The Nest suffers from the same issue that affects
every single (per zone) thermostat - it's trying to control several distinct areas (rooms) based on measuring the temperature in one place.
For heating control, you ideally want individual control of each radiator. There are now several option for fitting "intelligent" heads to all the rads - some are locally programmed, some have central control via wireless link. With the right choice of system, you can have individual control over each room in the house.
Personally I'm waiting to see what comes out of the
OpenTRV project.
Next step is to push the water round with a modulating pump (eg Grundfos Alpha). I did this in my flat and the result is a very quiet system as the pressure drops as the valves shut, rather then the normal pressure rises and the valves "hiss".
But you can't combine this with running directly off the boiler - boilers have minimum flow requirements (more so with combi boilers that are normally grossly oversized for the heating load). So you need a buffer tank to act as a neutral point - separating the variable flow rate round the heating from the fixed flow rate requirements of the boiler.
Should I ever get round to that stage of the project, I was also looking at controlling the flow temperature with an electronically controlled mixing valve - so rather than have a trickle of very hot water, the system could always have a reasonable flow of "suitably warm" water. Feedback from the individual rad valve positions deteriine if we need to crank up the temperature or can let it drop.
And of course, we can feed this back a stage and control how hot we heat the tank with the boiler.
The ultimate buffer tank would be a thermal store giving you mains pressure hot water without the restrictions imposed by a pressurised cylinder (aka unvented DHW cylinder).
It's also a convenient way to combine sources should you have the opportunity to add (say) solar thermal panels or wood burner.
As I said, I did the "all rads have TRV, plus modulating pump" as part of a thermal store install in my flat (which I rent out). I
wrote it up over on the Navitron forums. You'll notice the dates put the install during that really flippin cold spell at the end of 2010 - it was "pretty cold" working on that with no heat available.
