Blocking diode for radiator pump?

[malthep]

Hi

I have a central heating system installed but the controller cannot differentiate between hot water demand and central heating demand. Currently when hot water demand is on and CH is off the CH (radiator) pump has to be run.

The issue is that the boiler only has one input for heat demand so is linked to both HW and CH. The HW and CH motorised valves both are linked to the boiler demand wire, but each only return a signal when the respective valve is open, therefore the the CH pump is energised regardless.

Could I seperate the CH and HW return wires and only power the CH pump when the CH return demand is there? I assume perhaps placing a blocking diode between the HW link to the CH Pump would do this and have the CH demand power up as normal?

Many thanks for advice

 

[malthep]

View attachment 94625
I was assuming something like the sketch. I was thinking the heating signal would open the valve and turn on the room heating pump. The hot water signal through the stat would open the hot water valve. The contacts in either valves would turn on the boiler and pump.

Hi Aaron

Yes I think this sketch is pretty much correct. There is no pump on the boiler side but I assume there is one inside the boiler.

If you imagine that the two valves have the micro switch return power line connected together and then linked to the CH recirc pump and the boiler that is the the setup.

I need to isolate the micro switch returns and only power the CH pump when there is a demand for CH and not just HW.

Lucien has a great idea and if I’m right in interpretation I should link the CH pump with the activation signal to the motorised valve and not to the return wire?

 

[brianmoooore]

What is wrong with my suggestion in post 11?

Nothing whatsoever, other than I didn't read it before posting.

 

[Lucien Nunes]

That wasn't aimed at you, rather at the OP who I don't think had seen it either. It is the simple solution. As you say it is also possible to fire the boiler via a relay controlled by the valve microswitch, with its coil in parallel with the pump motor. But in this case it is unnecessary because the heating pump is in the secondary circulation and not throttled by the valve which is in the primary. Therefore it is not a problem that the pump will start at the moment the valve begins to open instead of when it is fully open.

if I’m right in interpretation I should link the CH pump with the activation signal to the motorised valve and not to the return wire?

Exactly right.

 

[malthep]

Connect the heating pump in parallel with the heating valve motor blue and brown, and leave the heating valve orange output just energising the boiler and circulating pump. That way the heating pump and valve will operate at the same time but the water valve will not backfeed the heating pump. If whatever thermostat is controlling the heating valve cannot handle the load of the pump then a relay will be needed, but with modern low wattage pumps it should be fine to connect directly,

Hi Lucien

Thanks for the reply, apologies in the delay in responding.

Am I

That wasn't aimed at you, rather at the OP who I don't think had seen it either. It is the simple solution. As you say it is also possible to fire the boiler via a relay controlled by the valve microswitch, with its coil in parallel with the pump motor. But in this case it is unnecessary because the heating pump is in the secondary circulation and not throttled by the valve which is in the primary. Therefore it is not a problem that the pump will start at the moment the valve begins to open instead of when it is fully open.



Exactly right.

perfect, I have done the wiring change…seems to be working correctly!

Many thanks

I’ll update this post if anything changes but my most sincere thanks to all, esp Lucien and Aaron