# CM67 Optimum Start algorithm

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• posted on February 8, 2004, 12:09 pm
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
Does anyone out there know what it actually does?
--
Cheers,
Set Square
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• posted on February 8, 2004, 1:09 pm
wrote:

http://content.honeywell.com/uk/homes/files/pag113.pdf
http://tinyurl.com/2yc23
If 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.
.andy
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• posted on February 8, 2004, 3:22 pm
In an earlier contribution to this discussion,

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.
--
Cheers,
Set Square
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• posted on February 8, 2004, 3:45 pm
wrote:

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.
.andy
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• posted on February 8, 2004, 7:32 pm
In an earlier contribution to this discussion,

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].

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!
--
Cheers,
Set Square
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• posted on February 8, 2004, 8:36 pm
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)

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• posted on February 8, 2004, 8:48 pm
On Sun, 8 Feb 2004 20:36:51 -0000, "Frank McGuire"

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.

.andy
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• posted on February 8, 2004, 8:41 pm
wrote:

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.

.andy
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• posted on February 8, 2004, 9:31 pm

My, oh my! It gets worse.
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• posted on February 8, 2004, 9:43 pm

feeding. They are very affectionate and would be pleased to give you a big hug.....
.andy
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• posted on February 8, 2004, 9:54 pm

What a weirdo!
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• posted on February 8, 2004, 11:37 pm

Proportional control has no time division. That would be PI control, which the CM67 has. Most commercial controllers now tend to have PID control.
Full explanation.

Proportional control is a corrective action that is proportional to the error. Remember that error is defined as the difference between the desired value (setpoint) and the actual measured value (process variable). Proportional control is referred to as either "gain" or "proportional band". Proportional band is the percent of change in the controlled variable that causes the controller output to move over 100% of its range. In other words, a proportional band of 100% means that if the measured variable moves over 100% of its range, the controller output signal will move over 100% of its range; or, if the measured variable moves 1% of its full range, then the control output moves 1%. In general, as the proportional band percentage gets smaller, the controller becomes more sensitive. It is possible to make it so sensitive that a very small change in the process variable measurement will result in very large controller output. For those who like to work in gain instead of proportional band, the relationship between these terms is simple: gain is the reciprocal of proportional band. If the proportional band is 100%, then gain is 1/100%, or 1. If proportional band is 50%, then gain is 1/50%, or 2.
A proportional-only controller cannot control most processes well because it never reaches the desired setpoint. This offset, defined as the difference between the desired setpoint and the actual process value that the controller can achieve for the process, is due to the mathematical nature of the proportional control mode. Because there is only one controller signal output value associated with the particular process variable value the controller is reading, the controller sticks to its rigid mathematical association between input and output values. It cannot recognize that there is a problem of not achieving the desired setpoint value with proportional-only control. Increasing the controller gain will reduce, but not eliminate, the offset, but too much gain will result in unstable control that will oscillate endlessly above and below the desired setpoint.
INTEGRAL Integral settings are defined in repeats per minute or minutes per repeat. A setting of 10 repeats per minute will instruct the controller to repeat the action 10 times each minute. In general, the more often the repeats, the faster the offset will be eliminated. Integral control is a control mode that acts only as long as an offset exists. <<<<<<
There are three basic modes of control:
1. Proportional - operates on off-set and never reaches setpoint 2. Integral - time. 4. Derivative - anticipation, measurting rate of change.
PID controllers, which incorporate all three, are generally better. Certainly more flexible. In short, the PI of the CM67: proportional gets it near the setpoint, but never reaches setpoint because it operates on "offset". A proportional controler can sit there all day and not move and the setpoint will not be reached. They need a prod. The Integral mode of control operates on time and pulses the offset (the differnce bewteen the setting the proportional mode has decided to stay on an the setpoint) up to setpoint, giving high accuracy.
Be careful not to have the proportional band too tight as "hunting" will occur, known is short cycling to you.
In a CM67, the control aspect that times the delayed start is separet from the PI temperature control.
The time proportions of the CM67, are a crude way of keeping an on-off boiler within the proportional band.
The CM67 can control an electric actuator. A better system would be a heat bank, heated by a basic on-off control boiler (these are cheaper and have simple more reliable basic controls. The CH flow and return pipes would have a modulated 3-way valve, moved up and down by and electric actuator. This can stop at any point in the travel. This way only PI control would be used and guarantee much more accuracy of setpoint temperature control. As many heat bank and thermal stores are recommended to be on 24/7, the boiler can be controlled only by the heat bank and the CM67 moves the modulated valve and switches on the pump when it calculates the start time.

It is not "bandwidth". It is "Proportional Band - width". The "width" is the range of the band, e.g., 2C. That is the setpoint and 1C either side.

< snip crap about snakes, which should all be banned >
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• posted on February 9, 2004, 12:53 am

Hunting is hunting. It is not short cycling but movement around the set point.

They could conceivably be related. the manufacturer doesn't say.

I think you are confusing the issue. From the control mathematics perspective this is PI control exactly the same. All that this achieves, although it will work is perhaps finer grain control of heat delivery from the heatbank to the radiators. Of itself, the heatbank is a red herring in this if the boiler is not part of the main control loop.
I would also be surprised if a valve for this application would be modulated with a ten minute or even 5 minute cycle time. but rather that it would have some form of electronic control requiring a much shorter cycle time -seconds or even sub second. Can you suggest a make and model that would have the characteristics to work with a controller like the CM6x which has a much longer time?

Obviously, and that is explained in the data sheets. However it is often written as a single word in controller data sheets.

This is a real life application using an industrial type of PID controller and served as a reasonable explanation of some of the principles.
I felt that it was a better illustration than simply cutting and pasting material from a web site
.andy
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• posted on February 9, 2004, 1:24 am
wrote:

It is a form of short cycling an easier for others to understand.

Controlwise they have to be eparte. The user doesn't know this nor care.

Not at all.

Not so. The heat bank can sit there 24/7 keeping itself to its own setpoint providing a nice constant heat source. Just a store of heat. The CM67 will then just switch on the CH pump and modulate the CH actuator. Modulating a 3-way valve gives far more precise temperature control than switching in and out a boiler.
Another way is that the thermal store can be switched on also by another CM67. The desired time must be earlier than the time of the heating. This is if you can replace the room temperature sensor with a water strap-on cylinder sensor of course.

All those cycle times in minutes would be redundant, as only PI control on an actuator is required.

Don't care They should not be allowed in the country. If a couple of those get out and breed in the wild.
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• posted on February 9, 2004, 4:38 pm

It's a totally different mechanism

Not true. The piece controlling the optimised start could make very good use of knowledge of the characteristics of the PI part. Whether it does or not is a matter for the manufacturer.

You are because the heatbank is only a source of heat if designed to be oversized by a reasonable amount so that the boiler behaviour goes out of the equation.

Exactly, but that's all.

I don't disagree with that notion but can you suggest a make and model that can be modulated in a precise "analogue" way (i.e. part opened) by a CM67 type of controller with a long cycle time?
There are certainly valves that can be modulated with a DC voltage (I have some) and I could imagine pulse width modulated using a much shorter cycle time - e.g. in the way that model control servos work.
If you can't suggest one that will work with the CM67 type of controller, then the argument becomes a bit academic because either a different type of controller is needed, which is not in the same price bracket, or you would be talking about a standard motorised zone valve and proportioned on/off control of it. That would be a bit better than controlling a boiler on/off, although with a lightweight heat exchange type, I don't think that the difference would be substantial.

Yes of course, but this is adding extra complication. As you said, a thermal store ought to be on 24/7 anyway.

The proportional pulse width output from the controller would be used, but can you suggest a valve that will take this and use it to operate the valve to arbitrary positions.

First of all, the particular snakes in question (Brazilian Rainbow Boa - Epichrates Cenchria Cenchria) range in the wild in the Brazilian rainforest and their habitat is rapidly disappearing together with their numbers in the wild. They do, however, breed and thrive perfectly well in captivity, given the right conditions of temperature and humidity (28 - 31 degrees and 70-80% humidity). In order for them to breed, the conditions have to be right and there needs to be a pattern of temperature drop and rise around the breeding time. The UK climate simply won't create the conditions for them to breed in the wild and they would be unlikely to live for very long in the wild anyway. Considering the massive species loss in the rainforests, anything that can be done to sustain them in captivity until their natural habitat issues can be resolved is a positive step.
.andy
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• posted on February 9, 2004, 4:59 pm

It constantly moves up and down in an on-off manner.

It could supply data to other control processes, but it is a separate control process.

And, the modulating 3-way valve has finer accuracy to maintain a setpoint temperature than switching on-off a boiler.

See above.

The cycle times are not a part of the Integral aspect of the controller AFAICS.

There are 230v electric modulating valves available. Look in the CM67 pdf, it has a diagram of one.

What I am on about is a "modulted" 3-port mixing valve, with an electric actuator. I have a few here.

It depends on how far you want to take it. A CM67 on a themal store will learn how long it takes the boiler to heat it up, and bring it on at the right time.

AFAIK, the cm67 when controlling the setpoint temperature, only applies PI control modes. The cycle times appear to be an underlying control layer which only allows the boiler and PI control on it at certain time periods.
< snip crap about snakes, which should all be banned.> <If Brazil is short of snakes then you should not be having one. >
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• posted on February 10, 2004, 8:00 am

That's a good technical explanation....:-)

Of course.

Yes, except that I was referring to the heatbank.

Glad to see that you clarified it.

I am referring to the specific method of operating the control valve - the signalling between the controller and the valve. The CM67 provides a pulse with a total width of anything from 5 to 20 minutes depending on the what is configured on the setup. In order to control the controlled device (boiler, motorised valve etc) it will be on for between 1 minute and 100% of that time while operating in the proportional band. On the fastest cycle time, that will equate to a range between 60 and 300 seconds of on time per 300 second cycle.
What I have specifically asked you to provide is the make and model of a valve which will operate in a modulated fashion given that type of control signal.
I would be surprised if such a thing exists. I know that there are voltage controlled modulating valves ( I have some radiator valves that can be controlled by 0-10v DC); and I could believe that there are some that would modulate based on a pulse width with a much shorter cycle time ( a second or less)

I just have. Presumably you are referring to the sheet I posted - http://tinyurl.com/2yc23
It doesn't say anything about it being modulating.
There is a thermal actuator (diagram c) controlling a boiler. This type of device is a thermally operated relay which controls the boiler on and off, not modulating.
There is an electric actuator (diagram h) which is an electric actuator. This is a motorised device to operate some kind of mechanical entity such as a damper. It even shows the limit switches which stop travel. This isn't a modulated device either.
Did you mean something different? Please provide the URL to the CM67 sheet that shows it if I have the wrong one.

I know. Please provide the make and model of them. I am simply asking you to provide details of a *modulated* device which will work with a CM67.

True.
The cycle time is about choosing a control signal suitable for the device being controlled e.g. boiler, zone valve, electric heater

.andy
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• posted on February 10, 2004, 9:50 am
wrote:

Good enough for you.

The heat bank supplies the constant hot water for modulation purposes.

A pulse? A signal going on and off continuously?

The control signal is 230V.

It does. A reversible motor. 230v on one terminal and it moves one way, same the other way. Stop the singal and it stays where it is.

It appears to be.

Any maker makes them. They are common.

AFAIK the CM67 can operate a 230v modulated valve. If that is not the case (specs vague on this from wjat I have seen), then I think the Landis & Staefa version can. If so, I would go for this controller not the CM67. I tend to prefer Landis & Staefa controls anyhow. They are much more reliable, better made, better user intefaces and generally cheaper too.

They should be sent back to the wild then.
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• posted on February 10, 2004, 10:26 am

The Landis & Staefa RVP will give proportional control by driving a servo on top of a mixing valve, measuring the flow, return, inside and outside temperatures, deriving the setting from a pre learnt heating curve for your house. Assuming your boiler already has a RVP the extra cost is about 200 quid for the valve and servo. With a rad based system I would recommend fitting this type of control as the temperature swings are eliminated by matching the heat input to the heat loss. But with UFH time constant you would really gain much from these controls.
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• posted on February 10, 2004, 2:38 pm

Your explanations are seldom good enough for me

This controller has a contact which makes and breaks with a duty cycle as described - e.g. 1 minute on, 4 off out of a total of 5. When this is controlling a supply to a device, it is pulse controlling it in this case with a long duty cycle.

That's the source of power to the controlled device. The controller is turning on and off.

I very much doubt it. An actuator should run from end to end reasonably quickly - certainly less than a minute.
You can't have an arrangement as you describe. This would require a controller which could stop and start the motor and another to reverse it and something to report back to the controller what the position is.
That is not a simple actuator and it requires a more complex controller than a CM67.
It is possible to have pulse width controlled actuators in other applications where the control to it is effectively open loop, but the pulse cycle time is vry much less and the switchover time in the cycle used to determine the stopping position - not the same thing and it also requires some electronics at the actuator to do the motor control. This is how model control servos work, for example.

I don't think so.

So please suggest a make and model that will work in a modulated fashion with a CM67.

OK, so now you are changing your tune.

OK. Please supply the part number of the L&S controller that will do this and a suitable modulating valve to go with it

.andy
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