Discuss Baxi system boiler and Opentherm in the Central Heating Forum area at PlumbersForums.net
Haven't tried it, but perhaps the normal Live overrides the Opentherm. I think I had to remove the 230v link for the Opentherm to work, however I can't remember.Its my understanding that a Nest only has room temperature as an input, therefore it does not know if there is a cylinder HW demand unlike an Evohome that has a cylinder stat sensor for input.
As far as I can see implimenting HW priority with system boilers and a Nest (in opentherm config), requires that the Nest measures the room temperature and adjusts flow temp with opentherm. Then HW priority is externally demanded by a simple cylinder mounted stat providing a 230V switched live that can be used to switch zone valves as appropriate and if supported by the boiler - request full temperature flow. (overriding any opentherm inputs).
So far I have found no documented way for a 230V live to request a Baxi 600/800 to go max flow temp when under opentherm control.
Some Ideal boilers manage it via a S2 switched live input, I was hoping to find a Baxi work arround no luck yet.
I am surprised that Baxi make it so difficult to impliment HW priority with Opentherm CH control on their system boilers.
Hi Ian, sorry to restart an old thread, I wondered if your system is still working fine.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 sorry to be so slow in responding.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
Reply to Baxi system boiler and Opentherm in the Central Heating Forum area at PlumbersForums.net
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