Heating pump runs continually

14 Apr 2012
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United Kingdom
Yes another one I'm afraid. I had what I thought was one problem but now appears to have been two all along.

I had a mv which wasn't working that well and a pump which was running all of the time. I have changed the actuator head and the mv now spins over and responds to the programmer correctly but the pump never stops, I had hoped this would be due to a faulty microswitch having read a lot of the threads on here.

The boiler is an Ideal Excel RS40 from the late 80's, the system has a honeywell V4073a mv, honeywell tank stat and a randall 922 programmer. It doesn't have a room stat which I intend to rectify but not until I resolve the current problem. Radiators all have thermostatic valves.

In order so far I have changed the valve head as above as well as the tank stat as I understood the pump could be fed from either. I have also had the chance to try a compatible danfoss programmer and have swapped out the boiler pcb neither of which has had any effect on the pump.

I am guesssing there must be some other stat on the boiler which is firing the pump but I have got as far as I can without asking for help.

If anyone could shed some light on this I would be very grateful
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You need to establish where the pump is getting its power from.

Mr. W
I agree entirely, I was hoping someone might be able to give me a clue where to look.

The wiring for the system hasn't been changed and with the actuator head and tank stat changed I thought that had it covered. I changed the pcb because I happened to have a known good one lying around and I was running out of ideas.

Kind Regards
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You need to establish where the pump is getting its power from.
Is the pump connected into a junction box, together with the valve, thermostats etc?

If so, remove the lid and find the terminal pair to which the pump brown wire is connected. There will be another one or two wires connected to the same terminal - possibly via a link to another terminal. So either two wires or three connected together. If three, one should be the orange wire form the motorized valve.

If three wires, including orange wire
The pump is being fed from either the motorized valve (via orange wire) or from the cylinder thermostat (third wire).

Disconnect the orange wire first to see if the pump stops (the boiler will not light on CH only).

If the pump still runs, turn the HW stat to minimum. Does the pump stop?

If it doesn't the cylinder thermostat is faulty.

If two wires only, excluding orange
The pump is connected directly to the boiler and controlled from there.

Disconnecting the second wire should stop the pump. In fact it won't run at all.

If this is the case, the problem lies in the boiler.
Thanks very much for your clear and concise help - just going to administer a good kick to my own backside for not doing that already.

The pump is connected directly to the boiler but I hadn't checked as I had read so many posts saying it was fed from either the valve or the tank stat.

The junction box is a bit of a cramped mess but I have traced the other wire from that connection all the way back to the boiler where it connects to a terminal labelled Lp - I think I can guess what that stands for!

So if it isn't the pcb it is an overrun stat or the like calling the pump?

I know when to quit I will give my heating man a bell.

Thanks once again

Kind Regards

Just in case anyone else has a similar problem, for completeness it was the boiler pump overrun thermostat.

The part was obsolete but I got hold of one which could be easily adapted.

Thanks very much for the help I was given


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