On Sun, 01 Oct 2017 13:48:40 +0200, in
, Axel Berger
 wrote:

>"Michael J. Mahon" wrote:
>> > I meant 0.000012 Hz.
>> Well *that* makes all the difference!  ;-)
>
>The point of PWM is not for the frequency to be high in any absulute
>sense (there is none) but too high for the system in question to follow.
>For floor heating a cycle length of about two hours is PWM and it is
>what el cheapo wall thermostats with internal feed back enforce.[1] The
>cycle length with an overshooting on/off thermostat would be more than
>ten hours. If you are faster than the natural cycle by an order of
>magnitude, then by definition what you get is PWM.
>
>[1] The valves don't like being switched too often. Ten times higher
>frequency means ten times lower life expectancy and would become rather
>a nuisance.


With on/off control and a situation like that, common practice is to
establish a "deadband" around the desired point and turn the system on
when below the low value and off when above the high value.

In the case of infloor heating, 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.

--
Jim H

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