Baxi system boiler and Opentherm

[Ian777]

I have been doing some further investigation of Opentherm controls for system boilers focused on controlling both Heating and the temperature of a hot water cylinder.

I have been looking for a cheaper solution to the excellent Evo Home and think I might have found it by using a couple of products from EPH that allow you to combine the Opentherm signals from a Room thermostat and a HW cylinder thermostat.

The two devices are a:

CP4i Opentherm Smart Thermostat (with an RF1A-OT control receiver)

CP4-HW-OT Opentherm programmable cylinder thermostat (with an RF1A-OT control receiver)

The two thermostats each come with a RF control boxs that can control a zone valve and be easily wired together to give hot water priority.

The RF control boxes can also be linked by RF so that one of the control boxes can be defined as the Opentherm Hub. The Hub will then send Opentherm commands from both thermostats to a System boiler over just two wires. (further room thermostats can also be added if more heating zones are required - sending its opentherm commands to the defined Hub)

As a bonus a Wifi gateway (GW03) is available that allows all devices to be controlled over the internet if required using the Ember app.

This solution seems to tick all my requirements to fully control a system boiler by Opentherm for:
central heating, cylinder hot water, relays for zone valve control, priority HW and internet access at a very reasonable price.

I am about to give it a go but am interested if anyone else has used these EPH thermostats in a combined Opentherm configuration giving priority hotwater with a a Baxi 600/800 system boiler.

 

[ShaunCorbs]

I’ve used eph controls and there great might want to email tech just to check product numbers etc

 

[Ian777]

I thought I would give an update in case anyone else wants to implement full Opentherm control of a Baxi 600/800 System boiler using a simple low-cost approach.

I have been using the EPH controls mentioned in my previous post for a while now controlling a Baxi 818 system boiler that is plumbed hot water priority using a 4043B (normally open) valve for heating and a 4043H (Normally closed) valve for hot water.

It’s all working perfectly, HW always uses the max set flow temperature which I have set to 70C and CH in my case typically ranges between 65 and 47C depending on room demand (it gets to 55C or below quite quickly). A range of Opentherm parameters such as max, min flow temps are user definable and you can monitor flow/return and boiler modulation via any connected thermostat.
(My Opentherm user set flow temps are Max 70C min 45C)

I chose the normally open valve on the central heating as it ensures there is always a “bypass” route for the boiler through the central heating circuit when the boiler goes into pump over run at the end of any heating cycle (HW or CH - rather than being restricted to the boilers limited internal bypass)
Should you wish, you can modify the wiring slightly and use two normally closed valves to easily use the above setup with a standard S plan arrangement. (This is detailed on the EPH website under hot water priority).

With the additional Wifi gateway I can control heating and hot water over the internet from the Ember app - which also works very well. A point to note is that for safety HW storage temperature can only be manually set at the EPH tank thermostat.

I have been very impressed how well this all works, it’s been completely reliable and controls the boiler very efficiently with the boiler on very low for long periods. It keeps the return flow temperatures well below 54c during room heating cycles and heats my current hot water cylinder using 70C in approx 30 mins.
(I should add in my case when heating the HW tank using 70C flow, the return temps are over 54C but thats probably due to my current tank and is only for 30 minutes)

All at a very reasonable price of around £200.

 

[Podengo]

I thought I would give an update in case anyone else wants to implement full Opentherm control of a Baxi 600/800 System boiler using a simple low-cost approach.

I have been using the EPH controls mentioned in my previous post for a while now controlling a Baxi 818 system boiler that is plumbed hot water priority using a 4043B (normally open) valve for heating and a 4043H (Normally closed) valve for hot water.

It’s all working perfectly, HW always uses the max set flow temperature which I have set to 70C and CH in my case typically ranges between 65 and 47C depending on room demand (it gets to 55C or below quite quickly). A range of Opentherm parameters such as max, min flow temps are user definable and you can monitor flow/return and boiler modulation via any connected thermostat.
(My Opentherm user set flow temps are Max 70C min 45C)

I chose the normally open valve on the central heating as it ensures there is always a “bypass” route for the boiler through the central heating circuit when the boiler goes into pump over run at the end of any heating cycle (HW or CH - rather than being restricted to the boilers limited internal bypass)
Should you wish, you can modify the wiring slightly and use two normally closed valves to easily use the above setup with a standard S plan arrangement. (This is detailed on the EPH website under hot water priority).

With the additional Wifi gateway I can control heating and hot water over the internet from the Ember app - which also works very well. A point to note is that for safety HW storage temperature can only be manually set at the EPH tank thermostat.

I have been very impressed how well this all works, it’s been completely reliable and controls the boiler very efficiently with the boiler on very low for long periods. It keeps the return flow temperatures well below 54c during room heating cycles and heats my current hot water cylinder using 70C in approx 30 mins.
(I should add in my case when heating the HW tank using 70C flow, the return temps are over 54C but thats probably due to my current tank and is only for 30 minutes)

All at a very reasonable price of around £200.

Hi Ian, sorry to restart an old thread, I wondered if your system is still working fine.

from what I have read Baxi are not members of opentherm association and their boilers aren’t fully compatible with open therm products as the opentherm signal from the controller has to have an ID for Baxi to recognise it.

It does sound like the set up you have from EPH works properly with the Baxi boiler though - could you confirm if you’ve found that to be the case?

TIA

 

[Ian777]

Hi Ian, sorry to restart an old thread, I wondered if your system is still working fine.

from what I have read Baxi are not members of opentherm association and their boilers aren’t fully compatible with open therm products as the opentherm signal from the controller has to have an ID for Baxi to recognise it.

It does sound like the set up you have from EPH works properly with the Baxi boiler though - could you confirm if you’ve found that to be the case?

TIA

Hi sorry to be so slow in responding.

The setup I described in my previous post has been totally reliable, I have not had a single problem with it since I installed the EPH controls back September 2022. I have been very impressed with the EPH setup for HW priority control of a Baxi system boiler.

Let me know if you want any more info
Rgds
Ian