Baxi system boiler and Opentherm

[Ian777]

Hello
Does anyone know if its possible to connect an Opentherm thermostat such as a Nest to a Baxi 800 System boiler.
I know its possible with a Baxi 800 Combi but I cannot find any information on achieving this with a system boiler.

I wondered if its possible to control the radiator flow temperature with say a Nest connected to the opentherm inputs.
(also controlling S plan CH & HW zone valves with its 240V outputs.)

Then for higher flow temperatures needed to heat the HW cylinder reliably to 60C, I would like to slightly modify the standard S plan to hot water priority with one relay (disabling CH zone valve when HW zone valve open) and add the Relay PCB kit from an Baxi IFOS System kit (without the weather comp sensor).
Its my understanding that 240V on the Relay PCB kit when installed temporarily overrides any current boiler flow temperature to 80C

Thus when the HW cylinder made a call for heat, the Zone valve to the HW cylinder would open and using a simple extra relay the central heating zone valve would close. (if it was open - the modified S plan). At the same time the HW cylinder call for heat would also cause the IFOS Relay PCB to boost boiler flow temperature to 80C.

Then when the HW call for heat ceased flow temperatures would be controlled by Openthem control and CH flow would be resumed.

Does anyone know if this would work
Many thanks
Ian

 

[gmartine]

Probably not but check with Baxi technical.

There's a heating engineer on YouTube called Andrew Millward (Watford) who's adept at these sort of custom fixes, he has a very helpful FaceBook group that you can join.

 

[ShaunCorbs]

Unfortunately no you can’t use two control methods with baxi either ifos or nest via open

You could use ifos with the wc sensor and then use nest in a dumb setting eg on or off

 

[Ian777]

Thanks for the responses,

I had hoped that the Relay PCB (from the IFOS kit) would be a way to stimulate a full heat demand for HW, regardless of whether you were using IFOS or Opentherm sensors.

Looking at the PCB layouts the Relay Pcb seems to short out the terminals on the System main boiler PCB that in the Combi are connected to the DHW NTC sensor. I am guessing the boiler figures out its in IFOS mode by geting weather sensor input then the IFOS control software looks on the DHW NTC sensor terminals for open or short ie a call for max temp flow.
In opentherm mode maybe the software does not look for/react to this input, as it does not expect it ?

However one final confusing piece of information from the Installation guide for an IFOS Kit suitable for Baxi System boilers. (page3)

"OpenTherm Controls: If an OpenTherm control is also to be connected to the boiler this must be taken into consideration as some devices affect the operation of the boiler control knobs or buttons. Depending on the device used it is possible that the OpenTherm control will assume the role of “master”, rendering the boiler controls inoperative or altering their function.
It is recommended that the IFOS is fitted and set prior to connecting any OpenTherm device. Consult the instructions supplied with such devices before setting the curve.
The Baxi uSense device is recommended for this type of control application."

So this would make for an interesting experiment ??
IFOS and Open Therm dual input was considered possible by BAXI, why would you ever have load compensation and weather compensation at the same time !!!

Any thoughts ?

 

[ShaunCorbs]

The boiler knows it’s in hw mode via a cable / 240v from the hot water call to the boiler terminal block

“A 230V switched live from the DHW cylinder thermostat (demand signal) must be connected to the spare position on terminal block M1”

And then a normally open relay connected to the same hw call breaking the brown 2 port for heating

That sorts the hw side out eg high fire and hw priority

As for the nest it’s worth a try in open but you might have to switch it to just 240v switching depending on findings in theory it might work but sometimes it doesn’t play ball etc baxi and opentherm

 

[Ian777]

Shaun thanks for your help,
I think I have missed something, I did not realise you could mix Opentherm and the normal switched live input.

Just to confirm are you saying that if I have an opentherm controller wired on low voltage M2 pins 1&2 this should "in theory" control CH flow temperatures. (assuming no switched live inputs on M1 pin1 )

If I then apply a HW demand as a switched 230V live to the normal switched live input on M1 pin1 the boiler will ignor any opentherm commands and fire. Do you know if this is definitely at max flow temperature or will the boiler fire at the set temperature on the CH dial - as though it was in normal switched live mode ?


Thanks again

 

[ShaunCorbs]

You can’t to my knowledge but the switch live eg hw demand to the ifos box must isolate it just to that

In theory if everything is wired correctly yes

Like I said above worst case use the nest in 240v switching and plug in the wc sensor and set the curve and it will do the same as open therm but better

 

[EvilDrPorkChop]

Opentherm works with the Baxi System boilers Done it with Evohome and works spot on.

 

[Ian777]

Thats great to hear, would you care to share more details of a typical installation.

In particular how exactly did you get the boiler to fire at higher temps to heat hot water while connected to Opentherm ?
Did you set up the system as hot water priority ?

I have also read that Baxi are not fully Opentherm compatible, did you have to setup anything special on the Evohome ?
Thanks for any information you can give, there is very little on the internet about Opentherm and Baxi 600/800 System boilers at present.
Thanks

 

[EvilDrPorkChop]

Thats great to hear, would you care to share more details of a typical installation.

In particular how exactly did you get the boiler to fire at higher temps to heat hot water while connected to Opentherm ?
Did you set up the system as hot water priority ?

I have also read that Baxi are not fully Opentherm compatible, did you have to setup anything special on the Evohome ?
Thanks for any information you can give, there is very little on the internet about Opentherm and Baxi 600/800 System boilers at present.
Thanks

Evohome controls the flow temp, so for HW it goes in Max (80oc flow). Then back down for CH.

Got HW Priority with a NO motorised valve for CH however there’s radiator controllers on all the rads.

When Opentherm is used the controls on the front of the boiler are redundant. You can’t set a max flow temp, it’s done by the controller which will be the boilers max 80oc.