Opentherm and S Plan with Evohome HW priority
Anyone got the above set up? So Opentherm Receiver, CH relay and HW relay (To use the built in HW priority features of the newer software).
Got an issue with the Opentherm receiver and CH relay where when there's a demand on the Opentherm receiver of less that 20% then the CH relay doesn't open. So all the lovely low temperature heat goes around the bypass valve and not to any of the radiators requiring it as the CH valve is closed.
Opentherm and Boiler are working great and talking fine.
Yes I have exactly the same and it works fine. Not really enough info to try to trouble shoot. You’re first option is trying to remind the CH BDR. Has OpenTherm recently been added?
A temporary fix is to lock open the CH zone valve (if you have HR92s fitted across the board)
Thanks. It's a S plan system with the CH and HW bound under system summary as 'Two 2 Port Valves'.Originally Posted by MajorDanby
The system is about 4 years old. I've upgraded to a Opentherm boiler, so I unpaired the old boiler relay and added in the Opentherm relay. When I noticed the issue, I unpaired the HW and CH relay and rebound them which made no difference.
The main issue is when there's a very low demand. So I've got HR92s throughout and when i've got a low demand of 8% say, the HR92 opens and the Boiler fires, but the CH relay fails to operate. Which results in low temp water just flowing around the bypass and the zone in question never getting any heat.
Out of interest have you checked yours when you've just got a very low demand? (You can see on System Summary) I thought my system was fine at first as when there's a larger demand it works fine. It's only when it heats up and it's trickling heat into zones the CH relay doesn't operate.
I'm an installer so I have quite a good knowledge of Evohome, but haven't experienced this issue before but I do little Opentherm installs with Evohome primary due to issues like this!
As you say as a temporary measure i've removed the CH relay and locked open the CH valve.
Could it be the load scaling bug that is affecting all the non BDRs in the system? So even though the OTB should not be affected by load scaling, I wonder if the load scaling parameters are affecting all the non HW BDRs in the system.
Why my theory? Because parameters like min On time and cycle time etc are applied to all non HW BDRs even though you would think they should only be applied to the Boiler relay.
I suspect it has something to do with TPI. Although in this case the Opentherm bridge isn't the issue, it's the CH relay that is. So in theory when its acting as a CH relay for HW priority it should act like the HW relay does as in just either ON or OFF rather than applying TPI. I think in this case any demand below 20% it isn't recognising, which I'm sure i've read previously when we were beta testing that a BDR91 will not react to anything less than 20%. The main problem here is that it defeats the object of Opentherm.
Yes that is the load scaling bug that I referred to. Trying playing around with the various load scaling parameters and see if the behaviour of the CH BDR changes with each change.
Like I said, load scaling should not affect the way the OTB works, but it is possible that is affects the CH BDR, if my theory is right.
The main issue is once you've paired the Opentherm Bridge it removes all the options of the Load Scaling features that came in the new update. I may try unpairing it and seeing if it allows me to views the options again.
I tried changing the BDR cycle rates and minimum on time yet made no difference, as you said think it comes down to the bug with demand being less than 20%.
I have encountered another bug that has been introduced with this firmware version but it relates to the HM80. There Honeywell's suggestion was to set the Mixer Valve as a Zone Valve. But similar to what you said, in Zone Valve mode, none of the Mixer Valve parameters are visible. So now if I need to make Mixer Valve parameters changes, I need to repair the device makes the necessary changes and then change the device type to Zone Valve.
So I would suggest forcing the system out of its default Load Scaling option before you pair the OTB and lose access to the Load Scaling menus.
So tried unpairing the Opentherm receiver, Load Scaling was already disabled. Repaired, and it's doing the same. As I say specifically when the loads are below 20%.
My only way around it at the minute is to change the Motorised valve to a NO rather NC, but I can't get one matching my existing. And tbh, the software should allow it and I shouldn't need to do it.
As a temporary fix just using the A B C terminals of a single BDR to get your HW priority going as well as operating the CH valve. As you probably know, mechanically locking it open doesn't actually open the valve completely.
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