My current heating hot/water setup is a 25 year old boiler (in good order, no plans to replace), a Randall 103 old-school timer, the water is gravity fed with big cold tank in the loft and a yellow cylinder in the airing cupb oard, a circulation pump, NO motorised valve and every radiator has a therm ostatic valve.
This is unsatisfactory as the Randall offers no flexibility at all, and the minimum run time for each on/off cycle is about an hour, and throughout th at on cycle the pump runs continuously.
I wish to install a nice Nest thermostat and have total control over the ho t water and heating and get rid of the stone-age Randall timer clock. I alr eady know that I will need to install a motorised valve in the airing cupbo ard plumbing.
The installation guides for Nest concentrate on simply replacing an existin g stat, which I don't have, so I'm a little unsure on what will need doing.
I'm after some advice on what will need wiring and how.
IMHO, best to change to a three way valve where the pump is used to do the hot water cylinder too. Allows easy control of both heating and hot water independently. Will also likely give faster re-heat of the water.
Going to need a fair bit of modification no matter what.
My cast iron lump (Ideal RS440) is older than the O/P's, it has a SEDBUK rating of 78.5%, most new (e.g. Worcester Bosch) condensers seem to be about 90.3%.
The difference is nowhere near double, for an 11.8% improvement I also have no plans to replace.
Let's consider the traditional approach and then refine it to add a bit more automation.
In order to get independent control of the heating and hot water, the solution requiring the least changes to the plumbing is to convert your system to a C-Plan system.
This requires a 2-channel programmer, a 2-port motorised valve with
3-terminal change-over auxiliary contacts in the HW circuit, a room stat to control the heating and a cylinder stat to control the domestic hot water temperature. See
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The wiring is very cunning, and you must stick religiously to the valve wiring shown in the diagram - otherwise it won't work.
So that's the basics. If you wish, you can combine the functions of the CH timer and room stat by using a programmable thermostat - either wired or wireless.
You can almost certainly use a Nest device instead. You mention that the instructions talk about replacing the thermostat. You need to interpret that in terms of where the thermostat would have been in a straight-forward C-Plan system. I don't know whether Nest can control the hot water too. If so, that part of it would presumably replace the cylinder stat.
Having said all that, C-Plan is a bit of a compromise. It's a lot better than you currently have since it provides independent control of HW and CH *and* provides boiler interlock - which means that it doesn't waste energy keeping the boiler and/or pump ticking over when all demands are satisfied.
Depending on how the plumbing is currently arranged, you *may* be able to convert your system to an S-Plan instead. This would mean that HW and CH were *both* pumped - enabling the HW to heat much more quickly rather than having to rely on gravity (thermo-syphon) circulation. To do that, you'd need a single hot outlet from the boiler, passing immediately through the pump before being split into two circuits. Each circuit would need its own 2-port motorised valve.
Yet another alternative is a Y-Plan - which is also fully pumped, and uses a single 3-port mid-position valve rather than two 2-port valves. The cylinder stat wiring is a bit more complicated for Y-Plan, and I don't know how easily you could "Nestify" it.
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