I have a W-plan system with an On/Off boiler, and I’d like to add variable flow temperature. I envisage something like attached sketch, where the pipestat is manually adjusted depending on outside temperature. That’s quite straightforward to install.
Before my present fancy boiler and controls were installed, which allow separate temperatures for HW v CH, I had the same idea for my condensing, but none modulating boiler.
What I found was, it was a total waste of time.......
The ordinary mechanical/bimetal stat, had far too wide an hysteresis (switch on, versus off point), that it would switch off on rising temperature, then the house would cool far to much, before it would click back on. What it needed, was a much more precise means of switching, such as could be provided by an electronic sensor, which had built in adjustable hysteresis. I had several such modules around, but never got around to employing one, before that boiler needed replacing by my present one.
Set the hysteresis too tight, and it would involve extra wear and tear on the boiler, too wide and the house cools too much. Best of all, i proper, separate temperatures, with modulation, and even better, the boiler being aware of how near it is to achieving the desired temperature, so it can throttle back it's output of heat.