Ian
You are unlikely to find RS232 interfaces on domestic heating equipment - it tends to be based round very simple mains switching components sequentially diverting energy to the required components, everything is either on or off.
In general the timer allows power in at the appropriate time to the stat (room or cylinder), the stat allows power to the motorised valve if the temperature is below its setpoint, the valve motors open and closes a pair of contacts allowing power to the boiler and pump, boiler cycles on and off under its internal stat until any one of the previous links is unmade. Cheap, robust and simple as....
For UFH the motorised valve is substituted with a 'wiring centre' that performs the same boiler interlocking function
Temperature control is provided by 'pulse modulating' the sytem! This makes it very easy to use a brick to provide interface at any point - particularly the on/off control of zones from either one wire temp sensor or a trad thermostat - low voltage zone valve actuators and stats are available. This applies to UFH, rads and DHW.
All the domestic underfloor installs I've seen (and it's quite a few) use simple wax capsule actuators (24 or 240V ) connected to a local 'wiring centre' Simply connect the appropriate DO of the brick (normally 0-3 [ Andy is 24V ennough for the brick gate drivers?]) into the thermostat terminals of the UFH wiring centre. If you wish to use the brick as an overide to existing controls then put it in series/parallel using DO 4 or 5 for the changeover contacts. Mange the brick output via the scheme of your choice.
Looking art systems as a whole, particularly with the goal of fuel ecomnomy: in the comercial world and on the continent far more sohisticated controls are the norm. These generally modulate the output of the heat genration plant (boiler) to only deliver what is needed for the job - downside is that they do it with valve gear and pumps - bulky and expensive.
In recent years (with the widespread popularity of the combi) the modulating burner has arrived in domestic sized packages (in the UK) and this technology is starting to be used by heating controls.
The two significant players in the UK market are Viessman and Vaillant (both German!!) who provide 'external' control modulatable boilers and modulating stats for domestic sized equipment that are acceptable to the lady of the house. The downside from our point of view is that the comms are proprietry and as such cannot be interfaced with also until Vaillant release the VR63 extender module in the summer the system does not accomadate multiple heating zones.
The good news - there is a standard called Opentherm in use/development to allow a common comms system and hence mix and match of different suppliers equipment and hence allow us to interface. The standard is starting to gain traction and there are a number of Opentherm units out there if you dig a little. Currently you have to buy into the standard but it will only be a matter of time till the details are out there.
The British manufactureers are of course all busy trying to ignore it - this is probably because the British public are so 'careful' when it comes to such things and the commerial custommers are already well catered for with Lonworks, Coster Cbus/ring and the like.
Alternative is to look at something like Heatmiser and use their networkable stats, add thier net monitor with its browser based interface.....
If you are ever doing a larger system then Coster bears a look and can be linked to the outside world
Simon (Willett)