Issue with Vailant Opentherm

[Bluestraw]

I'd appreciate some help on the following issue I'm experiencing please...

My boiler is a Vaillant Ecotec Plus 637 (system boiler), and I have the Opentherm module installed together with the Evohome Opentherm bridge. My installation is 'S plan' and I have HR92's on all radiators, yet I also have both CH and HW zone valves in order to enable HW priority.

The issue I have is that often the OT bridge is 'on' and the boiler is firing, even when no zone valve is open. Typically during an evening, everything will start off okay, and then the boiler will modulate the temp right down to maintain temperatures. One by one the HR92s will close off and eventually the CH zone will close due to no current heat demand. However, the OT bridge will still be asking the boiler to produce heat, albeit at at low flow temp (e.g. 47 deg). Because the zone valves are closed, the boiler will quickly overshoot the target temp as the water flows round the bypass loop, and then will turn off. This cycle continues and the anti-cycle lockout time grows. At some point a HR92 will call for heat again, yet the boiler is 'stuck' for a long time on an anti-cycle time, so the whole process becomes very inefficient.

I realise the ideal way to install OT is without a zone valve on the CH, but I don't see any other way to set up HW priority. In any case, even without a zone valve it looks like in my case the OT bridge is calling for heat even when no HR92 needs it, so even without that zone valve it'd be sending water unnecessarily round my heating system, albeit without the over-shooting / lockout that currently happens.

Would appreciate any advice on how to manage this? Also within about 1 month I will be extending the system as our extension is finished, and most likely will need a second evohome controller. I read the other post that said that OT control is still possible and would appreciate some advice on how to set this up.

Many thanks in advance!

Mark.

[Richard]

Top Brake would be best to comment here as he has a similar setup!

[top brake]

Hi

You're mixing control strategies here. Evohome does not offer a full integration for priority hot water so I'm not sure what you hope to gain by having an S Plan configuration with an OT bridge.

However it is possible to configure this, you'll need to take the feed from C on the DHW BDR to A on the heating BDR.

I would recommend removing the HEATING BDR and zone valve and rebounding as a HOT WATER ONLY 2 port valve for the DHW, then you can setup the hot water overrun to dump excess heat into the cylinder
This is the installation type I have at home.

Also need to understand how you've wired the zone valves and what if anything is connected to the boiler other that the OT bridge

[Bluestraw]

Hi

You're mixing control strategies here. Evohome does not offer a full integration for priority hot water so I'm not sure what you hope to gain by having an S Plan configuration with an OT bridge.

However it is possible to configure this, you'll need to take the feed from C on the DHW BDR to A on the heating BDR.

I would recommend removing the HEATING BDR and zone valve and rebounding as a HOT WATER ONLY 2 port valve for the DHW, then you can setup the hot water overrun to dump excess heat into the cylinder
This is the installation type I have at home.

Also need to understand how you've wired the zone valves and what if anything is connected to the boiler other that the OT bridge

Thanks for offering to help! That's exactly how it is set up - i.e. feed C to feed A. Richard (I think) from evohomeshop helped guide me on this. I'm a fan of hot water priority as my theory is that it can heat the water quickly at a higher temperature before falling-back to heating mode, whereas if both CH and HW are on together without this I'm not sure it would be so efficient?

Nothing is connected to the boiler aside from the OT bridge. Zone valves are wired directly to each BDR. But that goes back to my original question - even if I remove the heating BDR, surely the boiler is firing 'too much' if it's doing so today when the CH BDR is closed?

[top brake]

Sounds like the boiler is modulating so nothing fundamental wrong but I would bin the heating zone valve and this will let the boiler pump the water round the primary even when there's low load heat demand. The low load operation is a very interesting area and it's best to let the OpenTherm run free without an on off zone valve in the mix. Also set the pump from eco to comfort. Simple and fully modulation is best. If your worried about concurrent hot water and heat operation then schedule the cylinder timings during heating setback.

[Bluestraw]

Sounds like the boiler is modulating so nothing fundamental wrong but I would bin the heating zone valve and this will let the boiler pump the water round the primary even when there's low load heat demand. The low load operation is a very interesting area and it's best to let the OpenTherm run free without an on off zone valve in the mix. Also set the pump from eco to comfort. Simple and fully modulation is best. If your worried about concurrent hot water and heat operation then schedule the cylinder timings during heating setback.

Guess I can just 'lock' the heating zone valve open and re-bind the system without a CH valve, right? Saves me getting a plumber here...

I figured that having the water run around even with no demand was an odd functionality but suppose if it's at low heat levels then maybe there's some logic to have it 'ready' for a radiator when it opens a valve. Will also check the pump setting, can't remember what it's at right now.

So... once I do all this, what else would it take to add another evo controller in to the mix in future?

[top brake]

Yes just latch open and rebind accordingly

You can simply bind another evohome to the OT bridge.

[Bluestraw]

Thank you so much for the help. Will do just that.

Just two (hopefully final) questions - what difference does it make setting the pump to Comfort vs Eco. I have changed it per your suggestion but not sure exactly what it's done? Also, do I really need the overrun setting for HW since it's got a whole loop of heating to dump heat into if needed?

[top brake]

Pump to comfort keep the pump running at low demand making use of the residual heat in the flow and meaning that when the combustion does kick in at low load it runs for longer

When there is absolutely no heat demand the pump switches off

[top brake]

Hot water overrun gives the boiler a 'soft landing' when the cylinder is recharged. You may be able to reduce the DHW set point a few degrees.