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?
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
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)?
Not really. When you heat your house up to your desired temperature it looses heat to the outside world. With a "conventional" thermostat there is some hysteresis which causes it to switch off until the temperature drops sufficiently for it to cut in again, resulting in an erratic temperature. The programmer uses some clever logic to provide enough heat to balance the heat losses to the outside and maintain a much more steady temperature. It does this by varying the length of time to fire the boiler every ten minutes. When it's not very cold outside and you've arrived at the target temperature the heating periods are quite short but get longer as the weather gets colder.
Our CM927 behaves in a similar way and keeps the temperature at a steady value within half a degree of the target.
The late Mr Hall wrote an excellent post on this some years ago which will explain in detail what's happening and why:
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could try cutting the cycle rate down to 3 as there may well be benefits as far as boiler life is concerned although do keep an eye on how accurate the temperature is then maintained.
Cycling 6x an hour, 144x a day is a waste in efficency and equipment life, If this is a radiator HW set up its unbelievable, I cycle at 50f outside maybe 8x a day. Your wear and tear on equipment is startup, just look at surge load for motors.
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.
Cycling 6x an hour, 144x a day is a waste in efficency and equipment life, If this is a radiator HW set up its unbelievable, I cycle at 50f outside maybe 8x a day. Your wear and tear on equipment is startup, just look at surge load for motors.
Not withstanding the fact that a combination boiler seems to turn itself on and off everytime some runs a hot water tap to rinse their hands! Michael
That was a very useful post thank you Matthew. I "think" I now understand what is happening. This is a new house with a new boiler but I notice that the CM67 isn't sold any more - the developers/installers must have had a job lot to get rid of. It seems fine though. I have only just started putting the central heating on proper and it would appear that this thermostat can "learn" and I probably haven't given it time to do that. I'll leave it at the defaults for now and see how things go I think.
Yes, you won't get a better explanation than that provided by Andy Hall.
As a matter of interest, does your CM67 have Optimum Start and, if so, is it turned on?
[In case you're not familiar with it, Optimum Start is a feature whereby you specify the time by which the house should be warm rather than the time at which the heating should be switched on. The CM67 then decides when to switch on, so it does it later in mild weather and earlier in cold weather - helping to conserve fuel. It does this by 'learning' the rate of temperature rise when the heating is on, so - depending on the starting temperature - it knows how long it will take to get up to the required temperature. I have it on mine, and I *think* it's a good thing - but it does sometimes seem to turn the heating on *very* early in cold weather.]
| > That was a very useful post thank you Matthew. I "think" I now | > understand what is happening. This is a new house with a new boiler | > but I notice that the CM67 isn't sold any more - the | > developers/installers must have had a job lot to get rid of. It | > seems fine though. I have only just started putting the central | > heating on proper and it would appear that this thermostat can | > "learn" and I probably haven't given it time to do that. I'll leave | > it at the defaults for now and see how things go I think. | >
| > Angela | | Yes, you won't get a better explanation than that provided by Andy Hall. | | As a matter of interest, does your CM67 have Optimum Start and, if so, is it | turned on? | | [In case you're not familiar with it, Optimum Start is a feature whereby you | specify the time by which the house should be warm rather than the time at | which the heating should be switched on. The CM67 then decides when to | switch on, so it does it later in mild weather and earlier in cold weather - | helping to conserve fuel. It does this by 'learning' the rate of temperature | rise when the heating is on, so - depending on the starting temperature - it | knows how long it will take to get up to the required temperature. I have it | on mine, and I *think* it's a good thing - but it does sometimes seem to | turn the heating on *very* early in cold weather.] | -- | Cheers, | Roger
Yes it does have optimum start which will explian why it was on when I got up this morning but according to the timer it wasn't due for another 5 minutes. I'll keep an eye on timing to see if it is turning on too early!
On Sun, 19 Oct 2008 12:03:41 +0100 someone who may be "Roger Mills" wrote this:-
Has been used for more than two decades in large systems. Generally a very good thing, but one flaw with it is that if the fabric of the building becomes worse (open window for example) it will just learn to start earlier (without optimum start someone may investigate why the building is cold).
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