On Sun, 8 Feb 2004 19:32:48 -0000, "Set Square"
wrote:

In an earlier contribution to this discussion,
Andy Hall wrote:

On Sun, 8 Feb 2004 15:22:09 -0000, "Set Square"
wrote:


Thankyou for explaining cycles per hour too. Presumably with the
default 10 minute cycle (6 per hour) once it has done its thing
within the current cycle, it won't turn the boiler on again until
the start of the next 10 minute cycle even if the house has cooled
rapidly in the meantime because (say) some fool has opened all the
doors and windows?

That would be true unless the temperature has fallen so rapidly that
you reach the edge of the proportional band. At that point, the
controller will go to 100% on. Obviously this is another factor that
you could trade in deciding on the bandwidth setting. If the sudden
loss of heat situation happens a lot, then setting the bandwidth
narrower might be in order.

Ah, now I understand the proportional band width a bit better - I had
previously assumed that it had something to do with the gain - but it is
actually the temperature band within which the controller exercises what
might be called "time division proportionality".

Bandwidth is a bad word to use but is the usual one. Normally it's
used in connection with radio and network engineering to mean
something completely different. The way you've described it for
this application is exactly right, though.

Outside that band it is
either fully on or fully off (if I'm right).

Yes. Also within the band, don't forget that there is the minimum
on-time which has an influence.


I presume that there isn't any
user control over the gain applied to either the proportional or integral
feedback?

Not directly. There will be several factors, including the house
behaviour which will affect behaviour.


[I must admit that, having only worked in the past on analog
controllers for mechanical systems, I'm not too clear on how the principles
apply to something which has only an on/off output].


If you want to think of it electronically, in the proportional range
it is roughly equivalent to a switch applying charge or discharging a
capacitor and being switched each way to set a fixed voltage. If you
were to take the voltage on the capacitor and filter it to remove the
short term variation, you will end up with more or less DC voltage
which increases and decreases according to required heat.

This is really all that is happening. The house has a much longer
time constant than the cycle rate of the controller and so there is a
smoothing effect.

The proportional switching is only there because most boilers don't
have an analogue input to control the power level.

This is a very common method of industrial temperature control where
the process and the heater have a long time constant.

To give you another example, I keep snakes in a large vivarium. The
particular species requires a relatively narrow temperature range and
high humidity. They also require a temperature gradient of a few
degrees along the length of the enclosure so that they can choose the
temperature that they would like.

For the heating, I have a special kind of ceramic heater surrounded
by a guard in the enclosure at one end, and a temperature sensor
(platinum resistance probe) in the centre. There is an industrial
temperature controller which drives the heater in proportional mode
through a solid state relay. The cycle time is ten seconds which is
fine for this arrangement.

The controller is really intended to acquire and maintain the setpoint
as quickly as possible and has autolearning modes to do this. Most of
the time, the temperature is held at 29.5 degrees. However, there is
a second control system for the humidity. This consists of a
humidity probe and controller for that, the output of which is used to
drive a pump. The pump delivers water from a reservoir to fine mist
sprayers in the enclosure, wetting the bark chips on the floor. The
humidity rises and the controller cuts off. Precise control is not
so critical so this controller is a simple on/off with a hysteresis of
5% of humidity.

However, one effect of the humidifier part running is to drop the
temperature. As the water evaporates, it requires its latent heat to
do so and there is a cooling effect. The thermal control adjusts and
brings the temperature back within a minute or so without
overshooting.

The snakes seem happy with the arrangement, which reminds me, I must
go and feed them.







Presumably also, the
minimum ON time only applies once it has decided to turn the boiler
on. That is, it never runs the boiler for less than a minute.

If you set it to the 10 minute cycle, yes. Depending on the nature
of the house and system, setting a longer cycle might be worth doing -
e.g. if it's an older (say cast iron) boiler and not overpowered for
the house.

My house *is* well insulated, and the boiler is at least adequate. It is a
10-year-old Baxi Solo 70/4 PF - which *does* have a cast iron heat
exchanger, albeit of fairly low water holding capacity such that pump
over-run is required. Could you please explain in slightly more detail what
the likely effect would be of setting the cycles per hour to (say) 3 rather
than 6. Should I also increase the minimum ON time to more than one minute?
[For example, for oil boilers the leaflet says to set to 3 cycles/hr and 4
minutes min ON time].


Another factor that I should have mentioned is the boiler thermostat.
If the boiler is oversized or the heat is not being disposed of
quickly enough, that will come into play and have an impact on the
system. You would notice this if the boiler has a tendency to short
cycle without the controller in operation. Ideally, when starting
from cold, the boiler thermostat shouldn't come into play until you
are getting towards the set point if at all.


The boiler is on permanently when the system starts up from cold. When it
gets hot, but the room stat set point is not yet reached, the boiler *does*
cycle on its own stat - but I've always regarded this as normal. Do you
regard it as a problem?

One more point, while I think about it. One of the features on the CM67 is
something called "pump exercise" which purports to run the pump for 15
seconds each day. This is presumably to stop it seizing up if not used
during the summer. I can't quite figure out how this is supposed to work
since, on most systems the boiler and pump are either connected in parallel,
or the pump is controlled by the boiler to provide over-run. Either way, you
can't have the pump on without the boiler also being on - unless, I suppose,
you turn the boiler stat down to zero. Is this what you're supposed to do?
[This is of purely academic interest in my case because the boiler and pump
run all year for DHW anyway].

Thanks for sharing your expertise - it is very educational!

..andy

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