Not sure myself to be honest.Not sure I agree with this.
If you install a 2 channel programmer and make sure its wired correctly then your statement isn't relevant.
Or have I missed something?
But here's my rationale.
I have looked at the S-Plan wiring diagram and timer and RF receiver are still wired in parallel so they will still be able to operate independent of each other.
You can't actually wire them in series because they are both rated 240 VAC and you only have 240 VAC available.
If they were in series you could consider the timer to be in overall control because there are four permutations for two switches only one of which (ON + ON) allows current to flow. So when the timer is off everything is off. This has been the basic perception of the owner, not unreasonably, until I explained it to her.
When they are parallel the RF receiver can override the timer as it does at the moment.
If you remove the RF receiver from the picture, and just have the new timer, you can switch the HW and CH on and off independently with the new timer unit. CH only, HW only, CH + HW or neither.
But regardless of which state the timer is in when you bring the RF receiver back into play it can request heat from the boiler
independently and switch it on. As it does at present. Irrespective of the timer status.
So the way I envisage it going when the new timer is fitted is to leave the CH permanently off on the new timer ie: HW only and using the programmable RF receiver to control the CH status all year round switching it off in summer.
That's the plan at least. I will monitor it as usual and see how if it behaves as I anticipate.
This is unlikely to happen until summer.
At the moment, weather being what it is, both CH and HW are in demand and if anything goes wrong it might as well be
in summer when you don't need CH.