Does anyone know if the Drayton Digistat employ proportional control (I
think that's what it is referred to as). e.g. if I have the temp set
point at 22, below that it remains on and above that it remains off but
when the room is up to 22 the stat cycles the boiler on/off, the length
of time between cycles being dependent on the rate of temperature
Don't know about that particular model, but the mechanism you describe is
not how my Danfoss unit works. When heating up, it will shut off 0.1C over
the set point (0.1C is its minimum tolerance). As the temperature falls,
*no matter how fast*, it cycles the boiler in proportion to the temperature
shortfall, in a 2C band. So, if it is 1C low, you get 50% firing. In
practice, it checks the temperature every 30 seconds, and it cannot fall
quickly in that time, so the first on cycle is 5%. The next will typically
be 10%, and so on until equilibrium is reached. Personally, I think a more
sensible logic would be to start to progressively cycle the boiler off
during warm-up so that you would get full heat, gradually reducing to just
enough heat. By overshooting, you get full heat, then virtually no heat,
then a bit more until the same equilibrium is reached (probably slower). I
can definitely *feel* the times when the boiler is not on enough.
A similar logic is used in the predictive warm-up feature. I tell the unit
whether the boiler can achieve a 4C temperature rise in 30/60/90/120 mins,
and it wakes up that much earlier and examines the actual difference. The
real switch-on time is determined by what proportion of 4C the actual
temperature shortfall is. If it is 2C short, the boiler will come on in
15/30/45/60 mins, for example. Some units actually "learn" how fast the
system responds, so you don't have to.
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