Strange Optimizing Programmer Action

[D_Hailsham]

My CH is controlled by a Honeywell CM927 Wireless Optimizing Programmer. So far it has worked "as it says on the tin" and the house temperature hardly varies.

The first ON time is 0630hrs temperature 20C. The last time is 2230hrs temperature 5C. We usually get up between 0630 and 0645 and the house is comfortable.

This morning the wife got up at 0615 as she could not sleep and went downstairs. Minutes later she came back and said that the heating was not on and the house was freezing.

I felt the bedroom rad and it was cold so I went and looked at the programmer. It showed a Room temperature of 19C while the target temperature was 5C. I turned the temperature up until the target was 21C and the boiler came on.

As far as I understand it with optimizing controls the boiler should have come on in time to bring the temperature up to 20C by 0630, but it didn't.

I checked that the optimization facility was turned on, and it was.

Has any one come across this before or have an explanation?

Also can anyone explain what the "proportional bandwidth" setting is for and when to change it?

Lastly, there is a temperature offset for cases when the programmer is in a warmer/hotter location. Which way do you adjust it? e.g if the location is in the kitchen, which is always 2C warmer than the rest of the house, do you set the Offset to +2 or -2?

 

[joncox]

I don't think you have a problem with your programmer - you were only asking for a 1 degree rise (from 19 to 20) in 15 minutes (from 06:15 to 06:30). It came on immediately when you upped the temperature to 21 because it sensed it would have 'more work to do'. I bet it was just about to come on anyway when your wife got up.

BTW, if 19 degrees = 'freezing' in your wife's book and 20 degrees = 'comfortable' in your book, then you must had some domestic fun trying to agree on the programmes to use!

The point of optimisation is to ensure that you don't waste gas by turning on early if the house retains its warmth overnight. If your house got colder, then the CM927 would kick off earlier.

FWIW, my 3 week old CM927 typically kicks off the CH about 45 minutes before the first set time, to deliver a temperature rise of 2 degrees.

The optimisation seems to work better than on its predecessor (Honeywell CM67) which I had previously. That tended to come on much earlier.

No idea re: proportional bandwidth - hopefully a proper heating engineer will answer.

I'd also like to know the answer to your final question - the installation manual for the unit isn't at all clear.

 

[UF6]

The proportional bandwidth or as more usually called the prop band is the amount of change in measured variable required to drive the device from on to off, open to shut, high to low. (or vice-versa) etc.

This is usually a figure in units of the measured variable, say, "the prop band is 2 degrees centigrade"

The prop band is applied to the prevailing measured variable and the setpoint. And (depending on manufacturer) either sits across the setpoint or (in the case of heating control) below it.

Hopefully an example will help.

The required boiler flow temperature is 80DegC.
The prop band is 2DegC.
Assume the prop band is across the setpoint.
Assume on/off control.
The boiler will switch off when the flow temperature rises above 81DegC and switch back on when the flow temperature falls below 79DegC

This is basic on/off control. In this example the prop band could also be called the hysterisis band.

With modulating control the device would ramp from off to fully on, low-fire to high-fire, 0% to 100% etc.
So when a modulating combi burner is varied by the boiler thermal controls, it modulates through the setpoint's prop band.
Modulating control usually (but doesn't have to) look at the time factor as well. i.e. how quickly the setpoint is (or isn't) being achieved. It integrates the output over time.
This is called PI control.

The final term you can add is PID control. This is a way for the controller to react very quickly to (either) sudden changes in setpoint or measured variable.
For instance, the boiler flow temp suddenly plummets because the seperately zoned upstairs heating comes on.
This will override the I term and quicly ramp the burner to maximum. To try to reduce the error as quickly as possible.

 

[D_Hailsham]

I did think that it might be because the temperature difference of 1C was less than the proportional bandwidth. Everything I have read on the net about PI control systems is full of heavy maths which I forgot about when I left school years ago.

Yes, we do have the occasional "domestics" of the "radiators are cold so I must be cold" variety. The interesting thing is that they have reduced since I put in the CM927; must have been due to the temperature swing with the old mechanical stat.

It is interesting to compare the way the CM927 works compared to the Drayton RF3i.

With the Honeywell you set the time when you want the house to be at the set temperature, e.g 21C at 0630. The heating will come on no more than 2 hours early, depending on overnight room temperature.

In the case of the Drayton you have to set the time when the heating will come on, presumable on the coldest day. The drayton then compares the actual temperature at turn on with the required temp at get up and then decides if it needs to delay the start. There is a max delay of 1 hour.

Just seen a reply from UF6, but haven't read it yet!

 

[D_Hailsham]

The proportional bandwidth or as more usually called the prop band is the amount of change in measured variable required to drive the device from on to off, open to shut, high to low. (or vice-versa) etc.

This is usually a figure in units of the measured variable, say, "the prop band is 2 degrees centigrade"

The prop band is applied to the prevailing measured variable and the setpoint. And (depending on manufacturer) either sits across the setpoint or (in the case of heating control) below it.

This sounds sensible to me, I had assumed that a bandwidth of 2C meant 78C-82C, if the set point was 80. Though I could not see why you would expect the temperature to exceed the set point!

Hopefully an example will help.

The required boiler flow temperature is 80DegC.
The prop band is 2DegC.
Assume the prop band is across the setpoint.
Assume on/off control.
The boiler will switch off when the flow temperature rises above 81DegC and switch back on when the flow temperature falls below 79DegC

This is basic on/off control. In this example the prop band could also be called the hysterisis band.

The Honeywell literature has a setting to increase the proprtional bandwidth and says that this might be usefull in well insulated housed with oversized rads. Can't see the logic of that, can you explain?


With modulating control the device would ramp from off to fully on, low-fire to high-fire, 0% to 100% etc.
So when a modulating combi burner is varied by the boiler thermal controls, it modulates through the setpoint's prop band.
Modulating control usually (but doesn't have to) look at the time factor as well. i.e. how quickly the setpoint is (or isn't) being achieved. It integrates the output over time.

This is called PI control.

Then it will need PI control if it is to have any "intelligence" and be an "optimizing controller". Ideally, I suppose you really need the Programmer to talk directly to the boiler modulator, so it turns it up and down rather than just on and off. I looked into this and Honeywell make one, but it's not available in England, which uses the Opentherm Protocol. Of course you need a boiler which talks "Opentherm" such as the Broag Avanta and others. My boiler will need replacing sometime in the next few years so I will wait.

The final term you can add is PID control. This is a way for the controller to react very quickly to (either) sudden changes in setpoint or measured variable.
For instance, the boiler flow temp suddenly plummets because the seperately zoned upstairs heating comes on.
This will override the I term and quicly ramp the burner to maximum. To try to reduce the error as quickly as possible.

 

[UF6]

This sounds sensible to me, I had assumed that a bandwidth of 2C meant 78C-82C, if the set point was 80. Though I could not see why you would expect the temperature to exceed the set point!

One of the worst forms of control is on/off control of flow temperature. This will lead to short-cycling if there is not much lag in the heating system.

The Honeywell literature has a setting to increase the proprtional bandwidth and says that this might be usefull in well insulated housed with oversized rads. Can't see the logic of that, can you explain?

With a given setpoint, by increasing the prop band, you will introduce some delay into the system. System heats up, switches off and has further to 'fall' before switching back on again. An instance such as this is better served with an anti-recycle time-delay. System reaches setpoint, switches off and cannot switch on again for (say) 30 minutes. The classic example is a room stat located in the hall. The heating cycles off and then back on again when the front door is opened.

Then it will need PI control if it is to have any "intelligence" and be an "optimizing controller". Ideally, I suppose you really need the Programmer to talk directly to the boiler modulator, so it turns it up and down rather than just on and off. I looked into this and Honeywell make one, but it's not available in England, which uses the Opentherm Protocol. Of course you need a boiler which talks "Opentherm" such as the Broag Avanta and others. My boiler will need replacing sometime in the next few years so I will wait.

Yes, ideally the flow temperature should track the room temperature. This is called 'feed-forward' and is ideal to prevent the space overheting. Say "If we don't reduce the flow temperature, the room will overheat; but don't reduce it too much or the room will get cold"