Opentherm with S Plan System

[gingerprince69]

Hi

Can anyone advise how Opentherm wiring should work with an S Plan system (1 heating zone)? I'm looking to add smart control and was going Hive but learned that my boiler (Logic+ Combi 24) supports Opentherm so that might be a better option and lead towards Nest which supports Ooentherm. However, I'm struggling to understand how this would be wired with an S Plan system.

From my understanding, in S Plan it's the Honeywell valves that call for heat once the valves are open. However, with Opentherm the only wiring diagrams I've seen have the Nest connected directly to the boiler OT connections. So as I see it, that would fire the boiler before valves had opened, and would be dangerous, so there has to be some way to trigger from the valves. Is this possible, or is Opentherm only really practical for a Combi setup?

(For clarity, my boiler is a Combi but being used as a system. The DHW side is simply a tap direct from the boiler in the garage. Not sure why it's like that, only recently bought the house).

Also, does Opentherm make a worthwhile difference over and above going with a smart control system anyway? (Don't have TRVs yet so that's another job).

Thanks

 

[stem]

I have been watching this post hoping someone would reply first with a definitive comment. AFAIK Nest OpenTherm isn't suitable for use with S-Plan systems. But I would love for someone to confirm otherwise.

As you say, all of the Heat link diagrams show OT connected to a single zone combi, and I can't say that I have ever seen an installation with connections to both the dry switch and the OT terminals. If it was possible I would expect Nest to advertise the fact, especially as it's a feature that the comparable Hive can't provide.

Also, as you have figured, because the boiler is triggered by the switches inside the motorised valves, the boiler won't know from that if it's the hot water or central heating valve that is open. The Heat link will know if the heating is on or not, so the boiler could still know via the OT connection, but whether this is overridden by a switched live directly in from the motorised valves, I don't know.

It gets more interesting with new installations that require two heating zones, because a third possibility is then added, and another Heat link also. And can two Heat links share the same OT boiler terminals? Wouldn't have thought so.

Therefore, I have only used Nest OT on combi's without any zone valves, and the standard switching terminals for systems with motorised valves.

When I get the opportunity, I will ask users how they find OT after installation on an existing system and if they have they noticed any improvements in comfort or running costs. Most haven't, they just mention all the app features and that the thermostat looks nice on their wall. One guy (sound engineer) with a technical interest, commented that it does modulate the boiler temperature and thinks there may be a slight energy saving, but wasn't too sure.

 

[gingerprince69]

Thanks stem

After a bit more reading around this forum last night, I saw that suggested improvements in energy savings are only a few percent (seen 1-2%, 6%). Also some suggestion that manufacturers are removing OpenTherm from newer boilers, so whilst it sounds "shiny and new" it's been around for years and hasn't really taken off in the UK.

So I'll be going with the Hive and fitting TRVs (probably not smart as we don't change requirements around the house often enough to need different heating requirements) which would deliver most of the savings anyway.

 

[fezster]

I have been watching this post hoping someone would reply first with a definitive comment.

Same!

I can't say that I have ever seen an installation with connections to both the dry switch and the OT terminals.

I could be wrong (and I dont have a reference to where I saw this) but I'm sure I have read that some boilers allow the dry switch / 230V input to fire the boiler *and* the OT terminals to modulate it. Whether the nest would work in this fashion, I dont know.

It gets more interesting with new installations that require two heating zones, because a third possibility is then added, and another Heat link also. And can two Heat links share the same OT boiler terminals? Wouldn't have thought so.

You cannot have multi-zone nest with an OT boiler. As you say, it would not work. This can only be achieved where you have a single controller taking inputs from multiple zones and then making a decision on how to modulate the boiler temperature. Evohome does this, for example.

 

[DavidLewis]

I could be wrong (and I dont have a reference to where I saw this) but I'm sure I have read that some boilers allow the dry switch / 230V input to fire the boiler *and* the OT terminals to modulate it.

And I could be wrong too, but I think (because I did look into it a couple of years ago) my Viessmann system boiler will allow the (flat-out, high-temperature) DHW priority mode to be triggered using the DHW switch input, while (but not simultaneously) also allowing the CH to be controlled using the OT terminals.

So when the boiler is plumbed as W-plan with an external 3-port diverter, with no microswitches in it (for good reliability!), the DHW cylinder stat can be wired to move the diverter and signal the boiler's DHW switch to heat the hot water. And when the cylinder stat is satisfied, the diverter springs back to CH position and the boiler is modulated and controlled by the Opentherm CH signal again.

Perhaps this isn't all that relevant to the OP's situation though....

 

[TeeKay]

I know this is an old thread but it was one of the threads I came across when looking for an answer to whether a Nest would work with OpenTherm and S plan : The answer is : Yes, it does on the 3rd Generation Nest! It may do on other nest versions also. but I only have the 3rd gen and so that is all I can comment on.

The key bit of information I couldn't find when scouring the web, was if the the Nest still activates the relays for CH & DHW even when it is configured for OpenTherm, and it turns out it does. This is crucial as the outputs are needed to power the zone valves.

I hope this helps someone else looking for the same info, sometime in the future.

 

[Sim777]

I know this is an old thread but it was one of the threads I came across when looking for an answer to whether a Nest would work with OpenTherm and S plan : The answer is : Yes, it does on the 3rd Generation Nest! It may do on other nest versions also. but I only have the 3rd gen and so that is all I can comment on.

The key bit of information I couldn't find when scouring the web, was if the the Nest still activates the relays for CH & DHW even when it is configured for OpenTherm, and it turns out it does. This is crucial as the outputs are needed to power the zone valves.

I hope this helps someone else looking for the same info, sometime in the future.

Are you saying you have your S plan wired as standard S plan and then you connected open therm as well? Does it mean that on DHW boiler fires on full output and on CH it is OT? I thought this would not be possible? Where did you get that info please?

 

[ericmark]

I have watched a video on EPH thermostats and how they can be set as master/slave, but since not using gas, not looked too deep.

 

[jackthom]

I’m guessing the thermostat would set the boiler to a lower temperature when there is demand for heating and then immediately reset it to a high temp when that is satisfied, which would take care of any subsequent DHW requirement.
There’s no way AFAIK for the Nest to know when both heating and DHW were in demand so some sort of compromise setting for the water temperature wouldn’t be possible.
The above is pure guesswork; I’d be interested to hear if anyone has experience of this sort of setup.

 

[D_Hailsham]

If an Opentherm thermostat is connected to the OT terminals of a boiler it takes control of the boiler's flow temperature. This is OK for central heating but no good for HW heating, which requires a flow temperature of 70C or more. This is why HW priority is essential if OT is used.

 

[jackthom]

If an Opentherm thermostat is connected to the OT terminals of a boiler it takes control of the boiler's flow temperature. This is OK for central heating but no good for HW heating, which requires a flow temperature of 70C or more. This is why HW priority is essential if OT is used.

Would that be easily achieved by adding an opentherm cylinder thermostat, I'm guessing it probably isn't that easy.

 

[D_Hailsham]

Would that be easily achieved by adding an opentherm cylinder thermostat, I'm guessing it probably isn't that easy.

An OT thermostat isn't necessary as the boiler needs to run at a fixed high temperature when HW heating is required. The problem is that you can't have CH and HW running at the same time as they have distinct flow temperature requirements. It's (relatively) simple with a combi boiler as it is mipossible to have both on at the same time. The problem occurs when there is a hot water cylinder.. A W-plan helps (hot water priority) as it uses a diverter valve (looks like a mid-position) but has only two positions CH and HW. This overcomes the 'either/or' problem, but you still need to stop OT controlling the flow temperature. Some boilers -mainly continental- have the technology built into the boiler.

 

[jackthom]

I’m not really into Opentherm as my boiler is a Vaillant and for a year or two I’ve been controlling it’s flow temperature using a raspberry pi running ebusd and connected with an ebus usb interface. The heating flow temp is kept relatively low and chosen with reference to the local outside temp

During periods of combined demand for both heating and DHW the flow is set hot enough to at least get the cylinder on the way to being satisfied and only goes back up to the full 70+ degrees when the heating is no longer required.

With another relay I could inhibit the heating and go for DHW priority but things seem to work ok at the moment.

 

[Deleted member 221031]

Hi

Can anyone advise how Opentherm wiring should work with an S Plan system (1 heating zone)? I'm looking to add smart control and was going Hive but learned that my boiler (Logic+ Combi 24) supports Opentherm so that might be a better option and lead towards Nest which supports Ooentherm. However, I'm struggling to understand how this would be wired with an S Plan system.

From my understanding, in S Plan it's the Honeywell valves that call for heat once the valves are open. However, with Opentherm the only wiring diagrams I've seen have the Nest connected directly to the boiler OT connections. So as I see it, that would fire the boiler before valves had opened, and would be dangerous, so there has to be some way to trigger from the valves. Is this possible, or is Opentherm only really practical for a Combi setup?

(For clarity, my boiler is a Combi but being used as a system. The DHW side is simply a tap direct from the boiler in the garage. Not sure why it's like that, only recently bought the house).

Also, does Opentherm make a worthwhile difference over and above going with a smart control system anyway? (Don't have TRVs yet so that's another job).

Thanks

You can do it with evohome. You need the OT bridge to. It’s not the cheapest system though

 

[Bucki82]

If an Opentherm thermostat is connected to the OT terminals of a boiler it takes control of the boiler's flow temperature. This is OK for central heating but no good for HW heating, which requires a flow temperature of 70C or more. This is why HW priority is essential if OT is used.

Hello Hailsham

I have Ideal Vogue Max s18 system boiler which supports PDHW and Opentherm.

I was not aware that u cannot have OT on a S PLAN and just learning about it. Been looking at YouTube someone replacing the CH zone valve from Normally Open to a Normally Closed (NC) to make it work with Opentherm.

It seems to be doable with NEST but I got myself the TADO Wireless Thermostat 3v+ (EU model) which supports opentherm.

However, being told that it's not possible to achieve PDHW and Opentherm with my setup.


Any ideas or suggestions??