Jim H wrote:
> you need a sensor in the floor as well
> as in the room and when the floor reaches a certain temperature, the
> system turns off so the floor doesn't get so hot that the room temp
> overshoots significantly.

While this may be desirable you don't actually need it. I've just moved
to a flat in a newly built house and had my first experience with floor
heating. It took me a bit to understand how the current system works.

I have a bimetal swith with a large hysteresis. It switches on at about
the nominal temperature and off about three degrees (six in Fahrenheit)
higher. There also is a heating resistor in the case making it switch
off in about an hour or so even if the outside temperature does not
rise.

According to regulation textbooks this setup switches on an off about
ten times as frequently as a highly overshooting setup using a simple
switch without hysterisis would and the amplitude of the swings drops by
a similar amount. The downsides are:

1) The offset between nominal set point and actual average gets larger.
This is easily compensated for by moving the scale for setting the
temperature. (As water temperature follows outside temperature, the
difference between the regulated inside temperature and that reached
with heating full on stays about constant, so the regulator stay inside
a limited parameter space.)

2) The system takes longer to follow large changes in the desired set
point.
As this is rare, you can compeksate manually by setting it higher on
coming home and turning down after a while.

It still means that emulating this cheap standard off the shelf unit in
software can get quite tricky for someone who has not had any trainig in
how to do it.


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