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Andrew Smith
Andrew Smith, Engineer
Category: UK Plumbing
Satisfied Customers: 3633
Experience:  Domestic Gas, Commercial Gas, LPG, Unvented Hot Water, Part P Electrical, Commercial Catering
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Rick, my central heating system would only turn on when the

Customer Question

Hi Rick, my central heating system would only turn on when the hot water system was on. I have read around the potential issues but need some help fixing the problem. I have an S plan system with 2 2-way zone valves controlled by a programmer.
When the central heating was switched on the valve did not move so I assumed it was broken. However, subsequent testing showed that if, using the programmer, I switch the hot water on (HW valve switches on) the heating on (nothing happens), then turn the hot water off whilst the HW valve goes off, the CH heating valve goes on.
So actually the valve is functional after all. I assume there must be a wiring issuer around the programmer?
Submitted: 1 year ago.
Category: UK Plumbing
Expert:  Brian replied 1 year ago.

This is rather strange if wires break or become loose in the terminal box they usually fuse the unit or just dont work.

I would think its more likely to be a programmer fault.

Yes a drain down will be required to change the zone valve or a partial one at least. Unless they can freeze the pipework but its not usual to do this

Customer: replied 1 year ago.
Hi Brian, thank you for your reply.I have just moved into the house so don't think the wires have broken or come loose but more likely have just always been wired wrong. Can you recommend the best way for me to proceed in order to narrow down why the programmer is not controlling the zone valves correctly. I think the thermostats and zone valves are working. It's just that they are not responding quite how they should. i.e. As I mentioned, when they are both switched on at the programmer, only the HW zone valve will be on. And then when I turn the hot water off at the programmer the HW zone valve turns off and the CH goes from off to on. I have not seen any description of an S plan that works like this. The house is only 12 years old so I would expect a fairly standard set up. Any ideas? Thanks again, Richard.
Expert:  Brian replied 1 year ago.

Aha that makes more sense that its been wired wrong.

I think you are going to have to get an S plan wiring plan and work through the diagram. this is certainly what I would do.

But before you do this can you tell me what programmer you have and how is that wired.

Customer: replied 1 year ago.
A Drayton Lifestyle LP722 into a Potterton Suprima.
Expert:  Brian replied 1 year ago.

That looks a little confusing with the taped up connection block.

I wonder if its something as simple as the programmer is set to gravity instead of pumped.

If you take the programmer of the wall you will see a link plug it needs to be set at P i will give you a link to the wiring as well you can check that while the thing is off the wall.

Customer: replied 1 year ago.
Hi Brian, the programmer is set to pump. I have attached a photo of the programmer wiring as I wasn't sure from the Drayton instructions which one it should be.On a separate note, I had assumed the CH valve was stuck open as when it was on (even though zone valve was still off) and the HW was on, the CH was on. Also, when I took the actuator off the CH valve and adjusted it manually the CH stayed on when it was in both positions.Today, it occurred to me that as a final check I could turn the HW on, on its own. The CH did not come on. I assumed if the boiler was on and the CH valve was stuck open the CH would come on. Does this mean that the valve might not be stuck open or that it probably is stuck open but the CH pump is separate from HW pump, which is why it didn't come on?If the valve is stuck open, can that be dangerous? Or just problematic.Thanks again for your help.
Expert:  Brian replied 1 year ago.

if you have an S plan system there is no reason to have two pumps. if the Central heating is stuck open there is no danger just uncomfortable for you.

When you moved the CH zone valve did you hold it closed or just move the spindle into position. It needs to be held closed to stop the water flow.

I am going to opt out for the moment so that other engineers can read where we are, perhaps they may be able to help you further.

Customer: replied 1 year ago.
I just moved the spindle. Should I hold it in position by hand? (I don't think it moved even though I wasn't holding it).
Expert:  Andrew Smith replied 1 year ago.

It has to be wired like this;

Expert:  Andrew Smith replied 1 year ago.

Check the programmer wiring first to where it goes to in the juction box

Customer: replied 1 year ago.
Cheers ***** ***** have a chance to check it all on Saturday. I'll let you know how I get on.As well as having doubts about whether the wiring is correct I cannot figure out what is going on with the CH zone valve. When I have the CH and HW switched on, the CH zone valve does not open but the CH still comes on. So I assumed the valve was stuck open. But I tried just switching the HW on and the CH did not come on. Does that mean the valve might not be stuck open?
Expert:  Andrew Smith replied 1 year ago.

It means you cannot have heating without hot water, check the wiring in the junction box and then come back to me

Customer: replied 1 year ago.
Hi Andrew, apologies for the delay in checking the wiring. I wanted to check in daylight, as would have the electricity off and have been away at the weekends.There are 3 cables coming into the boiler, one from the programmer and two others. I am not sure exactly how that tallies withe S plan wiring diagram that you gave me so I thought I would do my best to explain how things are connected.The first cable is from the programmer. N, L and Earth all look fine. The number 3 (black wire with brown cover) from the programmer is connected to a black wire with red cover from one of the other white cables. And number 4 (grey wire with brown cover) is connected to a blue wire (with red cover) from the same, other white cable.The other cable also has Earth, a black wire (that is knotted and not connected) plus a brown wire that goes to SWL (on side labelled 'mains' on boiler inputs).The third cable (yellow tape around the end) goes to N and L (on 'mains' side of boiler inputs, the same as per programmer wires). A black wire with red cover goes to L on 'pump' side of boiler inputs and another black wire goes to N on 'pump' side of boiler inputs.Hopefully that makes sense. Please let me know if you need any more info or if you know which cable is which so I can compare the connections to your diagram.Thanks very much, Richard
Customer: replied 1 year ago.
A photo of the connections to boiler...
Customer: replied 1 year ago.
I guess the question I have is how to rationalise the fact that there are three cables coming into the boiler but the diagram shows wires coming from the programmer, both zone valves and both thermostats. So do two of the cables coming into the boiler contain wires coming from the zone valve and thermostat too? Best Regards,Richard.
Expert:  Andrew Smith replied 1 year ago.

The ones on the right are power and switched live for the boiler.

The ones on the left supply the pump.

The zone valves will both turn the boiler on via the switched live when calling for heat or hot water.

The thermostats control power to the zone valves.

The programmer controls power to the thermostats (water and room)

Customer: replied 1 year ago.
Thanks Andrew, I think I understand how the system works in theory. However, I cannot identify the wires coming into the boiler. As I say, there are two white cables coming to the boiler, as well as the cable coming from the programmer.The programmer's wires (3 and 4) are attached to wires from a plain white cable (the other cable has a piece of yellow tape around it). So would this cable go to both thermostats and the other cable (with the yellow tape round it) go to both zone valves?Best Regards, Richard.
Customer: replied 1 year ago.
Hi Andrew, I was wondering if you knew how I can identify which wires are coming from the thermostat and which from the zone valves as there are just two white cables coming into the boiler (one plain and one with yellow tape wrapped around it - detailed wire colours above if useful). Best Regards, Richard.
Expert:  Andrew Smith replied 1 year ago.

The power to the boiler comes from the motorised valves (linked through the grey and orange) and will go to the SL (switched live) connection. The other cable will be a permanent live.