S-Plan vs Y-Plan for Evohome ?

[dty]

I'm clearly missing something. I'll have to give it more thought.

[DBMandrake]

Yes, part of it could be done in the controller firmware, but not tying it in to the boiler overrun like that.

Indeed. If it was done in software it wouldn't use the pump overrun, since the Evohome isn't aware of the pump overrun or its current status. That was more of a convenient hack on my part to avoid adding yet another timer and make use of the one already there, also based on the fact that opening the valves when the pump is not running is not useful anyway.

However the Evohome does know if there is a non-zero heating demand at all times - even during the time periods where TPI would have cycled the heating zone valve closed. So it only needs to be programmed so that if the system configuration is S-Plan with THREE relays or S-Plan OpenTherm with TWO relays, make an additional "Boiler TPI only" configuration option available to the user, (can't think of a good name for it off hand) and probably make it the default as well.

When chosen, TPI of the heating relay is disabled when the hot water relay is off, and only allowed when it is on. To fully emulate what I've done with my wiring it would also require a user adjustable "heating overrun" setting in minutes - similar to how a three relay configuration currently gives you access to a hot water overrun setting.

The purpose of that would be to also keep the heating zone valve open for X number of minutes after all heating demand ceases (but only if there is no hot water demand of course) to let latent heat dissipate, rather than only keeping it open during TPI cycles. You would set this setting a bit longer than your pump overrun so that the zone valve remained open for a while but ultimately closed when the system was not in use.

In theory this would only do something useful if you had one or more uncontrolled or manually controlled radiators, but I think more people than not probably DO have an uncontrolled radiator still in the system so keeping it open for a few minutes would allow the latent heat to be transferred to that radiator instead of being wasted in the boiler, and allow the boiler to cool down more quickly on system shutdown.

The only downside I can see is that it's a more complicated install for a sparky. With the standard wiring the BDRs effectively just replace normal boiler controls. This now goes way beyond "connect the green wire to the brown wire".

Yes - a paint by numbers sparky would probably find it a bit confusing to trace out the circuit, although with a circuit diagram in front of them they should understand it. It's only two small modifications on top of hot water priority wiring which I believe is in use in some installations.

But the real answer is to provide the functionality in software - then the standard relay wiring can be used and wired up without any confusion. Probably wishful thinking on my part given the lack of concrete new features or meaningful updates in the last year and a half though. I'm just happy that I could achieve the desired result without any additional components by just doing a little bit of clever rewiring.

[bruce_miranda]

So how would things need to change in case you have OT controlling the boiler. Does Evohome control the CH valve with TPI even with an OT bridge?

[DBMandrake]

So how would things need to change in case you have OT controlling the boiler. Does Evohome control the CH valve with TPI even with an OT bridge?

I believe so yes - but not having access to an OpenTherm system I don't know for sure. Someone with OpenTherm + 2 BDR91's would need to verify it.

[bruce_miranda]

I do have OT and 2 BDR91s, which were previously controlling the CH and HW valves in a simple S plan configuration. I recently changed the wiring to use the Pump overrun to control the CH valve and got rid of the 2 port configuration and disconnected the CH BDR91. Now my CH valve is open whenever the pump is running.

[HenGus]

I do have OT and 2 BDR91s, which were previously controlling the CH and HW valves in a simple S plan configuration. I recently changed the wiring to use the Pump overrun to control the CH valve and got rid of the 2 port configuration and disconnected the CH BDR91. Now my CH valve is open whenever the pump is running.

Boiler tech support teams tell me that the preferred configuration is 'Y' or 'W' - not a modified 'S' plan if HWP is required. I am currently looking at an installation quote which retains the HW zone valve but replaces the CH valve with an inline ON/OFF valve. When HW is required, the HW valve opens which, in turn, closes the CH on/off valve. A 'Y' system in all but name.

[jvallis]

I do have OT and 2 BDR91s, which were previously controlling the CH and HW valves in a simple S plan configuration. I recently changed the wiring to use the Pump overrun to control the CH valve and got rid of the 2 port configuration and disconnected the CH BDR91. Now my CH valve is open whenever the pump is running.

Sorry for reviving an old thread, but it's a rather important one! I will be installing evohome in a set up very similar to yours: ie S-Plan with OT on a Vaillant 637 and VR33.

Since I won't have HR92's on the 4 towel rads, I will need to use the CH valve via a BDR, but can you explain the wiring for the Vaillant's integrated pump back to the CH valve as I can't figure it out at all...

[bruce_miranda]

Also think about what happens when the Boiler runs on its own for cycles like Frost protection. You will need to make sure you have a water loop before any zone valves via a ABV or similar.

[mtmcgavock]

Sorry for reviving an old thread, but it's a rather important one! I will be installing evohome in a set up very similar to yours: ie S-Plan with OT on a Vaillant 637 and VR33.

Since I won't have HR92's on the 4 towel rads, I will need to use the CH valve via a BDR, but can you explain the wiring for the Vaillant's integrated pump back to the CH valve as I can't figure it out at all...

As you're using the OT receiver and not any S/L you'll not need to use the Orange/Grey wires which would usually fire the boiler as the OT relay will do this for you. When the boiler fires, the pump will run and over run as required.

Also think about what happens when the Boiler runs on its own for cycles like Frost protection. You will need to make sure you have a water loop before any zone valves via a ABV or similar.

The Vaillants have an integrated ABV in the boiler. Frost protection will kick in and warm the water in the boiler to 30oc when the water drops to 5oc from memory. Obviously this doesn't include protection for any of the CH system just the boiler.

[jvallis]

As you're using the OT receiver and not any S/L you'll not need to use the Orange/Grey wires which would usually fire the boiler as the OT relay will do this for you. When the boiler fires, the pump will run and over run as required.

That's the issue - DBMandrake found a way by linking the CH valve to the pump timer, so as to allow any heat during the pump overrun to be dumped into the radiator circuit. With the Vaillant specifically, there's a mysterious setting in d.27 and d.28 that sets 'switching accessory relay 1|2 in the accessory module' to 'circulation pump' or 'ext. pump', but I have no idea what this 'accessory module' is and where the wiring terminal is... I was hoping someone had figured this out in using a Vaillant system boiler with an external pump as they're a pretty well known boiler...

EDIT: Found it. It's a VR40 '2 in 7 module'. £29. And a lot cheaper than 4x more HR92s on the towel rads to do a full-on Fig 4. I will report back with experience of using both a VR33 and VR40, once I actually install it.

The Vaillants have an integrated ABV in the boiler. Frost protection will kick in and warm the water in the boiler to 30oc when the water drops to 5oc from memory. Obviously this doesn't include protection for any of the CH system just the boiler.

evohome is a defacto frost stat too. It will never allow any zone to go below 5C iirc.