....ensur boiler stat is at LOW until satisfied that your system is circulatin again...

there are differant ways to clear air locks... try rocking the pump...this means..turn pump on then off in 4 se cycles....this creates surges and may move the air...

simply open your drain for short periods whilst system on and boile stat off..this again creates surges...

back fill using a garden hose connected to the drain..

try reversing your pum

-- gastec

Followed your advice-two vent plugs in the attic opened plus pum switched on/off a few times and the air lock cleared, and now workin normally.

Thanks all for the advice

-- SWW

I've had to drain down my oldish open system several times in its life for various things - new TRVs, pump, house extension etc and each and every time this is the only way I've managed to get rid of air pockets. To make things easier after the first couple of times I added a filling loop rather similar to a sealed system at the lowest point in the pipework - easy for me as I have a cellar.