Nest thermostat cannot initiate heating, no error codes

[df89]

Have lived in a property with the same nest thermostat + heatlink (3rd gen)for 3 years (UK). Nest is for heating and hot water. The heat link was never wired up correctly to heat the water, until last week when we got it fixed during a boiler service. Everything has worked as it should, plus had the water heating after this change.

2 days after the service, the heat stopped working via the thermostat/schedule.  (Also had the power off that same day for a few hours for unrelated work, in case that affected it). Manual mode works (ie heat link button), but scheduled heating or turning up the thermostat doesn't fire up boiler. Scheduled water heating does still work (boiler fired up for that just now).

When turning up thermostat for heat, there is a click, but no other sound like usual and no boiler firing up. No error codes or messages that something is wrong. Status light on heat link is green.

In technical info, Vin is 11V, lower than the recommended range (though I don't know what it was before). Other values ok: Battery 3.9, lin 200mA.  The thermostat base has T1 & T2 terminals to connect to heat link.

[Frustratedddd · Accepted solution]

The common question with an issue like this one. Have you any motorised valves in the system, and if so are they working?

Sounds like the display is talking to the Heat Link, and when working in manual mode it fires up, the valve could be an issue. You have a solid green light on the Heat Link, so that's correct. Not an oil boiler by any chance?

[df89]

Thanks! It is an oil boiler yes.

Only motorised valve I can see inside is for the hot water (at least it says for hot water only on the side of it- grey box in the back)

Thermostat is directly below the heatlink on the wall downstairs

  

(I found a thread elsewhere of the same issue, at least same tech info, and their problem was a faulty heat link motherboard 🤷‍)

[Frustratedddd]

I don’t think the Heat Link itself is the issue, but may well be wrong.

That can be easily checked if you have a little screwdriver skills and a neon screwdriver or tester? Neon is easiest.

Oil boilers are wired a little different. Who wired the Heat Link?

Can you upload a picture of the Heat Link terminations?

[df89]

It's dropping quality but left to right they are N, L, 1,2,3,4,5,6, OT1, OT2, [arrow?], T1,T2

 

Local heating company rewired it to get it heating the water tank. As it was when I moved in, everything worked except heating water (I think it was reverse issue in that it would click but do nothing when trying to boost water) 

[Frustratedddd]

Yes I know the terminal numbers it’s the connections we need to look at. 
Terminals 1-3 are your heating, and terminals 4-6 are your water. 
You also have a wired connection to the display.

Assuming everything has been working before the water connections, I would assume they could be the problem. I would think the valve could be at fault, but you really need to get the person back to confirm.

[df89]

If anyone reads this with a similar issue- it was indeed the motorised valve. It has 3 positions (radiators, water, both) and it couldn't get into 1 or 2 of them because it was old/hasn't been wired right for some years. As i have learned, the valve moves into position and that will trigger the boiler to fire up. Makes sense once you know!

Thanks for the responses!

[Frustratedddd]

Good to hear you back up and running, and as expected the valve is the issue.

Sadly so many users think the stat is the problem and spend hours going round in circles. A simple test with a neon screwdriver can confirm the Heat Link is switching, but people just blame the thermostat usually. You are sorted, and that's the best result.