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I've been struggling to configure my sundial S plan plus in Evohome for a few weeks and after much research and reading of many posts on this site I seem to have figured out the best way to do it. So to save others time I thought it would be useful to put this down in a focussed post with an appropriate title that others should be able to find easily. Here goes….
A 'sundial S plan plus' typically has three zone valves, one to control hot water and two to control two heating zones, often upstairs and downstairs. This is the case in our house. The sundial S plan (ie without the 'plus') has one CH zone valve and one HW zone valve. Also the S plan plus can have more than 2 CH zone valves if you have more than 2 CH zones but I'll not consider that here, just the S plan plus with two CH zone valves.
We have just purchased an evohome system including HR92s for all radiators and are in the process of installing the system. The evohome documentation covers how to configure evohome for a sundial S plan but not a sundial S plan plus. So how should it be configured for our S plan plus ?
The first question is how many zones to configure. It seems to me that one should default to one CH zone and only add more zones if some areas need different temperatures at different times. Geography or room type considerations (ie what rooms are close to each other or rooms that have the same function) should not play a part in the zoning plan. Therefore we have divided the upstairs into four zones (bathrooms, front beds, back beds and landing). The downstairs is divided into three zones (kitchen, corridor and sitting rooms).
The next question then is how to configure the evohome system for those zones where the underlying plumbing is a sundial S plan plus system since the evohome does not intrinsically support an sundial S plan plus with three 2 port valves (V4043s). I was hoping the documentation would give some guidance on how to setup a sundial S plan plus but sadly it seemed silent on this. Perhaps this could be added in a later addition.
In the end I realised that a zoned control system such as evohome means there is now far less need to control the two CH zone valves independently since the evohome system will take care of turning each evohome zone on and off as required through the HR92s or other actuator. Independent control of the zone valves was more important in the old setup with mechanical TRVs because they were far less able to control temperature. This meant one needed to control the flow of hot water to them to make sure they were off when they were supposed to be off.
Therefore I have concluded that I should configure the evohome system for a sundial S plan (notionally one CH valve and one hot water valve) even though the underlying plumbing is for a sundial S plan plus system (two CH valves and one HW valve). The question then is how you link evohome into the multiple CH zone valves. I think it is just a case of connecting the switched feed from the CH BDR91 to the motor feeds (brown wire) of both the two V4043 CH zone valves so the zone valves are wired in parallel.
I did think of dropping the zone valves completely and setting up the none HW BDR91 as the 'boiler demand' relay to directly fire the boiler (and the pump) but then thought doing that would mean hot water flowing around the central heating circuit when the CH is off but the HW is on. For instance in summer. That will lead to some inefficiency and so decided to leave the zone valves in place. This would be more important if any of the radiators have mechanical TRVs rather than HR92s.
One of the disadvantages of not having a boiler demand BDR91 is that you cannot configure boiler cycle times and min durations which means you are stuck with the default - 6 times and hour and a minimum of 1 minute. However this doesn't seem like a big loss.
Is that the best way to configure a sundial S plan plus in evohome ?
A 'sundial S plan plus' typically has three zone valves, one to control hot water and two to control two heating zones, often upstairs and downstairs. This is the case in our house. The sundial S plan (ie without the 'plus') has one CH zone valve and one HW zone valve. Also the S plan plus can have more than 2 CH zone valves if you have more than 2 CH zones but I'll not consider that here, just the S plan plus with two CH zone valves.
We have just purchased an evohome system including HR92s for all radiators and are in the process of installing the system. The evohome documentation covers how to configure evohome for a sundial S plan but not a sundial S plan plus. So how should it be configured for our S plan plus ?
The first question is how many zones to configure. It seems to me that one should default to one CH zone and only add more zones if some areas need different temperatures at different times. Geography or room type considerations (ie what rooms are close to each other or rooms that have the same function) should not play a part in the zoning plan. Therefore we have divided the upstairs into four zones (bathrooms, front beds, back beds and landing). The downstairs is divided into three zones (kitchen, corridor and sitting rooms).
The next question then is how to configure the evohome system for those zones where the underlying plumbing is a sundial S plan plus system since the evohome does not intrinsically support an sundial S plan plus with three 2 port valves (V4043s). I was hoping the documentation would give some guidance on how to setup a sundial S plan plus but sadly it seemed silent on this. Perhaps this could be added in a later addition.
In the end I realised that a zoned control system such as evohome means there is now far less need to control the two CH zone valves independently since the evohome system will take care of turning each evohome zone on and off as required through the HR92s or other actuator. Independent control of the zone valves was more important in the old setup with mechanical TRVs because they were far less able to control temperature. This meant one needed to control the flow of hot water to them to make sure they were off when they were supposed to be off.
Therefore I have concluded that I should configure the evohome system for a sundial S plan (notionally one CH valve and one hot water valve) even though the underlying plumbing is for a sundial S plan plus system (two CH valves and one HW valve). The question then is how you link evohome into the multiple CH zone valves. I think it is just a case of connecting the switched feed from the CH BDR91 to the motor feeds (brown wire) of both the two V4043 CH zone valves so the zone valves are wired in parallel.
I did think of dropping the zone valves completely and setting up the none HW BDR91 as the 'boiler demand' relay to directly fire the boiler (and the pump) but then thought doing that would mean hot water flowing around the central heating circuit when the CH is off but the HW is on. For instance in summer. That will lead to some inefficiency and so decided to leave the zone valves in place. This would be more important if any of the radiators have mechanical TRVs rather than HR92s.
One of the disadvantages of not having a boiler demand BDR91 is that you cannot configure boiler cycle times and min durations which means you are stuck with the default - 6 times and hour and a minimum of 1 minute. However this doesn't seem like a big loss.
Is that the best way to configure a sundial S plan plus in evohome ?
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