Top floor of town house gets hot, other radiators cold, ideas?

[Hariseldon]

Although your pump is turning, it may not be working as well as it should, perhaps the impellar is damaged or broken. It's possible that what you have is hot water convecting to the top floor and you aren't getting full circulation

You may well be right but I think it is unlikely, because removing a rad a month ago caused the same problem and shifting an air lock then, not only got everything working but got the rads on the lowest floor to work properly for the first time.
Removing a lower floor rad brought this problem on and it would be a bit of coincidence if the pump had suffered a problem at the same moment. I will have a go at letting some water out of the affected rads tomorrow and see what happens.

 

[Hariseldon]

Did not get back from work till late. There are drain points on all three rads on the ground floor. Presumably there for a reason, closed all non working rads off and bled some water off from ground floor 1 . Flicked heating on, top floor came on as before, bled a little water off on found floor 1, hot water!
On floor 3 opened up a rad, it got hot from the return pipe and then opened up rad on opposite side of wall, it too got hot from return, all this with just one rad fully on ,on top floor, thought I had progress but boiler turned itself off and now rads on floor 3 have cooled off with rad on floor 4 staying hot with boiler kicking in from time to time, despite thermostat on floor 2 turned full on.

Now having some tea and will then turn system off and drain off a rad on floor 2 then floor 3 and see if that helps.

 

[stani]

re-read the replies regarding the pump not working, the reason your upstairs rads are hot is purely convection the reason your down rads are not working is because the pump is not working properly, or bleed the pump it may be airlocked.

 

[Hariseldon]

re-read the replies regarding the pump not working, the reason your upstairs rads are hot is purely convection the reason your down rads are not working is because the pump is not working properly, or bleed the pump it may be airlocked.

I have bled the pump. Moving a radiator a month ago gave the same problem, when the lock was shifted not only was heating restored as before but the rads on floor 1 all worked perfectly, thus moving a rad now gives the same problem albeit shifting an air lock is proving difficult.
You may be correct that the pump has a problem but seems surprising that it should fail at the exact same time as another rad is moved and if five rads on one floor can heat up from cold within 2 minutes without a pump by convection alone does seem surprising.