Nest Generation 3 Thermostat only running central heating when the hot water is also active.

[DSM10000]

As per the title.

Friends had a Nest 3 smart thermostat installed yesterday and all seemed well when the Electrician showed them how the system worked. However,later that evening, the central heating did not turn on at 16:30 as scheduled, it did however turn on at 17:30 when the hot water was scheduled.

The App showed that the heating was on at 16:30 but the boiler did not run until an hour later when the schedule called for water heating. The central heating then worked until the hot water schedule finished at 20:00. The only way to keep the central heating on was to use the hot water boost function on the App or by pressing the button on the heat link controller box. It is an Ariston 30Kw boiler (not a Combi type)

They have left a message with the Electrician but haven't heard back from him as yet.

Are there any obvious wiring checks I can make? They are not skilled in DIY

TIA

 

[st13phil]

Logically it appears that the boiler isn’t “seeing” demand from the heating circuit on its own. Sounds like your friends are going to reliant upon the sparky coming back to rectify whatever wiring fault he’s introduced.

 

[Bobby Dazzler]

Check that the original control unit is set to ALWAYS ON for both water and heating. My guess is that it’s not for Central Heating or both.

 

[DSM10000]

Check that the original control unit is set to ALWAYS ON for both water and heating. My guess is that it’s not for Central Heating or both.

The original control unit has been replaced by the Nest control box.

They have been through the App , all is set as it should be, and heating is enabled on the thermostat. It seems strange that using the Boost option will switch both on, I would have imagined that this would still use the individual command circuits?

 

[Bobby Dazzler]

The original control unit has been replaced by the Nest control box.

They have been through the App , all is set as it should be, and heating is enabled on the thermostat. It seems strange that using the Boost option will switch both on, I would have imagined that this would still use the individual command circuits?

Unless it’s changed on the very latest version of Neat Thermistat, the original thermostat is usually replaced by the Nest Thermostat, but the original “box” which is used to control the timer for the hot water and central heating - including the boost button - is left in place and must be left in the ALWAYS ON position/setting for the central heating.

 

[DSM10000]

Unless it’s changed on the very latest version of Neat Thermistat, the original thermostat is usually replaced by the Nest Thermostat, but the original “box” which is used to control the timer for the hot water and central heating - including the boost button - is left in place and must be left in the ALWAYS ON position/setting for the central heating.

Ah no. On the Generation 3 type it comes with a electronics box called a "Heat link" that replaces the original control unit

 

[Doodle]

ISTR encountering something similar to this when I wired up my Nest unit a few years ago. I ended up comparing the wiring diagrams and discovered that my timer unit had a couple of internal connections that didn't exist in the Nest unit, so I had to reinstate them via extra link wires in the junction box (after no small amount of trying to decipher the rat's nest of wiring)

You can see the extra links on the top side of the connections, but it sounds like this is not something your friend will be wanting to get involved in.

 

[DSM10000]

ISTR encountering something similar to this when I wired up my Nest unit a few years ago. I ended up comparing the wiring diagrams and discovered that my timer unit had a couple of internal connections that didn't exist in the Nest unit, so I had to reinstate them via extra link wires in the junction box (after no small amount of trying to decipher the rat's nest of wiring)

You can see the extra links on the top side of the connections, but it sounds like this is not something your friend will be wanting to get involved in.

Thank you for your reply That is the earlier Generation 2 Nest?

The Generation 3 replaced the existing controller with the Heat Link and all of the wiring goes into that unit. I haven't seen inside one but looking on line at the wiring diagrams there are separate cables to command CH and HW. My guess is that the CH cable isn't connected properly.

 

[Doodle]

No, that wiring is nothing to do with the Nest. This house has an old Y-plan system boiler in it, so that is legacy infrastructure that links the boiler, the motorised diverter valve, the central heating pump and the hot water tank thermostat. It also connects to the controller, which previously was a Drayton timer with a Honeywell room stat linked to it.

A 3rd gen Nest has replaced the Drayton/Honeywell pairing, connected to the same wiring but it can only provide triggering to say "I want central heating/hot water". That box of wiring has to remain to provide the necessary logic in terms of boiler on/off, CH pump on/off, diverter position and feedback from the tank thermostat to either provide central heating, hot water or both simultaneously.

The Drayton timer had internal circuitry to link supply for the CH and HW circuits internally, so this control wiring is based on that premise. That internal circuitry does not exist on the HeatLink, so only one triggering circuit is powered which means you lose independent control for the other.

Hope that all makes sense? I'm not aware of what setup your friend has and what the Nest has replaced but if it's a similar unit then this could well be the problem. It wouldn't surprise me if a general sparky missed this as it's quite a heating-specific situation; it was only when I consulted the Drayton wiring diagrams because I assumed I'd make a mistake in matching the wires that I realised what was going on.

 

[W1ghty]

I wonder if there is a central heating 3 way zone valve that has been wrongly wired or failed open . He might be able to manually lock the lever on the side of it and then see if the central heating works by itself ?
This thing

 

[Delderek]

3 way zone valve stuck, would be my guess as well.

 

[ToeKnee]

My first thought was 3 way valve, I've had it happen to me but still best to get the electrics checked I guess.

 

[DSM10000]

I wonder if there is a central heating 3 way zone valve that has been wrongly wired or failed open . He might be able to manually lock the lever on the side of it and then see if the central heating works by itself ?
This thing
View attachment 150414

There will be a three way valve but it was working perfectly before the thermostat was changed

If CH is selected the app says it is operating but the holier does not fire.

 

[DSM10000]

3 way zone valve stuck, would be my guess as well.

It isn't the problem.

The app shows the boiler is commanded but the boiler doesn't fire up unless hot water is also selected.

 

[DSM10000]

No, that wiring is nothing to do with the Nest. This house has an old Y-plan system boiler in it, so that is legacy infrastructure that links the boiler, the motorised diverter valve, the central heating pump and the hot water tank thermostat. It also connects to the controller, which previously was a Drayton timer with a Honeywell room stat linked to it.

A 3rd gen Nest has replaced the Drayton/Honeywell pairing, connected to the same wiring but it can only provide triggering to say "I want central heating/hot water". That box of wiring has to remain to provide the necessary logic in terms of boiler on/off, CH pump on/off, diverter position and feedback from the tank thermostat to either provide central heating, hot water or both simultaneously.

The Drayton timer had internal circuitry to link supply for the CH and HW circuits internally, so this control wiring is based on that premise. That internal circuitry does not exist on the HeatLink, so only one triggering circuit is powered which means you lose independent control for the other.

Hope that all makes sense? I'm not aware of what setup your friend has and what the Nest has replaced but if it's a similar unit then this could well be the problem. It wouldn't surprise me if a general sparky missed this as it's quite a heating-specific situation; it was only when I consulted the Drayton wiring diagrams because I assumed I'd make a mistake in matching the wires that I realised what was going on.

Thank you for that.

It is almost certainly a wiring issue.

They have sent me pictures of the controller that was replaced

The wiring diagram for the Heat link shows separate control wiring for CH and for HW.

 

[DSM10000]

3 way zone valve stuck, would be my guess as well.

If it was stuck then you would only get CH or HW?

They get both but only when HW is selected.

The app shows that the CH is on via the schedule (when the HW isn't due to be on) but the boiler does not fire up.

 

[Doodle]

That's a pretty poor circuit diagram from Potterton - as explicitly printed it cannot work because what is providing power to the #5 connection? I suspect there's an internal link between L and #5 to provide power to the CH circuit, a quick prod on the controller connections with a meter would confirm.

A quick Google suggests my suspicions about an internal link to #5 may be correct:

Potterton EP2002 to Hive Active heating and hot water

Hi, Any idea how that old Potterton and the hive Receiver will come together? I know that the L link is not necessary but I’m bit confused about the yellow cable in B? I thought L-L N-N 5-4 4-3 3-2 2-1

www.diynot.com

 

[DSM10000]

That's a pretty poor circuit diagram from Potterton - as explicitly printed it cannot work because what is providing power to the #5 connection? I suspect there's an internal link between L and #5 to provide power to the CH circuit, a quick prod on the controller connections with a meter would confirm.

A quick Google suggests my suspicions about an internal link to #5 may be correct:

Potterton EP2002 to Hive Active heating and hot water

Hi, Any idea how that old Potterton and the hive Receiver will come together? I know that the L link is not necessary but I’m bit confused about the yellow cable in B? I thought L-L N-N 5-4 4-3 3-2 2-1

www.diynot.com

So are we thinking that there needs to be another link /jumper cable wired somewhere in the Nest Heat Link controller?

Unfortunately I don't have an image for the way that the original Potterton controller was wired in our friends previously working system for comparison.

On further discussion with our friends the Electrician had a problem getting the Nest controller to switch the boiler on at all after the install. He had to change something but what is unknown. He showed it working (ie the radiators were hot) before he left and in good faith he assumed it was working but as no schedules had been set he used the manual boost button on the Nest unit.

 

[W1ghty]

So are we thinking that there needs to be another link /jumper cable wired somewhere in the Nest Heat Link controller?

Unfortunately I don't have an image for the way that the original Potterton controller was wired in our friends previously working system for comparison.

On further discussion with our friends the Electrician had a problem getting the Nest controller to switch the boiler on at all after the install. He had to change something but what is unknown. He showed it working (ie the radiators were hot) before he left and in good faith he assumed it was working but as no schedules had been set he used the manual boost button on the Nest unit.

Does this link help ?

Changing Potterton EP3001 to a new Nest Thermostat

Hi all, New here but been lurking for a while; you've helped with many a DIY task to date so I'm indebted already! I recently bought a Nest Thermostat for our quite old but fully working boiler (Potterton Profile 60e). It is controlled via a Potterton EP3001 controller, and feeding into that...

www.diynot.com

 

[W1ghty]

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Potterton ep2002 to nest