Nest thermostat not firing up Baxi 105 boiler

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Hi guys,

I've wired up the Nest thermostat (3rd generation) to my combi boiler, which is a Baxi 105. The heatlink is showing up a green light, so all perfectly fine there.

However, when changing the temperature on the thermostat, it says it's warming up, but the boiler is not firing up at all, even changing to a higher temperature, still not kicking in...

On the boiler, there is a tuner to change the hot water and radiator temperature, but if I turn up the radiator to full, then it will start heating up, further than what is instructed by the Nest thermostat, so it looks like the thermostat is not quite taking over the heating control of the boiler?

Is there something I need to do? There are no errors showing up on the thermostat or in the app, so not sure what to do at this point.

I've attached a photo of the boiler that is firing up when the tuner is on full.

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CBW

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It sounds like it’s working to me. Leave the boiler radiator thermostat on full, turn a nest down if the heating goes off, then I’d say it’s ok. Where was the boiler thermostat set to?
 
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Hey, thanks for replying!

I've put the boiler thermostat on full and the Nest thermostat to 9 degrees, but still heating up the radiators.

Can you clarify what you mean on the last bit?
 

CBW

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Can you clarify what you mean on the last bit?
The bit you’re calling the tuner is a thermostat for the boiler, but doesn’t matter right now, as it sounds like it’s not working as you describe.
  • What thermostat was in previously?
  • If there was a previous thermostat have you located nest in place of this? Have you disconnected or removed it?
  • Where have you wired to on boiler and Nest? Photos would help
 
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Ah, gotcha. The previous thermostat was downstairs in the hallway, but it was a Honeywell R6660D thermostat which is wireless. I've got the Nest thermostat powered by a power adapter as there's no common wire from the boiler to the hallway.

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Hope that helps!
 
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The tuner as you call it, is the boiler water heating thermostat, the nest doesnt control the temp of the water in the system, this thermostat does, you need to set the boiler on board timer to constantly on and let the nest over ride it
 
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CBW

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If you didn’t remove the previous thermostat and receiver then this might still be controlling the heating. If it’s still connected turn the Honeywell up to maximum and see if the Nest then works as it should.

Side note: The flex cable (white) down to your Nest should be continuous, not 8 inches of the coloured cores beneath :eek:
 
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If you didn’t remove the previous thermostat and receiver then this might still be controlling the heating. If it’s still connected turn the Honeywell up to maximum and see if the Nest then works as it should.

Side note: The flex cable (white) down to your Nest should be continuous, not 8 inches of the coloured cores beneath :eek:

The Honeywell thermostat was broken, so I opted to replace it with a modern one, hence the installation of the Nest one.

Would you say the wiring is incorrect? If so, what should the wiring be?

I think it’s meant mate, that the Nest thermostat is powered via usb, the heatlink looks hard wired from boiler spur.

Yeah, that's correct, we have a USB cable powering it through the wall.
 

CBW

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I get the thermostat has been replaced with the Nest, what happened to the wiring from the Honeywell receiver?

Only flex (white cable) should be on show, not the coloured cores, eg the brown, the blue etc.

Post a photo of wiring of Nest cover removed.
 
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On the Honeywell receiver, it had the wires N/L to N/L then A/B to 2/3 on the Heatlink.

Pictures below

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CBW

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It seems wired correctly. The clock on the front set to constant? Could be a duff receiver. Are you able to test the wiring for voltages?
 
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Yep. The timer has all slots on, but in the middle, there's a small flip, if it's on 0, then heating light comes off on the boiler, but if it's in the middle or on 1, then it comes on, but I don't think it ticks at all. Don't think the timer is doing anything relating to the thermostat.

Which wires specifically should be tested?
 

CBW

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The clock should be on 1 - it’s related to the boiler as in on/off or timed, because it’s still connected it remains related to the thermostat - so if it was set to 1 (constant) then the nest would override any other timed periods.

Wires 2 and 3 to make sure 230 volts is in and out.
 

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