Ultimate Central Heating Thermostat

[REB.Barnett@xxxxxxx]

Winter is nearly here, and I have a spare 5 minutes, so I've started on the
spec for the house thermostat I've always wanted (no commercial product I
can afford does what I want). My requirements are moderately sophisticated,
and there's a chance what I want won't work, so if anyone has any comments
or experience to share I'd appreciate it.

*Multi-zone via Dallas one wire temperature monitors
*Predicts rate of rise and fall of house temp in order to anticipate when
to
run boiler
*Specify house warm by time and warm until time
Support for multiple programmes - week mode, weekend mode, alternate week
mode
Display to show current temp, set point, boiler on/off
Buttons to override on/off, adjust set point
RJ45 network connection to provide web interface to thermostat and Comfort
Zone inputs from Comfort to permit occupancy/alarm/telephone control
Zone outputs to allow 12v control of boiler (e.g. via Comfort)
*Additional outputs to allow independant heating of other rooms as required
Serial input from Comfort, to permit Comfort to use thermostat web
interface

I've asterisked a couple of the features I think I want, because I wondered
if anyone on the list has any experience of getting these sorts of things
to
work. My current thermostat does a reasonable job of anticipating the rate
at which the house heats and cools, yet often seems to let the temperature
fall 1/2 degree too low before switching on. I'm thinking of implementing
some sort of weighted average of temp increase rates held against ambient
temperature, so the system will know that if the current temp is 18C, it
will take x mins to reach 20C. That way I can set it to have the house at
20C by 7:30am, and on colder days the system will come on earlier to
compensate.

I'm keen on making it multizone simply because my current single zone
system
is easily fooled by heat from external sources - e.g. coal fire in lounge,
oven in kitchen. I'm not sure how best to implement multi-zone. One plan
would be to have one 'current' zone that the thermostat attempts to
maintain
at the set point. The trouble with that though might be that different
rooms
need different set points depending on where the temp probes are located.
Also if the 'current' zone were to be switched based on monitoring zone
activity from Comfort, it might make the 'rate of temp increase' algorithms
a touch tricky.

Another solution might be to have the system to keep an eye on all zones.
If
some start escalating out of control (e.g. lounge and kitchen temps soar),
but other zones remain in keeping with established parameters for rate of
heating and cooling, then perhaps the thermostat could selectively ignore
zones once it determines it has lost control of their temperature. Some
safeguard would be needed to ensure that once any zone reached a maximum
safe set point the heating is shut down on the assumption that 'a fault has
occurred'.

Finally, the conservatory is on the main central heating system, but
naturally cools more quickly in the winter when the heating is off,
producing a draft into other warmer rooms. To combat this, we use a
convector heater to supplement the central heating in that one room. If the
thermostat were set to associate an X10 address with a particular zone, it
could ensure that zone was maintained warm when the main system was off.
Not
sure if this makes it all too complicated though.

Ray Barnett
Systems Developer






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