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

 

[snowhead]

The pump and boiler would normally run with demand from either or both Water and Heating.

A non demanding valve should be closed so no heat goes through that.

Has it always been like this or has it changed recently?

Does the heating come on with only demand from the Water and or does the water come on with only demand from the heating?

 

[malthep]

Hi

Thanks for the reply.

So if the CH is on the valve is powered and opens then the return line powers the pump when the valve is open. The CH is heated via a heat exchanger and the circulation is made by the CH pump.

When the HW is on, the heat exchanger is not used so the CH valve stays shut but the CH pump still gets powered and runs even tho there is no heat from the heat exchanger going that way.

If that makes sense??!

The pump and boiler would normally run with demand from either or both Water and Heating.

A non demanding valve should be closed so no heat goes through that.

Has it always been like this or has it changed recently?

Does the heating come on with only demand from the Water and or does the water come on with only demand from the heating?

 

[malthep]

Just to be clear, the output from the boiler goes into one side of the heat exchanger and the HW tank only. The other side of the heat exchanger has the CH recirculating pump so should only really run when the CH is on not when the HW is on as the closed loop of the boiler system should pump this round the HW tank.

I think the whole issue is that there is only one demand to the boiler for heat either HW or CH so the pump CH recirculating pump runs regardless of the demand.

Just wondering if I can wire it so that the CH pump only runs when the CH demand is there and not just the HW demand….but they seem to connected to one wire for the boiler to know to turn on

 

[Aaron b]

When you say heat exchanger is this an external unit?

Is the boiler heating the cylinder directly? How is the hot water taken off the cylinder? Are they 2 port or 3 port valves?

 

[malthep]

Hi

Yes the heat exchanger is an external unit. One side is linked to the closed loop of the boiler and the other side is attached to the CH pipes, the circulation pump is on this side also.

The closed loop has 2 motorised valves in it, one controls flow to the external heat exchanger and the other controls flow into the HW tank coil, these can both be open or either open depending on the demand from the nest controller, however welhen either is open the electrics link powers the CH pump.

It’s not really an issue just seems silly to run the pump if only the HW is on.

 

[Aaron b]

On a 2 port valve there's a contact that will close when the valve is fully open. It's between grey and orange, a live can be connected to the grey core and the orange core can be used to switch a load like your pump. If you only want your CH pump to run when the valve is open you could use this switch. Hopefully I have understood you issue correctly.

 

[123]

I'm struggling to follow this one...is there a pump for the HW as well? Or is the pump solely for CH and HW is on gravity? A diode won't work as it's an AC supply. A relay might but changing what sounds like a pumped system to a gravity one sounds like trouble

 

[brianmoooore]

We really need more details on both the plumbing and electrical layout.
As Aaron b has written above, most, but not all, 2 port valves have a blue and a brown wire that connect to the actuator motor, and a grey and an orange wire that connect to a NO microswitch, that closes when the valve is fully open, and is intended to be used to switch the associated pump and boiler.

 

[Aaron b]

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.

 

[Lucien Nunes]

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,

 

[malthep]

Thanks for the replies.

It would be easier to show a diagram really!

The output of the boiler is a closed system.

The motorised valves have the orange return line that powers the CH pump regardless of the CH or HW demand and also sends the demand to the boiler to turn on.

What I’d like is for the CH pump to only run when the CH valve is open. Problem still is there is only 1 wire to the boiler to demand heat regardless of the type of heating required (I.e CH or HW)

 

[brianmoooore]

If I've have this right, I think what you really need is a 2 port valve with two microswitches. AFAIK, these don't exist, so you will need the switch connected to both the pump and a small relay, which is used to turn on the boiler.

 

[Lucien Nunes]

What is wrong with my suggestion in post 11?

 

[Aaron b]

What is wrong with my suggestion in post 11?

I was suggesting the same, you put it more elegantly though .