When I refill the system the lowest radiator fills quite quickly but then nothing seems to be happening for ages. I vent the pump and go round each radiator in turn and open the bleed valve but very little air comes out until many hours later. Should I leave them all open and just keep checking for the first signs of water? I also open a bleed valve off the pipe to the hot water tank but don't leave it open.
As a last resort I switch on the pump for a while till it starts to get warm, this does seem to speed the process up but I realize I shouldn't need to do this.
I suppose there is an airlock somewhere in the system, probably in the boiler, so would it be safe to connect a hose between a mains pressure tap and the drain c*ck and blast mains pressure water in reverse through the system until it appears in the header tank?
I wouldn't leave the bleed valves open unattended. All it takes is for an airlock to suddenly clear and then you will be in trouble in the worst Sorcerer's Apprentice fashion.
When the system is full, does it work properly? Do all radiators warm up and are they poor at warming up and with cold spots around the bottom centre?
I am thinking of possible sludging here......
This is a bad idea, because if a pump runs dry, you can knacker the bearings quite quickly.
Since you have the system drained down, I would suggest a check for sludging. Pick a convenient radiator and place collecting vessels underneath the valve unions. I use cat litter trays from the supermarket for this. Take care because heating water and sludge is an indellible dye.
If there are signs of sludging, you can try a method of cleaning whcih I've used and described before a few times which involves taking each radiator outside and flushing with a hose or a pressure washer. While each radiator is removed you can flush with some fresh water through the pipework as well. It could be that you have a build up of crud in the pipes.
Be a bit careful if you attach a hose and blast water backwards because you could overflow the header tank.
If there is a sludging problem, after you have cleaned the worst out as described, you can use a chemical flusher and deal with the rest, before flushing once more and filling with inhibitor.
If there is no appreciable sludging, then I would try to find out why the airlocks are happening if that is what was happening. Obvious things are poor pipe runs. An open vented system should fill OK. Before converting mine to sealed operation, I would tend to fill the ground floor radiators first and then the first floor. Actually I still do.
Another option, if the boiler is relatively recent and will accept it, is to convert the system to sealed operation. This is remarkably easy to do and will resolve airlock problems completely. The system must be clean of course. If you search for a post by Ed Sirett, he has written a FAQ on how to do this.
How old is your system? Have you ever taken a radiator off for decorating and did black slugde drip out of it? I had an old system, drained it then removed each rad in turn and flushed them through with hose pipe, some OK but some really full of sludge. Re-filled and added flushing agent,left for a few days then drained and re-filled adding inhibitor. Much improved for a couple of years but have have new combi system since.
Yes, do this. One of the reasons I prefer sealed pressurised operation is that the standard filling loop does just this, making refilling a doddle. Check the security of the overflow first, though!
If you find where the airlock is likely to be, insert a bleed valve there. Look for large inverted U-bends.
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