I've just finally bitten the bullet and installed a Honeywell CM67 Programmable Room Stat in my hallway. I bought the version with Optimum Start capability - which purports to decide for itself when to switch on the heating in order to achieve the target temperature by the programmed start time.
This seems to be working, but I'm curious to know what algorithm it uses in order to decide when to turn on the heating. The literature doesn't make this clear, so I wonder whether any of you actually know?
Several possibilities present themselves to me:
it could assume a fixed (factory set) rate of rise of temperature, and apply this to the current temperature in order to work out long long it will take to get to the target temperature
it could depend on one of the parameters - such as Proportional Band Width - which can be set in the Installer Set-up Mode
if it's *really* clever, it could calibrate itself by observing the actual rate of temperature rise when the heating is on, and then apply this to its future predictions
you look at Adaptive Intelligent Recovery, it is basically doing what you describe in your third point - i.e. if you turn that feature on, the controller measures the rate of rise of temperature and uses it to calculate the next heating period's start time also accounting for the starting temperature. This is a relatively simple controller, so it appears that it only does this session by session. A really intelligent controller would store the temperature points by time over more days because the rate of change of temperature is not absolutely linear, although it is to a first approximation. Hence the method used is a lot better than doing nothing at all. All of that has to do with optimised starting.
The proportional band has to do with the behaviour of the controller (it is a proportional plus integral controller)
Proportional control is that the heat contribution over a period of time is adjusted to match the heat loss and maintain the temperature accurately. There are controllers that will do this in a genuinely analogue way by modulating the boiler output to match the requirement.
However, many boilers can only be controlled in an on/off fashion aand indeed this is also true of most zone valves if you were using that as a means of control. In order to control this type of device, the controller does so by using time proportions. A simple example might be that if you want 50% of the average output, the boiler is turned on for 5 minutes and off for 5 minutes. For 80%, it would be on for 8 and off for 2.
The CM67 does this but allows some additional settings.
- There is an cycle rate setting of 3,6,9,12 cycles per hour. If the entity being controlled can react quickly without any disadvantages (e.g. if you were controlling electric heating), then you could set 12 cycles per hour. At the other extreme, devices that show inefficiency or might have other problems if cycled too frequently would be run at three cycles per hour. In between you have typical gas heating, which is run at 6 cycles/hour i.e. cycles last ten minutes. So the behaviour would be as in my example above. Note that the minimum on time is set to 1 minute, which in effect will be 10%.
- Proportional bandwidth is the temperature range about the set point where proportional control operates. Within this range, the controller operates in this cycling mode. Below it, the output will always be on and above it will always be off.
Adjustment for this will depend on the heating system and thermal inertia of the house. Ideally what you want to have happen, is that when starting from cold, if the current temperature is below the proportional lower limit, then the optimisation should time operation to fire the boiler at 100%. However, as the temperature rises into the proportional band, the cycling control should begin to take effect. If possible, you want to avoid the temperature overshooting above the proportional band because a) you use more energy than needed and b) the temperature may tend to oscillate for a long time before settling. This is most likely to happen in well insulated properties with oversized heating. It is really a trade off between achieving the set point quickly and not overshooting. On this controller, you can vary the proportional bandwidth to help with that, in that it will enter cycling operation earlier on the rise and begin backing off the average heat output of the system.
The proportional bandwidth may be taken into account in the optimised start. That's not clear.
The best thing to do is to check the temperature rise behaviour by recording temperature readings (say) every ten minutes during start up for a few days. Plot a graph if you like. You can then experiment with proportional bandwidth if needed.
20 years ago I designed one of these controllers and the next stage in its development was to include the facility which you mention. The whole idea was well ahead of the market at that time, so I was struggling on new ground.
I got as far as working out a rough algorithm. It went something like...
Starting with a basic programmed in setting, then incrementing or decrementing preheat time value, based on whether it attained the temperature before or after the set time. Which equates roughly with your last suggested method.
It was all very experimental. There was a theory going around, that in order to achieve this function, that an outside temperature sensor would be needed to enable the calculation to be done satisfactorily. I wonder if your unit includes an outdoor temperature sensor?
I've done some work on this as well. Working purely on an internal reading and time, you can make a first approximation assumption that heat loss and thermal inertia effects are linear with time. That is roughly true and better than doing nothing. It becomes better if the controller can gather data and average over several heating sessions.
The CM67 type of controller doesn't use outside temperature sensing but some of the sophisticated boiler controllers have outside sensors.
On mine, there is an outside sensor which goes on a north facing wall out of the sun and gives the boiler a temperature signal based on resistance. There is an inside controller which senses temperature and hooks up to the boiler with a pulse width modulated digital code, in effect providing an analogue temperature value.
It is possible to set the proportional bandwidth and the relative sensitivities to each of these terms, with data being accumulated over several days. The boiler output is then linearly modulated from 3kW to full power by controlling the fan speed (influences gas input rate) and pump power level. The slope and end points of the boiler temperature vs the environment temperatures can be varied as well.
The controller behaviour can be logged to a PC so I can look at the temperatures and power levels over a period of time.
I've tried running with the outside sensing turned off and it does make some difference, although principally when there are fast moving changes in outside temperature.
Self adaptive optimum start, and optimum stop, controllers have been around on the commercial field for over 20 years. Microprocessor based too.
It is far better with an outside temperature sensor, But using just one inside temp sensor, a decent optimised start performance can be achieved.
The DCD controller has a number of user set time zones during the day. Each one is optimised on and off (I believe). The CM67 is only optimised on, in the morning period, so I believe. Subsequently the DCD controller saves a lot on energy.
Many thanks for your very full asnwer, Andy - and for the links. My CM67 is the hard-wired rather than RF model, and the leaflet which came with it is slightly different. The "Adaptive Intelligent Recovery" section in the RF model leaflet does say fairly explicitly that it uses the start-up performance this time around to decide what to do next time. My leaflet is a lot less explicit. Its equivalent paragraph is entitled "Optimisation (Variable Start Time)" and says: "The Programmable Thermostat will delay the start time to the last possible moment, so that the desired temperature is reached by the start of the program (sic) period. Simply set the times at which you would like to be warm and the Programmable Thermostat will switch on when required up to a maximum of 3 hours early."
I assume that the hard-wired version works in the same way as the RF version in this respect, but it seems strange that they use different words to describe it.
As you say, it appears that it bases its temperature rise rate value on what happened at the most recent startup period - rather than averaging it over a number of periods.
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? 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. However, it may - and often does - decide not to run it at all within a particular cycle - e.g. when the actual temp is way above the target temp, as in over-night conditions.
This does not stack up with my experience! My CM67 is programmed to provide a lunchtime boost of 21 degC from 12:30 to 13:30 having been set back to 18 degC for the morning period. The boiler usually fires up just before 12 in order to achieve the target temperature by 12:30. If you monitor the target temperature on the stat, it changes this from 18 to 21 at the point when it decides to fire up, rather than at the programmed time.
I think that if you poke around on the sites a little, you will find the CM61 or CM67. There are possibly a few differences in the RF version to save battery power, but functionally from when I looked them they are basically the same as the wired one.
It is a bit vague isn't it. Probably written by a technical author who didn't understand the product.
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.
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.
Correct. But that is where you have gone outside the proportional band anyway. On the way down, as you come into the band, the boiler would begin firing as soon as 10% heat or more is demanded.
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.
As some will remember, with the groups useful advice I fitted a CM67RF to my existing system a few weeks back. It's transformed the level of control and refinement compared to before (a TRV only system) but if like me you are new to these types of stat it does take a bit of mind set change to get the best out of programming the device e.g. one error I made was to set the target temp time too early because I was thinking along the lines of giving the house a chance to warm up before getting up. The CM67 started the system up in the middle of the night. A bit of tweaking sorted that.out.
I've not changed the factory settings but like most tuning (bikes, cars, IT systems etc.) I'm going along the path of changing one user setting at a time and checking the results over the next or so. I may move on the factory settings but I doubt it.
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].
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].
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!
I've just installed a Honeywell CM67(RF) and I've been trying to figure out how it is operating so the information in this thread has been useful to me. The boiler I have is a Vaillant Turbomax Plus. Once the house is up to the set temperature everything is fine, but it does seem to take a long time to get there even in 'mild' weather, set to 19.5 Degrees C
The boiler has a temperature control on it - should I set this to max and let the CM67 take over?
Does anyone know which has priority - the boiler's thermostat or the external (CM67)
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.
Yes. Also within the band, don't forget that there is the minimum on-time which has an influence.
Not directly. There will be several factors, including the house behaviour which will affect behaviour.
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.
Yes you should. Keep an eye open for short cycling when the controller is in the permanently on range. This would mean that you are not getting heat away from the boiler fast enough, often because pump setting is too low or radiators turned down too far.
The CM67 will give the boiler a demand for heat. The boiler will do that until that goes away or the water temperature reaches the level set by the boiler thermostat.
What you want to try to avoid is that the CM67 is creating a demand for heat and the boiler is turning on and off on its own thermostat. The system will still work, but you may get some inconsistent effects with the controller depending on the heat actually required due to the outside temperature. Ideally, you want if possible for the CM67 to do the main control.
Are these things ever advertised? I only learnt about the existance of programmer/thermostats where you can vary the temperature during the day from earlier posts in this ng. I have always wanted such a device so when I learnt that they existed I ordered one from plumbworld.
Installing and using the device has made me realise how slowly my house responds to temperature changes. Maybe 1C /hour either up or down.
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