CM67 Optimum Start algorithm

[Andy Hall]

Integrated in as part of it.

Just curious!

.andy

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[JH]

Likewise. Plus work commitments means the house is empty for a few days during the week ATM. Previously I'd frig around with the old timer to get the house warm on my return but it didn't really do the job i.e. the air was warm but the house still felt cold, much better now even without fine tuning the programs or factory settings. I'll be interested to see in the gas bill next time mind.

Thanks again to old CH hands in this group. Why on earth installers put the Mickey Mouse stats or in my case none at all can only be put down to cost, but for the sake of less than 90 quid it's false economy in my book.

Jules

[IMM]

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

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. this application is exactly right, though.

< snip crap about snakes, which should all be banned >

[IMM]

And ignorance. Most installers know sweet nothing of control.

[Andy Hall]

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

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

That was already covered.

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.

Well.... they were around before you and will probably be afterwards.

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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[IMM]

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.

[Anthony James]

Does anyone have any views on the relative merits of the Honeywell and ACL Drayton programmable stats? Much of a muchness?

[Tony Bryer]

Because people understand basic thermostats. I think the CM67 is a brilliant piece of work (especially being able to use the party button to kill the heating for a few hours if you go out) but you need to understand the concept of time/temperature variations rather than on/off and then how to override it when required.

It's not difficult but I put one in at my mum's place and she really does not like. With a £10 stat when the rads went cold she could just turn it up a bit more . Expensive controls that people cannot/will not master are a waste of money.

[Bungle & Zippy]

Sore losers

[Andy Hall]

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.

Don't be silly. Clearly you know very little about herpetology.

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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[The Most Rev. Peter Parsnip]

The Lord alerted my mind to the presence of this EVIL article by Harry Bloomfield, and I thusly replied:

You do like to sing your own praises, don't you?

[IMM]

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.>

[Phil Kyle]

Good, you must be nearly dead now then.

Modesty, Harry.

Christ that was boring.

[tony sayer]

So what does a Suprima having a lockout wobbly have on all this then?...

[Andy Hall]

You'd need to ask Professor Stanley Unwin that question.

I'm pretty sure that he was IMM's english language teature.....

.andy

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[tony sayer]

In article , Andy Hall writes

Better get the glass out for the seance then, he threw off his mortal coil a while ago.

Still, could you imagine the glass wizzing about with the answer:-)).....

[Andy Hall]

Doesn't bear thinking about, does it?

.andy

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[Andy Hall]

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.

Please reply to that specific point.

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 -

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

They didn't come from Brazil, they were captive bred in the UK.

.andy

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[IMM]

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.

[IMM]

You were on about individual radiator electronic control: