Evohome + Opentherm v Evohome + Boiler Outside Weather Compensation

[mtmcgavock]

I have HW Priority via a 2 valve bespoke system. The HW is on a standard motorised 2 port valve controlled by a BDR. CH has an inline power off open motorised valve. When HW is demand, the HW valve opens and the CH valve closes. Seems to work perfectly as my installer said it would.

Or you could just simply provide power to the A port on the BDR91 Heating receiver from port C on the BDR91 Hot Water. (Providing you have two NC MV in your setup).

[mdurkin]

Or you could just simply provide power to the A port on the BDR91 Heating receiver from port C on the BDR91 Hot Water. (Providing you have two NC MV in your setup).

Hi - yes I think I could wire with a couple of options to achieve hot water priority, power the other two BRD91s (UFH and heating) from the C port on the Hot water BRD91, or I think similarly there's a 2 way switch in the motorised valves that I could wire in a similar fashion (I think) - just wondered if evohome could just implement this in logic - pretty easy - was a bit surprised not to see it.

One other thing - my UFH is just off a BRD91 - just a zone valve. When the UFH comes on evohome opens the UFH zone valve as expected, but it also opens up the heating zone valve. Is this expected? I don't have the heating BRD91 double bound - I cleared it and rebound. Unless perhaps it's in the evohome controller where the binding is incorrect.
I was thinking that perhaps once a zone valve is set for 'heating' in the water kit configuration, it gets opened for any heating requirement - the TRVs calling or the UFH zone?

thoughts?
Matt

[HenGus]

That depends, or so I was told, on what is needed by the boiler to ‘inform’ the boiler of a hot water demand. Atag requires a 3 way diverter valve. This was a compromise/ cheaper way of avoiding the need to re-pipe the heating system from an original ‘S’ Plan.

[bruce_miranda]

I have pump over run set to 20 mins which matches my maximum wait time, so that the pump never switches off unless there is absolutely no heat demand. Also 20 mins is enough time to have the heat slowly ramp down.
The OT bridge assumes all boilers are Heat Only, so having studied the eBUS messages in detail, I can confirm they never send the HW demand flag which would make your d.22 show as 1. The only way to do HW priority is by wiring the BDR91 and the motorised valves appropriately. OT Bridge goes from 10 no demand to 90 full demand. Some Vaillant controllers start from 0, some go from 10.

[mdurkin]

I have pump over run set to 20 mins which matches my maximum wait time, so that the pump never switches off unless there is absolutely no heat demand. Also 20 mins is enough time to have the heat slowly ramp down.
The OT bridge assumes all boilers are Heat Only, so having studied the eBUS messages in detail, I can confirm they never send the HW demand flag which would make your d.22 show as 1. The only way to do HW priority is by wiring the BDR91 and the motorised valves appropriately. OT Bridge goes from 10 no demand to 90 full demand. Some Vaillant controllers start from 0, some go from 10.

Thanks - very useful - I've seen mine set between 10 and 90, with a few values in between. The house is cold so it's mostly just going full throttle atm. I have seen some middle values (ie opentherm calling for heat) when no BRD91 is open - in other words no water / heating / UFH zone valve open. That seemed a bit odd to me, though perhaps it was just a temporary state.

Any thoughts on the heating zone BRD91 coming on when the UFH BRD91 comes on - is that expected? It's not the end of the world as most rads have HR92s fitted, so little heat will be wasted, I just wondered if it was normal.
Cheers,
Matt

[mtmcgavock]

Hi - yes I think I could wire with a couple of options to achieve hot water priority, power the other two BRD91s (UFH and heating) from the C port on the Hot water BRD91, or I think similarly there's a 2 way switch in the motorised valves that I could wire in a similar fashion (I think) - just wondered if evohome could just implement this in logic - pretty easy - was a bit surprised not to see it.

One other thing - my UFH is just off a BRD91 - just a zone valve. When the UFH comes on evohome opens the UFH zone valve as expected, but it also opens up the heating zone valve. Is this expected? I don't have the heating BRD91 double bound - I cleared it and rebound. Unless perhaps it's in the evohome controller where the binding is incorrect.
I was thinking that perhaps once a zone valve is set for 'heating' in the water kit configuration, it gets opened for any heating requirement - the TRVs calling or the UFH zone?

thoughts?
Matt

Yes I have the same problem, although with a towel rail circuit. I have it set up in the same set up as you, when my Towel rails call for a demand the heating zone & Boiler Relay BDR91 calls for a demand also. Although in my case this isn't an issue as all my radiators have HR92s, so therefore the heat can't go anywhere.

I'm suspecting that the update later this year which allows you to have an 'Electric UFH' zone will just be the ability to open the BDR91 for that zone and not request a heat demand from either the heating zone valve or boiler relay. Which in that case would be brilliant to solve this issue (As you can then just use the S/L from the Microswitch to fire the boiler instead of boiler relay - however this would only be the case if you weren't using OT)

[mtmcgavock]

That depends, or so I was told, on what is needed by the boiler to ‘inform’ the boiler of a hot water demand. Atag requires a 3 way diverter valve. This was a compromise/ cheaper way of avoiding the need to re-pipe the heating system from an original ‘S’ Plan.

I'm not sure whether that would make any difference, seeming really what you are currently doing is the same thing? It's just shutting and closing valves. My way is just the same as yours in theory (One opens and the other shuts). Whether this be with a boiler relay or OT bridge.

However i'm unfamiliar with Atag, so unless they have some form of HW demand box (Like Viessman to go to max mode) i'm not sure what difference it would make.

[bruce_miranda]

Any CH demand will open the CH valve, be it HR92s or UFH. Hence why you don't really need a CH Valve.

In the Vaillant boiler, the controller send a Heat Demand message which tells the boiler what it is heating CH or HW. If you don't use a Vaillant controller the boiler works in CH mode only. The OT Bridge + VR33 never set the HW demand message. To work around this I use Domoticz + an eBUS interface that I built to provide a higher flow when heating HW and lower when doing CH only. Not ideal but works well.

[bruce_miranda]

I too saw the OT Bridge fire the boiler in advance of the zone valves and didn't like that. Hence now my CH Valve is wired to the pump, which always comes on before the boiler is fired and stays open even during pump over run.

[mdurkin]

I too saw the OT Bridge fire the boiler in advance of the zone valves and didn't like that. Hence now my CH Valve is wired to the pump, which always comes on before the boiler is fired and stays open even during pump over run.

I’ve got to be honest, I’m pretty unimpressed with this behaviour. It’s not just in advance of the zone valves, it does it after the valves are closed as the zones shut down in the evening - I watched it this evening. It sat there with full demand via opentherm for a while with all the valves closed.

I think my easiest option is to put back the ‘orange’ wire that is the feedback to the boiler (to pin 4 from memory) and remove the jumper on 3 to 4. This will shut off the boiler when no valves are open. I probably could find a switched output for the pump, but as my boiler is a system boiler, I don’t really want to looking for something that is designed to be self-contained within the boiler.

I’ll probably put this back in place. Later when i get time I might wire a hot water priority - right now I see little modulation of heat, so it’s heating the water effectively. I thought I had read that evohome modulates up the heat during hot water demand. If that is the case hot water priority would be better so that the tank is heated as quickly as possible before switching back to CH only (and presumably a lower demand via opentherm). Call me a cynic, but does this actually work (different opentherm temperature demand when there is hot water heating requirements)?