Angela wrote:
"ransley" wrote in message
...
On Oct 18, 3:51 pm, "Angela" wrote:
I have a Honeywell CM67 on my central heating and I have noticed that it
is
calling for heat several times an hour even though it states that it is at
the correct temperature. When it is at the correct temp it only calls for
heat for a minute or two. This seems a real waste. I noticed on the
installer menu that there are 2 settings which may affect this. The first
is the cycle rate and the default is 6, which is about as often as the
thermostat is calling for heat when it doesn't appear to need so. Could
this be why? If it is, can this be set to 3 ( this seems to be the lowest)
to reduce the times or does it need to be 6? Also there is a minimum ON
time which is set to 1 minute as a default and this can be set from 1 to 5
minutes. Does that make a difference?

Advice is appreciated.

Angela

Heating systems run more efficently the longer the cycle and wider the
swing in temp, short cycling as you are doing is wearing out all
components that switch on, since only so many cycles are built in to
any equipment. The first maybe 5 minutes any small heating system is
on is the most inefficent, it has to warm up, try both at max swing
and temp run time and see what happens, if temp swings to much for
comfort reduce it until you are happy

Sorry, can I just clarify that have I have understood, I should reduce the
cycle rate to 3 and the min time on to max (5 minutes)?

If you read Andy's post that Matthew linked to you should get a feeling
for what the stat is doing.

Personally, I would not be too worried about having the stat call for
heat six times / hour. Most stats will call for heat at least that often
unless the house is very well insulated, or the amount of temperature
swing permitted is rather large.

The default settings of the stat should work well enough on many systems
(and the stat will "learn" the typical operational performance of your
system and adapt its control to allow for the observed behaviour).

You may be able to tune for better operation if you are aware of
particular factors that affect your system. For example, if your boiler
is an older high water content type with a cast iron heat exchanger, you
may want to increase the minimum burn time a little since it will take a
while to get all that thermal mass up to temperature.

Optimising stats are good at getting accurate temperature and time
control out of relatively crude heating systems. However they are
sometimes less beneficial when used with boilers that already include
sophisticated control systems, since you can end up with unexpected
interactions between them.

Modern lightweight condensing modulating boilers may for example work
well with fewer cycles per hour since they will implement their own
closed loop control system to modulate the flow temperature of the water
in response to the actual heating load. Under light loads (i.e. return
water coming back at a highish temperature), these boilers will reduce
the flow temperature (promoting increased condensing efficiency), which
should allow longer burns at low power. However experimentation would be
required to select the optimal settings.

--
Cheers,

John.

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