System Boiler (Y-plan) with Google Nest 3rd Gen and OpenTherm?

[albert85]

Hi

I've got an Ideal Logic+ System Boiler (installed by my plumber as a Y-plan currently in the simple on/off mode) with Google Nest 3rd Gen.

Is it possible to upgrade my system so that the Nest 3rd Gen controls the boiler via the OpenTherm protocol for both the heating and for the hot water tank?

The Google Nest installation manual doesn't contain a wiring diagram for my situation. It only contains the on/off Y-plan setup I've got currently running (page 23) and an OpenTherm setup (page 24) that doesn't show how the Y-plan valve might be controlled to divert water between either heating or hot water tank in my setup once the boiler is controlled via OpenTherm.

Is this possible if I purchase additional equipment? My goal is to have both the hot water and heating controllable via the Nest app, and have the heating temperature controlled via OpenTherm to get a more even room temperature.

 

[hometech]

the protocol works by deciding how much input you need at a point in time. There is a very simple logic to the operation and is often referred to incorrectly as intelligent. So imagine that the 2 zones you have need different inputs so zone 1 needs 10Kw and zone 2 only needs 5Kw. What does the boiler decide is best to turn the boiler up or down? How can it even do that? Then add the hot water needs and the variable input. Which does it give priority to and in which order? The other thing worth a mention. The opentherm protocol is a set of standards that CAN be implemented but DON'T HAVE TO BE implemented. This will vary by brand as many have their own versions of the standard. Compatibility and interoperability are different

 

[albert85]

Thanks for the reply!

I'm interested to know if specifically the Ideal Logic+ System Boiler can be made to work in OpenTherm mode (for both controlling heating and hot water) with Google Nest 3rd generation and a Y-plan setup, and if so, how?

My plumber had not installed OpenTherm before and so if I want him to do it I need to give him some more pointers

 

[hometech]

Thanks for the reply!

I'm interested to know if specifically the Ideal Logic+ System Boiler can be made to work in OpenTherm mode (for both controlling heating and hot water) with Google Nest 3rd generation and a Y-plan setup, and if so, how?

My plumber had not installed OpenTherm before and so if I want him to do it I need to give him some more pointers

What are you expecting to happen? control zones without closing the valve but instead turn the water temp down?
on that individual circuit...while keeping the other circuit(s) operating at a higher or lower flow temp?
but with the water coming from the same feed/return?
Then add DHW control...so does the boiler switch the valve to heat the coil in the tank...while keeping the heating circuits in operation or does it heat the water to the temp you want...while deciding how much water needs heating....and how much you are using..
Do you see the complexity of what you are suggesting?

Opentherm modulates the burner to adjust CH flow temp to prevent stop/start and create a slow, steady heating profile.
Imagine you have a car that only has full throttle or no throttle as a means of controlling speed in traffic.

So it will operate S or Y plan and modulate the CH flow temp (opentherm) but the S/Y valves will be either open or shut as will the case with the DHW valve. So you fill a bath, water needs heated, max power applied, no CH in operation. HW store up to temp, CH back on line and working with opentherm. Close a valve, less flow, higher return temp, modulation kicks in etc...

My personal home system has CH and DHW as different needs/demands so has different boilers. I removed the zones S plan) and TRV's and sized the rads based upon heat loss of the room based on 45 degree temp flow.
The heating is automated (on 24/7) so I have no room thermostats, it works on air intake temp and return temp.
It controls the CH flow temp directly. Result is the house has a constant temp and humidity in every single room and landing and the gas reduction is around 50% less than it was with a storage system with zones.
Unlimited hot water on demand is great...

 

[albert85]

Your two boiler setup sounds nice

What I wanted to happen was for the hot water cylinder heating to take priority always, so any time the hot water cylinder temperature drops below target the system should try to heat that immediately (even stopping central heating if sending the very hot water towards central heating is not asked for at that time)

And then for the central heating flow temp to be modulated to keep the room temperature contained in a 0.5 degree range or better (currently my on/off system manages to keep it in a 1 degree range)

Also I was hoping for the radiators to not get as hot as they currently do and instead stay on for longer on a lower temperature (because I think it might create a more pleasant humidity level that way, could be wrong on that)

From your answer it sounds like maybe I'm expecting entirely too much out of a single boiler, and I either accept the limitations of a traditional on/off system, or I go for a two boiler setup like you have

p.s. i've attached the temperature profile my on/off system is able to achieve over a 24 hour period for other interested readers (target temp for day is 2.5 degrees, night is 19 degrees which is why it drops down to that)

 

[Jerry]

Hi Albert85,

As far as I am aware, only combi boilers seem to work with opentherm.

The nest controls the heating directly on an opentherm combi boiler by low voltage. If you have two heating zones (e.g heating and hot water), then you have zone valve/s. The only way the Nest can control this is via mains voltage. This is then your normal on/off type controls. As opposed to modulating via opentherm.

I believe that with Ideal boilers, if any mains voltage is detected on the switch live to the boiler (i.e the bit that turns it on/off), then the opentherm is ignored.

However, there may be a way of doing it. You may be able to run low voltage through the brown and orange or two 2 port valves with 2 seperate Nests etc. There may be a aftermarket gadget that can do this but....its a lot of time and money for a small improvement.

You also stated that you want to keep the temperature within 0.5 degrees. i dont beleive you can obtain this. Think about the solar gain from the north and south facing aspects of your property. Think about the movement of air within the property when you cook, open windows, open doors etc. I doube you can maintain a 0.5 degree difference between the ceiling and the floor.

 

[albert85]

Thanks for the replies. I think I'm beginning to see why OpenTherm hasn't taken off more. The extra complexity doesn't seem to be worth it for the small improvement it can offer over the simple on/off system

 

[hometech]

Thanks for the replies. I think I'm beginning to see why OpenTherm hasn't taken off more. The extra complexity doesn't seem to be worth it for the small improvement it can offer over the simple on/off system

it makes a massive improvement (being able to modulate the boiler) but like all de facto standards, companies want to control things.
Like Linux and windows. Use Linux and you have no security concerns, it is always up to date and it costs nothing as its opensource. Its older than windows but it requires a higher level of understanding (well it used to) to use and manage it. Why don't people use Linux then? Because MS spends a fortune to convince you otherwise.

What you want to achieve can be put together but you would use valve control rather than gas burner control.
A few raspberry pi zeros or arduion's and relays and temp probes would work and you will find a massive community who have already done this and will show you how to do it.

 

[Jerry]

I have one customer who has done the above. He has used his professional technical knowledge to access the Honeywell Evohome wireless network that controls his wireless TRV's. He built an app that allows him to track everything. It must have taken him a very long time and he was rightly proud.

However, 99.9999% of people just want it to work, maybe save them some money and not have to think about it.

Boilers modulate, controls do their thing and manufacturers are always concerned about giving access to their products. Especially if it can turn into a bomb or poison you in your sleep.

 

[Basher]

Thanks for the replies. I think I'm beginning to see why OpenTherm hasn't taken off more. The extra complexity doesn't seem to be worth it for the small improvement it can offer over the simple on/off system

To attempt to answer your original post regarding the circuit for opentherm, my immediate reaction on reading the Nest manual you linked, was that the opentherm connections on P24 are to be used in addition to all the other connections for the system (in your case Y-plan on P23). This seems to be borne out by a thread (but S-plan) I've seen, but some people who have done this have had trouble with demand for hot water only not firing the boiler. Might be worth looking through this (admittedly from a few years ago):

Nest and Opentherm - Page 1 - Homes, Gardens and DIY - PistonHeads UK

 

[hometech]

I have one customer who has done the above. He has used his professional technical knowledge to access the Honeywell Evohome wireless network that controls his wireless TRV's. He built an app that allows him to track everything. It must have taken him a very long time and he was rightly proud.

However, 99.9999% of people just want it to work, maybe save them some money and not have to think about it.

Boilers modulate, controls do their thing and manufacturers are always concerned about giving access to their products. Especially if it can turn into a bomb or poison you in your sleep.

Is this what is know as the Jihadi plumbing defence LOL. Please explain how a boiler becomes a bomb with the added ability to poison while you sleep. You don't write the Russian stories for the BBC by any chance?

 

[albert85]

To attempt to answer your original post regarding the circuit for opentherm, my immediate reaction on reading the Nest manual you linked, was that the opentherm connections on P24 are to be used in addition to all the other connections for the system (in your case Y-plan on P23). This seems to be borne out by a thread (but S-plan) I've seen, but some people who have done this have had trouble with demand for hot water only not firing the boiler. Might be worth looking through this (admittedly from a few years ago):

Thanks Basher for digging that out.

From that thread it sounds like if I configure the Nest in on/off mode for hot water, and in OpenTherm mode for central heating, I might be able to get it working, so long as the Ideal Logic+ system boiler can sensibly handle switched live (on/off) and OpenTherm inputs at the same time.

Regarding 'sensibly handle' multiple inputs, the questions to understand I think are what happens if:
a) Nest is calling for heat via switched live and not via OpenTherm. I assume the boiler will prioritise switched live at that point and fire on the flow temperature configured in the boiler.
b) Nest is calling for heat via OpenTherm but not via switched live. I assume the boiler will then fire on the temperature requested via OpenTherm.
c) Nest is calling for heat via both OpenTherm and switched live at the same time. I assume that in this case switched live will be prioritised and the boiler will fire on the flow temperature configured in the boiler.
d) Nest is not calling for heat via either interface. I assume boiler will stay off as expected in this case.

I've asked Ideal support to clarify how their Ideal Logic+ system boiler specifically will handle these four scenarios and will update here if I get useful input from them.


@hometech thanks for pointing out the DYI option with the raspberry pis. It sounds like a fun project but I'm still looking for a more accessible setup so it can be serviced by most heating engineers without my input. Else my house will have to come with its own GitHub repository if I ever decide to sell it...

 

[hometech]

Thanks Basher for digging that out.

From that thread it sounds like if I configure the Nest in on/off mode for hot water, and in OpenTherm mode for central heating, I might be able to get it working, so long as the Ideal Logic+ system boiler can sensibly handle switched live (on/off) and OpenTherm inputs at the same time.

Regarding 'sensibly handle' multiple inputs, the questions to understand I think are what happens if:
a) Nest is calling for heat via switched live and not via OpenTherm. I assume the boiler will prioritise switched live at that point and fire on the flow temperature configured in the boiler.
b) Nest is calling for heat via OpenTherm but not via switched live. I assume the boiler will then fire on the temperature requested via OpenTherm.
c) Nest is calling for heat via both OpenTherm and switched live at the same time. I assume that in this case switched live will be prioritised and the boiler will fire on the flow temperature configured in the boiler.
d) Nest is not calling for heat via either interface. I assume boiler will stay off as expected in this case.

I've asked Ideal support to clarify how their Ideal Logic+ system boiler specifically will handle these four scenarios and will update here if I get useful input from them.


@hometech thanks for pointing out the DYI option with the raspberry pis. It sounds like a fun project but I'm still looking for a more accessible setup so it can be serviced by most heating engineers without my input. Else my house will have to come with its own GitHub repository if I ever decide to sell it...

Your boiler has a manual, why not your home?
I would suggest you look around your locality for a raspberry pi club. There are lost of 10yr olds who could do this for you. Its really very simple stuff but it looks like magic to the those who never use a multimeter lol.
Is your boiler a sentient being? How would it handle two instructions simultaneously but being able to decide which one to choose?
The nest instructions are for an S plan OR Y plan OR opentherm...its not a pick and mix lol

 

[albert85]

Had a reply from Ideal. Sounds like with their boiler its not possible to use OpenTherm and switched-live at the same time.

Response from Ideal support: "if you was to have both controllers wired in each terminal then boiler wouldn’t fire as the link would be broken in the circuits, the only way the links would be made on either switched live or open therm would be to have a demand on. As stated earlier, if this was to be the case then the switched live would take priority making your open therm void."


For future readers, I can confirm that in my Nest settings when setting up a system boiler,

• I first get to pick if heating should be controlled with OpenTherm or on/off
• If I pick OpenTherm (see attached first image), I then get to pick if I want hot water to be controlled via 'OpenTherm' or 'on/off' or 'not controlled' (see attached last three images)
• And if I pick the heating to be controlled via 'on/off' then the offered hot water control options are either 'on/off' or 'not controlled'

And judging from the thread linked by Basher the Nest seems to output as you would expect from these settings. Just thought I'd leave that hear in case it helps someone

 

[hometech]

Raspberry-Pi Home Heating Controller

Raspberry-Pi Home Heating Controller: Have you ever wanted to remotely control your home heating and hot water from anywhere, but did not want to pay over £200 for a commercial system ? Using off-the-shelf hardware and shareware code, you can build your own DIY controller for less than …

www.instructables.com

 

[Jerry]

Is this what is know as the Jihadi plumbing defence LOL. Please explain how a boiler becomes a bomb with the added ability to poison while you sleep. You don't write the Russian stories for the BBC by any chance?

Are you high?

 

[Jerry]

Your boiler has a manual, why not your home?
I would suggest you look around your locality for a raspberry pi club. There are lost of 10yr olds who could do this for you. Its really very simple stuff but it looks like magic to the those who never use a multimeter lol.
Is your boiler a sentient being? How would it handle two instructions simultaneously but being able to decide which one to choose?
The nest instructions are for an S plan OR Y plan OR opentherm...its not a pick and mix lol

Go and find a 10 year old! What an answer! You sound like Groucho Marx

You are disconnected from reality.

It's so easy. All you have to do is buy rasperbby pi. Learn how to turn it on, connect it to your computer and you are away! No learning the computer language you can just download the prewritten scripts made by 10-year-old (who looooooove boilers and heating. So much better than Fortnight). You won't have to learn, stress, swear and scream at this tiny little computer at all. It will all be fine and easy and plain sailing. Figuring out a multimeter will be the hardest part. It will be the only doohickey you have to buy and you can easily do it in an evening.

You have no idea what you are talking about. Please stop lying to people. The more you write, the more ludicrous you sound.

TO THE ORIGINAL POSTER - if you really want to go down this route, I have a lot of respect for your desire to understand and control your heating system. Please be aware the cost/benefit ratio will be in the negative. BUT, you will learn a lot along the way.

HOMETECH - please stop misleading people. I am sure you have a wonderful heating system but what you are so flippantly discussing is beyond the reach of nearly everyone. Including nearly every registered insured qualified experience profession heating engineer who does this for a living. I pity the person who comes to service your boiler.

 

[Basher]

The nest instructions are for an S plan OR Y plan OR opentherm...its not a pick and mix lol

if you are correct, please could you explain how Nest can control the heating and hot water when its internal switches are not connected to anything (as per the Opentherm schematic on P24)?

 

[Jerry]

Raspberry-Pi Home Heating Controller

Raspberry-Pi Home Heating Controller: Have you ever wanted to remotely control your home heating and hot water from anywhere, but did not want to pay over £200 for a commercial system ? Using off-the-shelf hardware and shareware code, you can build your own DIY controller for less than …

www.instructables.com

You are insane mate.

Let me just copy and paste a little of the instructions for everyone to read:

Software :

Connect to each pi-heating-sensor in turn,

ssh [email protected]

We can now start to install the packages and software that will read the 1-wire thermometer data and make it available the the heating controller hub.

again there is a script to do this :

curl "https://raw.githubusercontent.com/JeffreyPowell/pi-config/master/pi-heating-remote-install.sh" > pi-heating-remote-install.sh && sudo bash pi-heating-remote-install.sh

once everything is installed, reboot :

sudo shutdown -r now

now we need to edit the config file ~/pi-heating-remote/configs/sensors

but first we need the discover the serial numbers of the connected 1-wire thermometers

ll /sys/bus/w1/devices/

Will show that I have two 1-wire thermometers connected, 28-0000056e625e and 28-0000056ead51.

drwxr-xr-x 2 root root 0 Jan 25 15:23 .

drwxr-xr-x 4 root root 0 Jan 24 16:17 ..

lrwxrwxrwx 1 root root 0 Jan 24 16:43 28-0000056e625e -> ../../../devices/w1_bus_master1/28-0000056e625e

lrwxrwxrwx 1 root root 0 Jan 24 16:43 28-0000056ead51 -> ../../../devices/w1_bus_master1/28-0000056ead51

lrwxrwxrwx 1 root root 0 Jan 25 15:23 w1_bus_master1 -> ../../../devices/w1_bus_master1
Now edit the config file and insert these serial numbers.

vi home/pi/pi-heating-remote/configs/sensors

Delete the default settings and insert the serial number and a name for each 1-wire thermometer.

remove all this :

28-000005cf873e = External

28-000005d0065c = Conservatory

28-000005d01a5a = Conservatory Rad
and add your data, e.g. :

28-0000056e625e = Lounge

28-0000056ead51 = Garden
save and close the config file.
​

Did i mention there was soldering involved?

Muppet

 

[albert85]

I don't find the DYI option ridiculous thats why I thanked hometech for suggesting it

I have a physics degree and develop software at work, so it would be a fun project for me and totally within reach, as it will be for thousands others.

As a software engineer I'm also keenly aware of the responsibilities that come with maintaining legacy software for many years. With that said I don't think it would be too hard to set it up in a way where there is a straightforward way to convert it back to the well known on/off system if desired at any point.

Anyway, for me personally I've decided I want something more off the shelve where I can pay a heating engineer to maintain it, but I could totally see myself make a different choice in the future

The more options the better

 

[hometech]

You are insane mate.

Let me just copy and paste a little of the instructions for everyone to read:

Software :

Connect to each pi-heating-sensor in turn,


We can now start to install the packages and software that will read the 1-wire thermometer data and make it available the the heating controller hub.

again there is a script to do this :


once everything is installed, reboot :


now we need to edit the config file ~/pi-heating-remote/configs/sensors

but first we need the discover the serial numbers of the connected 1-wire thermometers


Will show that I have two 1-wire thermometers connected, 28-0000056e625e and 28-0000056ead51.

drwxr-xr-x 2 root root 0 Jan 25 15:23 .

drwxr-xr-x 4 root root 0 Jan 24 16:17 ..

lrwxrwxrwx 1 root root 0 Jan 24 16:43 28-0000056e625e -> ../../../devices/w1_bus_master1/28-0000056e625e

lrwxrwxrwx 1 root root 0 Jan 24 16:43 28-0000056ead51 -> ../../../devices/w1_bus_master1/28-0000056ead51

lrwxrwxrwx 1 root root 0 Jan 25 15:23 w1_bus_master1 -> ../../../devices/w1_bus_master1
Now edit the config file and insert these serial numbers.


Delete the default settings and insert the serial number and a name for each 1-wire thermometer.

remove all this :

28-000005cf873e = External

28-000005d0065c = Conservatory

28-000005d01a5a = Conservatory Rad
and add your data, e.g. :

28-0000056e625e = Lounge

28-0000056ead51 = Garden
save and close the config file.
​

Did i mention there was soldering involved?

Muppet

You not having any education whatsoever in computing (using windows is not computing) is not a reflection of the current levels of education of an average 10 yr old, it is a reflection on yourself. Most of the clubs that are running around the country are populated by youngsters, this is also taught in schools which is why I suggested using youngsters to do this job. The internet and youtube have millions of how to guides so don't heap your inability and lack of knowledge and your obvious frustration onto the cognitive abilities of others. It seems that there is now a pride taken in ones inability, a badge of honour to proclaim your unwillingness to learn and to shout with pride that you know very little...and proud of that fact.

 

[hometech]

Go and find a 10 year old! What an answer! You sound like Groucho Marx

You are disconnected from reality.

It's so easy. All you have to do is buy rasperbby pi. Learn how to turn it on, connect it to your computer and you are away! No learning the computer language you can just download the prewritten scripts made by 10-year-old (who looooooove boilers and heating. So much better than Fortnight). You won't have to learn, stress, swear and scream at this tiny little computer at all. It will all be fine and easy and plain sailing. Figuring out a multimeter will be the hardest part. It will be the only doohickey you have to buy and you can easily do it in an evening.

You have no idea what you are talking about. Please stop lying to people. The more you write, the more ludicrous you sound.

TO THE ORIGINAL POSTER - if you really want to go down this route, I have a lot of respect for your desire to understand and control your heating system. Please be aware the cost/benefit ratio will be in the negative. BUT, you will learn a lot along the way.

HOMETECH - please stop misleading people. I am sure you have a wonderful heating system but what you are so flippantly discussing is beyond the reach of nearly everyone. Including nearly every registered insured qualified experience profession heating engineer who does this for a living. I pity the person who comes to service your boiler.

wonderful system by design, not chance. I started with basics on understanding the property. I then improved where I could or it was calculated it would be beneficial. Selecting the equipment took a lot of time but knowing what I needed meant 95% progress had been made. After that It was a lot of effort in monitoring, experimentation and observation. I discovered an awful lot in the process, a knowledge I am willing to share. Maths and physics...its all you need.....

 

[hometech]

Are you high?

As high as the pressure inside your combi LOL. Give me one, just one example of a combi boiler exploding under normal usage. Just one

 

[albert85]

I decided to leave the system in the simple on/off mode for now, which is how my heating engineer has left it. He will come back to issue the gas safety certificate and has offered to make any installation tweaks I still want at the same time.

One thing I'm confused about is that he left my old Honeywell controller to work alongside the Nest Heat Link base station. I would have thought that the Honeywell controller is no longer needed if the Nest is wired in Y-plan on/off mode (page 23 of the nest installation guide.) With the cylinder thermostat directly connected to terminal 4 on the Nest.

I have attached some photos of my installation as it is wired now. Am I missing something and in fact the Honeywell controller is still required? Or can I ask my heating engineer to remove the old Honeywell controller? I'd like to leave the system in the simplest possible way, with as few components that can fail or confuse future installers.

 

[Basher]

I decided to leave the system in the simple on/off mode for now, which is how my heating engineer has left it. He will come back to issue the gas safety certificate and has offered to make any installation tweaks I still want at the same time.

One thing I'm confused about is that he left my old Honeywell controller to work alongside the Nest Heat Link base station. I would have thought that the Honeywell controller is no longer needed if the Nest is wired in Y-plan on/off mode (page 23 of the nest installation guide.) With the cylinder thermostat directly connected to terminal 4 on the Nest.

I have attached some photos of my installation as it is wired now. Am I missing something and in fact the Honeywell controller is still required? Or can I ask my heating engineer to remove the old Honeywell controller? I'd like to leave the system in the simplest possible way, with as few components that can fail or confuse future installers.

Is there an isolator (eg FCU) there for the heating controls? If not, perhaps your heating engineer left the old controller there as the simplest way of providing that 'switch'? It seems to be set to be permanently 'on'.

Or do you have another way of locally isolating the system?

 

[EvilDrPorkChop]

I decided to leave the system in the simple on/off mode for now, which is how my heating engineer has left it. He will come back to issue the gas safety certificate and has offered to make any installation tweaks I still want at the same time.

One thing I'm confused about is that he left my old Honeywell controller to work alongside the Nest Heat Link base station. I would have thought that the Honeywell controller is no longer needed if the Nest is wired in Y-plan on/off mode (page 23 of the nest installation guide.) With the cylinder thermostat directly connected to terminal 4 on the Nest.

I have attached some photos of my installation as it is wired now. Am I missing something and in fact the Honeywell controller is still required? Or can I ask my heating engineer to remove the old Honeywell controller? I'd like to leave the system in the simplest possible way, with as few components that can fail or confuse future installers.

Sorry I haven't read the thread fully. But why can't you use Opentherm without the S/L to the Ideal?

I have a system boiler using Opentherm and it works perfectly, but another brand. When you refer to Y Plan, you ideally to use Opentherm you should either use a Diverter rather than a Mid position (Referred to as W plan I believe) Or S plan using 2 two port valves.

 

[albert85]

@Basher that sounds like a likely explanation.

Judging from the attached photo I can see the cylinder thermostat wire going into the wiring centre, then to this controller, and then I think it goes directly to the boiler + Y-plan valve I (I think the Nest is bypassed because if I define a heating schedule in the Nest it doesn't appear to stop the boiler from heating the hot water cylinder.)

Do you think it would be possible to replace this Honeywell controller with a fused switch, and then go into the Nest's terminal 4, so that I can use the Nest to control the hot water schedule, in addition to the heating schedule? I like the idea of being able to turn the entire system off remotely, including the hot water.

 

[albert85]

Sorry I haven't read the thread fully. But why can't you use Opentherm without the S/L to the Ideal?

I have a system boiler using Opentherm and it works perfectly, but another brand. When you refer to Y Plan, you ideally to use Opentherm you should either use a Diverter rather than a Mid position (Referred to as W plan I believe) Or S plan using 2 two port valves.

Haha just when I had resigned myself to the idea that I won't be able to get it to work 🙂

Are you using a Nest 3rd gen thermostat? Do you mind telling me a bit more about how your thermostat controls the valves?

 

[EvilDrPorkChop]

Haha just when I had resigned myself to the idea that I won't be able to get a system boiler to work with Nest + OpenTherm 🙂

Are you using a Nest 3rd gen thermostat? Do you mind telling me a bit more about how your thermostat controls the valves?

It should work fine. Basically you need HW priority, so that it heats the HW first before any CH demand. This is so that it can demand a high flow temp to reheat the HW quick, then switch over to CH with lower flow temps.

I'm using Honeywell Evohome, but it's the same principal.

Regarding your timer, think your CH engineer has bodged it up. I think you need to start a fresh with the wiring.

In terms of my valve set up. I have 2 port valves, I have multiple zones. Basically when the HW is demanded the power to the other valves is killed so that the higher flow temperatures can't go round. So you could technically do it with the 3 port Mid position valve using some sort of relay, but really you'd do it using the W plan.

 

[EvilDrPorkChop]

Sorry, you'd have to have some way of detecting the HW temp too. I'm not sure how Nest operates with this? Evohome measures the tank temp.

 

[Basher]

Do you think it would be possible to replace this Honeywell controller with a fused switch

This is really a question of how the fixed wiring in your property is configured.
If the supply to your heating system is a spur from a ring, it absolutely needs an FCU on that spur before being connected to the heating components.
If you have a dedicated radial from the CU to the heating, there should at least be a switch in the vicinity, but again better a FCU, fused to protect the cables in the heating system.

So the answer is "yes"!

You might ask your heating engineer what was the reason for leaving the Honeywell in position?

 

[Basher]

Sorry, you'd have to have some way of detecting the HW temp too. I'm not sure how Nest operates with this? Evohome measures the tank temp.

Nest has a binary hot/cold input from the thermostat, not a temp input from a thermocouple.
(I have Evohome 👍)

 

[Basher]

Haha just when I had resigned myself to the idea that I won't be able to get it to work 🙂

As you see I'm no expert on this, but I still don't understand why you can't follow both the Y-plan and the Opentherm schematics, but disconnect the (switched) feed from the cylinder stat and valve to boiler SL, make the boiler SL permanently Live, and then get Opentherm to control the boiler. Or maybe Opentherm can't turn the boiler on and off?

 

[EvilDrPorkChop]

Nest has a binary hot/cold input from the thermostat, not a temp input from a thermocouple.
(I have Evohome 👍)

Ah, see that would work. I didn’t realise you could do this with Nest. Open them should then just control the boiler.

 

[hometech]

I'm inclined to adopt the occam's razor principle.
Your "engineer" does not know why it works but it does
so don't touch it just in case it stops working