On Sun, 08 Feb 2004 13:26:54 GMT, Harry Bloomfield
wrote:

Set Square wrote on 08/02/2004 :-
This seems to be working, but I'm curious to know what algorithm it uses in
order to decide when to turn on the heating. The literature doesn't make
this clear, so I wonder whether any of you actually know?

20 years ago I designed one of these controllers and the next stage in
its development was to include the facility which you mention. The
whole idea was well ahead of the market at that time, so I was
struggling on new ground.

I got as far as working out a rough algorithm. It went something
like...

Starting with a basic programmed in setting, then incrementing or
decrementing preheat time value, based on whether it attained the
temperature before or after the set time. Which equates roughly with
your last suggested method.

It was all very experimental. There was a theory going around, that in
order to achieve this function, that an outside temperature sensor
would be needed to enable the calculation to be done satisfactorily. I
wonder if your unit includes an outdoor temperature sensor?

I've done some work on this as well. Working purely on an internal
reading and time, you can make a first approximation assumption that
heat loss and thermal inertia effects are linear with time. That is
roughly true and better than doing nothing. It becomes better if the
controller can gather data and average over several heating sessions.

The CM67 type of controller doesn't use outside temperature sensing
but some of the sophisticated boiler controllers have outside sensors.

On mine, there is an outside sensor which goes on a north facing wall
out of the sun and gives the boiler a temperature signal based on
resistance. There is an inside controller which senses temperature
and hooks up to the boiler with a pulse width modulated digital code,
in effect providing an analogue temperature value.

It is possible to set the proportional bandwidth and the relative
sensitivities to each of these terms, with data being accumulated over
several days. The boiler output is then linearly modulated from 3kW
to full power by controlling the fan speed (influences gas input rate)
and pump power level. The slope and end points of the boiler
temperature vs the environment temperatures can be varied as well.

The controller behaviour can be logged to a PC so I can look at the
temperatures and power levels over a period of time.

I've tried running with the outside sensing turned off and it does
make some difference, although principally when there are fast moving
changes in outside temperature.




..andy

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