Recently had ch upgrade which included upstairs/downstairs zones controlled by CM67 wireless stats. My understanding of the operation of these, in that a desired temperature is set for a given start time and the stat decides when to turn on the heating to achieve this temperature. But, my wife got up early this morning and reported that the heating came on at 5:00am. The set time for achieving 20c was 7:10am some 2 hours later! In writing this, it occurs to me that the ambient temp may have dropped below the min temp for that period (will check this over the weekend).
But the question still stands - how far in advance of the set time will these stats call for heat to achieve the required temperature?
Whilst I'm not sure specfically about the CM67, its replacement - the CM907 - restricts optimisation to a maxium of 3 hours advance.
How long has it been fitted? The CM907 starts off with an initial assumed rate rise of 3 C/hr and fine tunes it on a day-by-day basis - perhaps it needs some time to settle down?
As you say, it could just be the fact that the temperature dropped below whatever the 5am setting was/is at.
It seems the CM67 also restricts to a maximum of 3 hours however the way I read it is that it could well be using a less intelligent control function insofar that it has a single 'rate of rise' variable which is modified in accordance with the measured rises for any 'start-up' period. The CM907 on the other hand appears to maintain seperate morning/afternoon registers - perhaps the morning rise is different to the afternoon rise in practice (sounds feasible given the bigger temperature differentials?)
CM67: 'The CM67RF will adjust the start time at upward setpoint changes so the desired temperature is reached at the programmed time e.g 7:00 temperature
21oC.The chronotherm will monitor the start-up period and use this information to modify the start time calculation for the next upward setpoint change. The system will restrict the start time to a max of 3 hours before the programmed comfort time.'
CM907: 'The thermostat will adjust the start time in the morning/afternoon so the desired temperature is reached by the start of the program period e.g. Time
7:00, Temp 21 oC. The thermostat will monitor the accuracy of the start up and use this information to modify the calculation for the following day by changing the ramp rate (initial 3 K/hr) .The system will restrict the start time to a max of 3 hours.'
On the other hand, perhaps they operate the same and the descriptions were written by different people! One would however expect some improvement between models when it comes to relatively 'complex' processing and resulting accuracy/performance - and the CM67 must be getting on a bit now?
For both of these (67 and 907), my understanding would be that the start-up period should reduce as the system "learns" how long the temperature rise actually takes to achieve the programmed setting. Is that right?
In the meantime, I will reduce the "off period" temperature to something well low and see if this has any effect.
The one incident that did concern me was a situation enountered the other day. Went home at lunch time approx 1:50 pm to find the heating running. The set-point was 15c with the ambient displaying 18c IIRC. The next "on" period was due to start at 4:20 pm with a required temp 20c. So the heating was running 2-1/2 hours ahead of the set point. The system has been in for 2 months now.
I was thinking perhaps only a day or two so we can probably rule out any initial learning stage.
I have heard of people having trouble with the optimisation and disabling it as a result so perhaps you might not have much choice (I'm assuming the premature firing is undesirable). Our '907 seems to work well but I don't really get to see it in action in the morning however I have witnessed the afternoon start-up a few times and it seems to work well. Whether this is down to the 907 or our house/heating I don't know hence it probably is of much help...
I think my CM67RF has become confused in the past when I've turned the heating off for any length of time for whatever reason *at the boiler* and not at the stat. I guess it confuses the temp rise/time calculations. Seems to take a few days to settle down again afterwards.
As you say, with the current weather you should check if you're hitting minimum night time temperatures (regular here I'm afraid).
I think I have read somewhere that the CM range does not learn if it is less that 3 deg c beweent the demand tempratures. In other words, try letting it drop 10 deg C at night so it has to think...
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