Weird behaviour of CH pump

[MarkDavis]

I have a conventional CH system: gas boiler, Grundfos pump and 2 Honeywell motorized valves controlling separate circuits for radiators (with thermostat-timer control) and HW (timer control).

Here's my problem. The motorized valves close, the pump stops and the boiler shuts down as normal when the required room temperature is reached. But 3 minutes later the pump starts up again with both motorized valves still closed and the boiler off (there's no demand for heat). There's quite a bit of vibration and I find I can get the pump to stop, after a minute or so, if I manually open one of the motorized valves. It eventually stops anyway. It can't be right to have the pump on when both valves are closed, so I suppose this must be a wiring problem (the pump was recently replaced). Any other explanation?

 

[petertheplumber12]

Because you have two motorised valves you should have a bypass loop as well, this takes water back to the boiler if both valves are closed and the pump is on. I suspect you don't have one or the gate valve on it is closed. It need to have an automatic bypass valve fitted on it that should stop the problem of vibration. The pump switching on intermittently needs checking too.

 

[Hugh Jaleak]

What boiler have you got? Some boilers control the pump to provide an overrun when the boiler stops firing to dissipate the heat from the boiler, however to achieve this then a bypass should be fitted. (Bypass will be taken from after the pump, but before the motorised valves, straight into the return. Either be fitted with a gate valve or an auto bypass valve.)

 

[MarkDavis]

Thanks to both for helpful replies. It's a Glow-Worm Space-Saver 75, installed around 1984 (yes, I know we should have replaced it, but it keeps working and a 1-for-1 replacement by condensing boiler isn't that easy because it's mounted on an inside wall).

It does seem likely that the boiler is trying to cool itself down by keeping the pump on, but there is no by-pass circuit: the downstream pipe from the pump leads to a T-junction where each side goes directly to a motorized valve, for the CH and HW respectively. There is no other downstream route. So any cooling arrangement would have to open one of the MVs .. so maybe it IS a wiring problem??

 

[Agile]

You should really have an auto bypass valve!

Tony

 

[petertheplumber12]

Thanks to both for helpful replies. It's a Glow-Worm Space-Saver 75, installed around 1984 (yes, I know we should have replaced it, but it keeps working and a 1-for-1 replacement by condensing boiler isn't that easy because it's mounted on an inside wall).

It does seem likely that the boiler is trying to cool itself down by keeping the pump on, but there is no by-pass circuit: the downstream pipe from the pump leads to a T-junction where each side goes directly to a motorized valve, for the CH and HW respectively. There is no other downstream route. So any cooling arrangement would have to open one of the MVs .. so maybe it IS a wiring problem??

You must have a bypass loop with a auto valve.