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". Outside that band it is
either fully on or fully off (if I'm right). I presume that there isn't any
user control over the gain applied to either the proportional or integral
feedback? [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].



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!
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is Black Hole!