On Sun, 8 Feb 2004 12:09:44 -0000, "Set Square"
wrote:

I've just finally bitten the bullet and installed a Honeywell CM67
Programmable Room Stat in my hallway. I bought the version with Optimum
Start capability - which purports to decide for itself when to switch on the
heating in order to achieve the target temperature by the programmed start
time.

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?

Several possibilities present themselves to me:
* it could assume a fixed (factory set) rate of rise of temperature, and
apply this to the current temperature in order to work out long long it will
take to get to the target temperature
* it could depend on one of the parameters - such as Proportional Band
Width - which can be set in the Installer Set-up Mode
* if it's *really* clever, it could calibrate itself by observing the actual
rate of temperature rise when the heating is on, and then apply this to its
future predictions

Does anyone out there know what it actually does?



http://content.honeywell.com/uk/homes/files/pag113.pdf

http://tinyurl.com/2yc23


If you look at Adaptive Intelligent Recovery, it is basically doing
what you describe in your third point - i.e. if you turn that feature
on, the controller measures the rate of rise of temperature and uses
it to calculate the next heating period's start time also accounting
for the starting temperature. This is a relatively simple
controller, so it appears that it only does this session by session.
A really intelligent controller would store the temperature points by
time over more days because the rate of change of temperature is not
absolutely linear, although it is to a first approximation. Hence
the method used is a lot better than doing nothing at all.
All of that has to do with optimised starting.

The proportional band has to do with the behaviour of the controller
(it is a proportional plus integral controller)

Proportional control is that the heat contribution over a period of
time is adjusted to match the heat loss and maintain the temperature
accurately. There are controllers that will do this in a genuinely
analogue way by modulating the boiler output to match the requirement.

However, many boilers can only be controlled in an on/off fashion aand
indeed this is also true of most zone valves if you were using that as
a means of control. In order to control this type of device, the
controller does so by using time proportions. A simple example
might be that if you want 50% of the average output, the boiler is
turned on for 5 minutes and off for 5 minutes. For 80%, it would be
on for 8 and off for 2.

The CM67 does this but allows some additional settings.

- There is an cycle rate setting of 3,6,9,12 cycles per hour. If the
entity being controlled can react quickly without any disadvantages
(e.g. if you were controlling electric heating), then you could set 12
cycles per hour. At the other extreme, devices that show
inefficiency or might have other problems if cycled too frequently
would be run at three cycles per hour. In between you have typical
gas heating, which is run at 6 cycles/hour i.e. cycles last ten
minutes. So the behaviour would be as in my example above.
Note that the minimum on time is set to 1 minute, which in effect will
be 10%.


- Proportional bandwidth is the temperature range about the set point
where proportional control operates. Within this range, the
controller operates in this cycling mode. Below it, the output will
always be on and above it will always be off.

Adjustment for this will depend on the heating system and thermal
inertia of the house. Ideally what you want to have happen, is that
when starting from cold, if the current temperature is below the
proportional lower limit, then the optimisation should time operation
to fire the boiler at 100%. However, as the temperature rises into
the proportional band, the cycling control should begin to take
effect. If possible, you want to avoid the temperature overshooting
above the proportional band because a) you use more energy than needed
and b) the temperature may tend to oscillate for a long time before
settling. This is most likely to happen in well insulated properties
with oversized heating. It is really a trade off between achieving
the set point quickly and not overshooting.
On this controller, you can vary the proportional bandwidth to help
with that, in that it will enter cycling operation earlier on the rise
and begin backing off the average heat output of the system.

The proportional bandwidth may be taken into account in the optimised
start. That's not clear.

The best thing to do is to check the temperature rise behaviour by
recording temperature readings (say) every ten minutes during start up
for a few days. Plot a graph if you like.
You can then experiment with proportional bandwidth if needed.




..andy

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