Combi boiler with existing 2 zone heating

[clockworks]

I'm removing the Evohome kit from mum's house as she is moving next week (mum bought the kit from The Evohome Shop a few months ago). I'll be reinstalling it in the new house, which has a combi boiler and two heating zones.

I've had a good look at how the "new" house is set up. Ideal combi in the kitchen, 2x 2 port valves in an upstairs cupboard, 2x Danfoss programmers. One zone does a single radiator in the lounge, the other zone does the rest of the house.

The programmers are connected, via a wiring box, to the zone valve actuator motors. The switches in the 2 valves are commoned together, and a single cable goes down to the "stat" terminals on the boiler. So, the programmers open their own zone valve, and either or both zone valve(s) fire the boiler.

My original intention was to connect a BDR91 in place of each programmer at the wiring box, much as you would with an S Plan setup (she is moving from an S Plan, so has two BDR91s). I'm not sure that this would work though, as the instructions don't seem to mention binding HR92s as the sensors in an existing 2 zone setup?

Would this be overcomplicating it anyway? Would it be better to disconnect the zone valves completely (electrically, removing them from the pipework isn't an option as the house is rented), and use one BDR91 as a boiler relay? I assume that the zone valves can be manually locked open. All but one of the radiators will have an HR92 fitted.

[Richard]

Yep, HR92's on all rads and a single BDR91 as a boiler demand relay is the way to go here. What model of boiler is is? If its an Ideal Logic or Vouge these are OpenTherm compatible...

[clockworks]

Thanks.
Yes, it's an Ideal Logic combi - I'll have to google OpenTherm.

I assumed that the boiler would have an internal bypass valve, but the chap who came to check out the heating system before the move insisted that it should be left with one open radiator (despite it already having mechanical TRVs on all the rads when the house was built).

Having thought about disconnecting both 2 port valves, it's been suggested elsewhere that I leave them connected to the boiler, and use the BDR91 to switch the valve motors. I can see the logic behind this, as it would mean I don't have to touch the wiring to the boiler itself (rented property with annual gas safety checks), and the valves would be cycled (so not seize up). I assume that the relay in the BDR91 can handle the load of two valves?

[Richard]

I assumed that the boiler would have an internal bypass valve, but the chap who came to check out the heating system before the move insisted that it should be left with one open radiator (despite it already having mechanical TRVs on all the rads when the house was built).

Having thought about disconnecting both 2 port valves, it's been suggested elsewhere that I leave them connected to the boiler, and use the BDR91 to switch the valve motors. I can see the logic behind this, as it would mean I don't have to touch the wiring to the boiler itself (rented property with annual gas safety checks), and the valves would be cycled (so not seize up). I assume that the relay in the BDR91 can handle the load of two valves?

OK, so firstly its an S Plan Plus system currently so by the very nature of the system it needed an Automatic Bypass Valve before evohome was ever considered (where does the water currently go when the zone valves close?).

As for leaving the motorised zone valves opening and closing using relays under BDR91 control, I certainly do not recommend this as it will shorten their life expectancy... Much better to electrically disconnect and leave manually open in my experience and will result in a smoother experience.

[clockworks]

OK thanks. I'll stick with disconnecting the valves.
Good point on the bypass. I guess the heating chap who did the inspection was just playing it safe.