What I have is a back boiler that runs my central heating via a pump. The domestic hot water is gravity fed.

At present I have to turn the domestic hot water on via a timer switch this then gives me hot water for the central heating pump to circulate when the room thermostat dictates.

I now have a Nest thermostat, this will sometimes switch the pump on earlier depending on the temperature outside so I connected both HW and CH to come on together via the Nest thermostat but because of it's properties the house may get warm before the hot water tank. If the HW is switched on manually then so is the CH pump.

Can I change the link wire with a wire with a diode and if so can someone spec it? The reason I want to do this is to keep it compact, the HW and CH will come on via the Nest thermostat but when HW switched only the HW will come on as the Diode should stop any power going back to the Nest and pump.

Horstmann 425 Diadem diagram

Room stat has been replaced with the Nest thermostat, N & L to respective position on Nest stat, 4 & 5 to 2 & 3 on the Nest.

for the heating to work the Nest starts the CH pump but only a timer stars the boiler(been like that since installation years ago) I have linked 1 & 4 together this means the Nest switches the pump on and the link starts the boiler = good enough! What this also means is that if the boiler terminal 1 is switched on then by virtue of the link the CH pump terminal 4 is switched on.

This is why I asked the question, to put a diode in the link between 1 & 4 so that switching 1 doesn't switch 4 on. I'm only suggesting this to keep within the existing footprint of these controls.

1 is switched live to boiler,2 is live (feeds 1 when switched on HW) 3 left vacant for this application, 4 is switched live for CH pump, 5 is live(feeds 4 when switched by room stat(Nest))

  • Can you post a wiring diagram for your boiler system please? Jan 9, 2016 at 16:08
  • ThreePhaseEel, I have posted a diagram as requested.
    – w1ggan
    Jan 9, 2016 at 22:07
  • Can you describe what the different numbered terminals on your controller do? Jan 9, 2016 at 22:19
  • Sorry but I thought I had, here goes. 1 is switched live to boiler,2 is live (feeds 1 when switched on HW) 3 left vacant for this application, 4 is switched live for CH pump, 5 is live(feeds 4 when switched by room stat(Nest))
    – w1ggan
    Jan 9, 2016 at 22:37
  • 1
    No, you can't use a diode to control AC circuits. You can use a relay instead, but I have a couple questions before I can explain how. What's the thing on the right hand side of the diagram, that looks like a relay? If it is a relay, it actually is almost wired correctly do what you're asking, but not quite. Am I correct that the desired outcome is that: timer (which switches on 1) controls HW but not CH; and that NEST (which switches on 4) controls CH which in turn switches on HW? (Which means when the timer is on, HW is always on; when the timer is off, HW is only on when CH is on.)
    – gregmac
    Jan 10, 2016 at 7:44

2 Answers 2


Not sure if my system is exactly the same, but I have to run the pump and boiler to get HW and the pump, boiler & a valve to get heating. The valve is a simple looking bypass thing with just live, neutral and earth and is sprung return. Now according to NEST such a system isn't compatible. However, I've wired with the live going to 2 and 6 (that's right, not 5) on the heatlink. Along with the valve on 3 and a jumper going from 3 to 4. The boiler and the pump are then on 5. This gives me full heating and independent HW control from the NEST without any relays etc. So far it's working perfectly. I looked for ages to find an answer to this and couldn't find a satisfactory one for my application. I hope this helps someone else in the same frustrating position!


Since this is a mains (240VAC) system, a diode isn't going to do what you want. However, you can use a relay to turn the boiler on from the Nest without the boiler timer turning the pump on. Connect the relay coil from CH (Nest output) to N, the relay NO contact to HW, and the relay Common terminal to L. That way, the Nest's output turns on CH, which turns on the relay to energize HW independent of the timer; when the Nest isn't energizing CH, the timer is free to connect HW to L on its own to run the hot water.

  • ThreePhaseEel; it's a 240vac as used in England with live Neutral & Earth, I'm going to use a 240v relay c/w 240v coil.
    – w1ggan
    Jan 16, 2016 at 23:19
  • @w1ggan -- good point and I'll edit my answer, but how are you using the Nest with this?! Jan 16, 2016 at 23:29
  • I'm sorry but the thermostat may be throwing you a curved ball, I've tried to upload an image but I'm struggling. I'll paste this link to the layout it's page 3 Diadem/Tiara top image horstmann.co.uk/files/8613/9867/2099/… I was going to get a relay with 240vac coil & contacts. Connect Common to Live, NO to 1(CH), B on the coil to Neutral and 4(CH) to A on the coil. The Nest takes Live from terminal 5. When Nest calls for heat it switches a live to terminal 4(CH) so using the relay I'm hoping to connect H
    – w1ggan
    Jan 16, 2016 at 23:46
  • (continued) W when CH is on and when CH is off I can use HW via the timer. I'm looking for alternatives to my thoughts.
    – w1ggan
    Jan 16, 2016 at 23:46

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