Nest thermostat cannot turn on the boiler but the heat link can

[Samlolkss]

My nest thermostat seems to have stopped firing my boiler for heating, either in schedule or manual mode.

Having tried a reset on the nest, checking the heat link, running that manually, and turning up the thermostat to a high temperature, nothing seems to make the boiler fire for heating. It does fire for hot water, both in schedule and manual modes.

When increasing the nest heating temperature, it displays as heating, but doesn't fire the boiler at all.

But when I press heat link button, the boiler can heat for radiator.  Seems it is ok for hot water but not ok for heating.

I have already found the plumber to check the boiler and he said it works

[CoolingWizard]

Technically you need to make some voltage checks. The thermostat communicates with the Heat Link, the Heat Link controls the Boiler. You need to evaluate if the thermostat is communicating with the Heat Link or not.  Is the communications problem the thermostat or the Heat Link.  To test this the Nest Thermostat has T1 and T2 connection to the Heat Link, When the Nest Thermostat desire heat, it internally connects T1 to T2. That connection informs the Heat Link that heating is desired and the Heat Link initiates the Turning on of the Boiler. To test the Nest Thermostat, go to the Heat Heat Link, turn the power off.  open it and use a Volt Ohm Meter and disconnect T2, and measure continuity between T1 and T2. When the nest thermostat calls for heat, there will be continuity.  So have the thermostat call for heat, then go down and make your continuity test. If you have continuity, the thermostat is working. If the Thermostat is working, then you need to test the heat link. Reattach the T2 wire. Turn the power back on. If the heat link does not turn on the heat automatically with the nest thermostat calling for heat. We know the thermostat is working so the problem is going to be with the heat link itself.   On the other hand, if you did not get continuity with the thermostat, then the problem is the thermostat.

AC Cooling Wizard

[Samlolkss]

It seems too complicated for me to do such test or fix.  I should find plumber or electrician on it? Ths.  To be honest, it is really very strange as two weeks before, when turn off heating in nest thermostat and then turn on again, it will have heat.  Now, it didn’t at all.  Can Google or Nest can assign staff to have a check on it?  Ths

[CoolingWizard]

Over there in England, I’d have no idea how they do things personally. I just know technology and how everything works so I to reason I can advise you what to do. But if you think about it, the nest thermostat and heat link a pretty simplistic device. What happens is when the two cameras T one is connected to T2, that causes a heat link to turn on power to the the boiler.  So in theory, if you go to the heat link, take the cover off, and then connect the wire carefully between T1 and T2, and wait a moment the boiler should come on. If it does, the heat link is working.

Now opening all that up and touching wires is a little bit overwhelming for you, any boiler mechanic can follow the instructions I gave you and make the test for you.

AC Cooling Wizard

[Samlolkss]

But it works for hot water boil and Nest thermostat show heat link connected?

[UanaC]

Hello there, @Samlolkss ! Thanks for posting. I'm so sorry to hear that you're facing issues with your Nest thermostat, but no worries I'll be more than happy to help out. Can you please let me know if you have the Auto-Schedule feature on? You can find that in Settings -> Nest Sense-> Auto-Schedule. @CoolingWizard I want to thank you for your reply and help. Always appreciated!

Thanks in advance, Oana.

[Samlolkss]

I have already turn off auto schedule.  I noted that I can turn on heating by heat link manually.  But when I set the higher temp by thermostat, I heard the sound from heat link but boiler did not turn on for heating.  But it is ok for hot water boiler.  Should any issue on the communication between thermostat and heat link?  Ths

[Frustratedddd]

You might have a relay stuck!

Are you saying, when you press the Heat Link button the hot water works, but no heating at all?

But when I press heat link button, the boiler can heat for radiator. Seems it is ok for hot water but not ok for heating.

........or are you saying when you press the Heat Link button you get hot water AND heating?

How are you connected to the heat link and display, wireless or wired, or both?

[Samlolkss]

It is wired.  I can use heat link to turn on heating and hot water.  But just can use thermostat to turn on water boil but not for heating. 

[Frustratedddd]

OK, if wired check to see if you are also wireless. Settings, equipment, heat link.

check there you should have wired and wireless, if just wired make it wireless also.

[TudorL]

Thanks for all the information you've provided. Can you check if your heating system has a Y plan or an S plan? 

[Samlolkss]

I don’t know what is y plan or s plan

[TudorL]

The S and Y plans are the ways in which your heating system delivers the water from your boiler throughout your house. We'll need to perform a couple of tests. I'd like you to open the Nest app and go to the Heat Link and turn on the heating via the app and listen for a click. Wait for a minute or two then deactivate the heating via the app as well and check for the second click. Once that's done, please let me know if you've heard two clicks.  

[Samlolkss]

Yes, it has two clicks sound

[AlexD]

Thank you for the information. If you hear a click when activating and deactivating the heating that would indicate that the relay is working as expected. The app is sending the command over the internet to the thermostat and the thermostat is sending it further to the Heat Link which moves the Heat Link relay with an audible click. In that case I would recommend you get and installer to check the system and the valves at the location. It could be that a valve is getting stuck or something else is going on with the system.

Thank you for all the input and suggestions @Frustratedddd .

[Samlolkss]

Valves are radiator valves? Ths

[AlexD]

You can have a look here at our installation examples. Look at the S and Y plan examples. In short, if you have a system boiler with a hot water tank you will have one of these plans. The hot water and heating will be controlled by one or multiple valves at the location to which the Heat Link is wired. It is possible that one of these valves may be getting stuck or there could also be something else going on with the system. If you do get in touch with an installer or a Nest Pro you can reach out over the live chat or phone line while they are there and my colleagues can troubleshoot with them live.

You can also check the valves on the radiators themselves, if they have any. If the pipes to the radiators are warm but the radiators are not, they may be closed. 

[Samlolkss]

Thanks a lot and let me study first.  It means that I should find an installer to help on it?  It is really strange that heating can be turned on by heat link button manually

[AlexD]

The Heat Link launches both the heating and the hot water when manual mode is used. This would usually indicate a Y valve getting stuck as that valve handles heating and hot water. When the request is done from the Heat Link simultaneously for both sides, it may move the valve to a functioning position but it does not engage the heating side when used separately. Of course, there could be something else going on but that would be the most common cause for such a behavior. 

It would be best to get in touch with an installer or a Nest Pro as they cannot only check the thermostat but also any other part of the system like the valves, wiring, boiler, etc.. If you have a multimeter and are comfortable in performing some tests on the Heat Link I can send you the steps over email. But it would be best to have an installer handle the tests. Though do make sure, if possible, to call or get in touch over live chat with my colleagues so they can do the troubleshooting steps on the thermostat together to confirm if something is going on with it or not. You can also ask for a diagnostic report from them, a PDF document that contains all the details of what tests they have done and what they found is going on. 

[Frustratedddd]

Yes I agree also, but if the plumber has checked that?

I have already found the plumber to check the boiler and he said it works

[AlexD]

The boiler is certainly operational, since engaging the manual mode engages the hot water and heating. However there are many other parts that could be causing this behavior, most likely the S or Y plan valve in particular that would need to be checked. Also, it would be best to go through all the steps for a continuity test with the installer live with one of my colleagues. 

[Samlolkss]

yes, plumber said boiler is fine

[Samlolkss]

Thanks a lot and understand more.  Can I change the valves by myself?  Ths

[AlexD]

If you are comfortable in doing so you can certainly try though I would not recommend it as I don't know the complexity of the system at the location I can't say for sure. However I would recommend getting in touch with an installer and going through the whole system and the thermostat with my colleagues. 

[Samlolkss]

Thanks and understood.  After your detail explanation, I think the most probability is related to valves.  How to replace the valves with new one such as unblock valve?  Ths

[AlexD]

I can only assist with questions and troubleshooting related to our products. I am not specialized on other devices from other companies or familiar with the details of the system installed at the location. I would recommend you get in touch with your preferred installer or Nest Pro for advice and assistance on this. 

[TudorL]

Hi there, just checking in to see if there's anything else I can assist you with. 

[UanaC]

Hey there, I haven't heard from you in a while so I'll be locking this thread. If you have any new issues, updates or just a discussion topic, feel free to start a new thread in the community. Thanks, Oana

[Samlolkss]

When pressing heat link button one time, it works for water boil and also heating.

but when turn on heating by thermostat, no response and no heating.  But turn on water boil by thermostat, water will boil.  Just heating by thermostat has issues

[Frustratedddd]

……and the important question, wired wireless or both?

[Samlolkss]

Wired.  But in the thermostat, it showed wired and wireless

[Frustratedddd]

OK, by showing wired/wireless you now know it’s communicating from the heat link to the display and the T1-T2 connections you don’t have to bother about. You also know if the battery is showing a voltage above 3.7v (ideally 3.9v) the heat link T1-T2 connections are OK as it’s getting power from the heat link. You obviously check the voltage reading in the settings to prove. 

2 clicks sounds like both relays are operating, but it’s really hard to hear both relays open at the same time. The easy simple proof is with a neon screwdriver if you have one?

Prove your heat link is communicating with the display, go settings, hot water and Boost, you should hear a click in the heat link. Should that work you know it’s OK, and your issue is with the heat communication only! From there I probably would do a full factory reset.

[Samlolkss]

Yes, it works for water boil.  Just heating has issue.  I have already reset to factory default but still not work for heating.  When I set the heating at higher target temp.  The screen will show +2rs, but keep it even after two hours and the radiator not hot 😭