One radiator not heating up after system drain down

23 Oct 2008
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United Kingdom
Hi any help would be much apreciated. I have a fully pumped open vented system. Baxi bermuda back boiler with a honeywell 2 port zone valve to feed central heating. All radiators have TRV's fitted with standard lockshield valves.
My problem is that 1 radiator downstars will not heat up. All of the others are fine in the system. My problems started when I drained the system down to replace my faulty honeywell zone valve. I fitted replacement valve with removable head. I filled system back up and bled all radiators. Zone valve is working and all radiators heated up apart from this one.
I have checked that the TRV on this rad is working and I have drained and desludged this rad also. I have also tried to rebalance all of the rads but the problem still persists. Both the flow and return pipe supplying this rad are not blocked as I get a strong flow from each. One of pipes supplying the rad warms up a little which i assume is the flow but the rad and the return are stone cold. This radiator before I carried out the work would get hot.
Any suggestion please could it be a blockage somewhere.
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try turning all the other rads off and see if this pushes it through.
Hi Finley

IF all the rads were working fine before the drain down then it could be an air lock in the system. There are a couple of straight forward ways of trying sort it.

Firstly, as previously mentioned, close down all the other rads (making a note of the number of turns on each valve you close). Make sure your pump is on speed 3 and hope that the full force of the system will push the water through the system.

If that does not work.

Then, if there is a drain off point near the cold rad, with the F&E tank in normal operation (ie free to allow water into the system), connect a hose to the drain off and pull some water through the system. This might remove the air lock. No not try this with the boiler running.

If that does not work you could try turning the pump around and see if that does the trick.

By the way - the most common problem with the Honeywell 4043H zone valve is that the motor burns out, easy to check. If you supply a live in on the brown wire and nothing comes out on the ornage wire - then change the motor. The motor can be bought for about £10 and takes about five minutes to swap out - assuming you can get to the head of the valve.

Good luck
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Hey fellas thank you very much for your posts. A combination of both your posts has helped me clear the air from the system. The radiator is now lovely and hot
Nemo with regards to the honeywell zone valve, I replaced it because the motor had failed like you stated. I decided to replace it with the new improved design which has the removable head to avoid drainning the system down in the future. Got it brand new off ebay for 25 quid(bargain).
Thanks again for the prompt responses.

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