Swapping out Switchmaster room stat for digital stat. Advice needed!

[SaintH]

I'm thinking about swapping out my old dial Switchmaster room stat for a digital one. I have an open vented system with a Baxi Solo 24 HE boiler controlled by a Drayton 241 programmer which controls CH & HW separately.
I was thinking about going for a Honeywell DT90E which would just have the same function as I have now but have also noticed the CM907 programmable stat which appeals to me as I can set different temperatures at different times.
My question is as I already have the CH set on the timer, will there be any issue with running a programmable stat as well?
I would like the stat to change temp at different times but do not want to use the timer function if that makes sense? I.E the programmer CH is set to turn on between 6pm and 11 pm but would like the stat to set the heating at 18 degrees between 6 & 9 and 21 degrees between 9 & 11.
Could I leave the Drayton Programmer CH function 'on' all the time with the CM907 controlling when the boiler turns on, or would I need to set the programmer and the programmable stat BOTH to the same time to turn the heating on?

 

[stem]

You have some choices here:

1.You can leave the Drayton 241 in place and set it so that the central heating is permanently 'on' leaving the new thermostat to take control of the time and temperature settings.
2. Replace the Drayton 241 with a new single channel timeswitch for the hot water time control only and install the new programmable room thermostat to control the central heating.
3. Replace the Drayton 241 with a new controller that has a remote programmable room thermostat and can also provide control of the hot water timings.

You don't give details of what the existing programmer / thermostat are actually controlling, so I'm assuming your system has either 2-Port motorised valves or a 3-port motorised valve controlling it. Systems without motorised valves don't usually allow the heating to be 'on' without the hot water also being 'on'. So, some careful coordination of the timings would be required with options 1 and 2. [The Drayton 241, can be configured for these older types of system so that the hot water automatically comes 'on' when central heating is selected 'on'] However, if you can presently have the central heating running on its own without the hot water being 'on' also, then that isn't a problem for you.

 

[SaintH]

Thanks for the info. I have a Honeywell 3-port motorised valve in my system and can set the HW & CH independently, so I am hoping to go with option 1.

 

[stem]

OK, that's fine.