Replacing old Honeywell programmer and thermostat with nest

dipdm · 24 Jan 2026

Hi All

I tried replacing my old Honeywell programmer and thermostat with the nest 3rd gen heatlink and digital display thermostat. Unfortunately things didn't go to plan as the display did not switch on! It's a bit of a complicated system as the wiring from the programmer and the thermostat all meet at a junction box with wires also going directly towards the hot water cylinder. Can anyone suggest the wiring pattern? Thanks in advance!

dipdm · Sunday at 9:05 AM

Also to note I have motorised valves and it's vented

ericmark · Sunday at 12:24 PM

Nest Gen 3 can replace this

it will need bridge wires L to both Coms

and leave the old thermostat at full. The new thermostat should be powered from T1 and T2 but it seems you don't have those wires, so either fit them, or use a USB cradle, I don't like the USB cradle as if the boiler is on an UPS supply, then it will still not work with a power cut as no power to thermostat. But don't like Nest to start with, I have it fitted, and had to fit Wiser as well, as Nest fails to connect to TRV heads in a meaningful way.

ericmark · Monday at 3:10 PM

Sorry missed this

can't read what it is, and seems the wrong shape for a simple junction box. I wonder if 24 volt motorised valves? Most at 230 volts, but there were some 24 volt models which came with a plug in system. I know they existed but never worked with them.

With the old system both the programmer and the thermostat had to be where you can easily access them, but with the Nest Gen 3 you don't need easy access to the heat link, the thermostat acts as the HMI (human machine interface) so fitting the heat link where the water tank is maybe best option.

When you answer quote, or make a reference to me, like @dipdm I am doing here. So it alerts me that you have answered. If @stem is around, he may recognise that box?

denso13 · Monday at 5:01 PM

ericmark said:
Sorry missed this can't read what it is, and seems the wrong shape for a simple junction box. I wonder if 24 volt motorised valves? Most at 230 volts, but there were some 24 volt models which came with a plug in system. I know they existed but never worked with them.

It's a shower pump.

CountryFan · Monday at 5:10 PM

Where is the programmer located?

dipdm · Monday at 5:31 PM

@ericmark it's a shower pump @CountryFan the programmer is located next to the hot water cylinder and the thermostat is in the hallway downstairs

CountryFan · Monday at 8:47 PM

Ok, so you need the links in as per @ericmark. For the thermostat display to work, either usb fed or locate wires for the thermostat in the wiring centre and these can be taken heat link

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Replacing old Honeywell programmer and thermostat with nest

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