Downstairs radiators cold - pump is fine

[ChrisR]

A few thoughts: probability is that you do have sludge problems. If your pump really is one of those DAB's they hate it and wear out very fast - slowing down a lot.
Sludge very often fills the vanes in a pump so they do practically nothing.
Sludge tends to collect in downstairs rads, stopping them working.
Draining and refilling will very likely make it worse.

Filling while draining is an easy thing to try, but I wouldn't be hopeful of a lot of improvement.
Obviously you can't fill from the drain point while draining - unless you have two drain points, which changes things a lot. That would mean you can flush each rad in turn by using the rad valves and would be well worth trying. Obviously the water might prefer to go up into the header tank even if you're draining from another drain point, but if you check the integrity of the overflow up there first you should be able to avoid it being a problem.

If you have a small downstairs rad you can remove, that would 1) tell you how much black creamy muck you have in the rads 2) you could flush that rad in the garden with a hose and 3) you would have two connection points for hoses.

 

[Softus]

Talking to a friend at work he thought it would be ok to drain the water out of the system and just let it refill from the header tank at the same time. This way I would avoid any nasty air locks. I think I have read that you should prevent the system refilling and refill it from the bottom up using a hosepipe. The later sounds quite a bit harder, which would be best for me?

Yes - that sounds much too hard for me to.

Basically, if you drain a dirty system without cleaning out the F&E cistern, and then don't entirely flush it out out, then you risk debris from the cistern getting stuck on valves - a right royal pain in the ar*e.

A system should be designed and implemented to have vents in the right places, but if you get an air lock then so be it - you'll have to deal with it, and they're not that hard to sort.