Is my Honeywell CM927 wasting gas?

[D_Hailsham]

If I turn the system off it goes into frost mode and the boiler will not fire until it reaches that temperature (which it does as the house is empty for 5 days a week). What controls the limits at which this is determined?

Can you clarify what you are asking?

 

[DaveSteveBob]

[/quote]

Can anybody explain what the offset does.
My understanding is that it only changes the displayed
temperature without affecting actual settings, IE room
Temp 21 C / displayed temp 20 C (target temp unaffected).
Is this correct.

 

[staz1000]

Have you use the Temperature Offset setting to correct the CM927 for any discrepancy between actual and displayed room temperature?

The temperature it's reading is right so there's no need to. Even if there was, adjusting the offset wouldn't affect my problem, it would simply occur at a different temperature.

 

[D_Hailsham]

Can anybody explain what the offset does. My understanding is that it only changes the displayed
temperature without affecting actual settings, IE room
Temp 21 C / displayed temp 20 C (target temp unaffected).
Is this correct.

Correct. It means you can correct for any errors in the displayed room temperature. This could be because the stat is in a warm/cold part of the room.

It must have some effect or there wouldn't be any point having it.

Say your stat is in a cold part of the room and set to 21C. The room warms up and the main part reaches 21C, but the stat is in a cold part so it only reads 20C and consequently carries on heating up the room. When the stat eventually cuts off, the room temperature is over 22C. By changing the offset so the stat reads 21C (not 20C) when the main part of the room reads 21C, the overheating will not occur.

Obviously for this to happen the change must affect both the actual and required room temperature.

 

[MJN]

The display offset is a red herring to the OP's issue.

Mathew

 

[staz1000]

Aha I hadn't seen there was a second page...

If I turn the system off it goes into frost mode and the boiler will not fire until it reaches that temperature (which it does as the house is empty for 5 days a week). What controls the limits at which this is determined?

Can you clarify what you are asking?

When I meant was how much higher than the target temperature does the room need to be at to stop the programmer from cycling? I could test this myself but I'm not at home for a week now.

And is this temperature adjusted by the proportional bandwidth setting?

Correct. It means you can correct for any errors in the displayed room temperature. This could be because the stat is in a warm/cold part of the room.

It must have some effect or there wouldn't be any point having it.

I think you're misunderstanding the problem. It's not that it's the room going over the target temperature, that's not an issue, it's that once the temperature has been reached (or exceeded) the boiler will still be fired.

 

[DaveSteveBob]

Can anybody explain what the offset does. My understanding is that it only changes the displayed
temperature without affecting actual settings, IE room
Temp 21 C / displayed temp 20 C (target temp unaffected).
Is this correct.

Correct. It means you can correct for any errors in the displayed room temperature. This could be because the stat is in a warm/cold part of the room.

It must have some effect or there wouldn't be any point having it.

Say your stat is in a cold part of the room and set to 21C. The room warms up and the main part reaches 21C, but the stat is in a cold part so it only reads 20C and consequently carries on heating up the room. When the stat eventually cuts off, the room temperature is over 22C. By changing the offset so the stat reads 21C (not 20C) when the main part of the room reads 21C, the overheating will not occur.

Obviously for this to happen the change must affect both the actual and required room temperature.

So it does have an actual effect on the temp and not just the displayed temp?

 

[MJN]

When I meant was how much higher than the target temperature does the room need to be at to stop the programmer from cycling?

Assuming by cycling (which is a bit of a reserved term in this context) you mean seemingly turning the boiler on despite being at/above the setpoint then this is indeed the proportional bandwidth setting. Hence, a setting 2C will give a 4C window within which TPI will activate.

It's not that it's the room going over the target temperature, that's not an issue, it's that once the temperature has been reached (or exceeded) the boiler will still be fired.

I appreciate it may be counter intuitive but, as discussed, this is actually a good thing as it will prevent undershoot. If it waits until the temperature drops to the set point or below then by the time the boiler has fired and water circulated the room temperature will have dropped somewhat below hence the stat has arguably failed in its duty.

The TPI functionality of the CM series cannot be disabled (completely at least, you can set all relevent values to minimum to reduce it) and, to be honest, if you didn't want it you'd be better off with another stat as TPI is what the CM is all about (and is a key capability that you pay for).

I'd recommend sticking with it - if you try and view it's actions in a positive light you might start to appreciate its accurate temperature maintaining capability a bit more!

Mathew

 

[MJN]

So it does have an actual effect on the temp and not just the displayed temp?

Switching logic is based against the displayed temperature so if you tweak this with the offset adjustment then, yes, it will have an affect.

As D_Hailsham touched on, the offset is not intended to be used to calibrate true accuracy (the sensor is plenty accurate enough) but rather to correlate measurement with something more meaningful to the user, whether that be to account for a variation on siting but also if there is another thermometer in the room (again particularly if it sited differently).

Mathew

 

[staz1000]

Assuming by cycling (which is a bit of a reserved term in this context) you mean seemingly turning the boiler on despite being at/above the setpoint then this is indeed the proportional bandwidth setting. Hence, a setting 2C will give a 4C window within which TPI will activate.

I use the term 'cycle' because this is the term used in the installer guide.

The default Pb is 1.5 and it can be increased to 3 in 0.1 steps, but doing this would make my problem worse if I understand it right.

I appreciate it may be counter intuitive but, as discussed, this is actually a good thing as it will prevent undershoot. If it waits until the temperature drops to the set point or below then by the time the boiler has fired and water circulated the room temperature will have dropped somewhat below hence the stat has arguably failed in its duty.

The TPI functionality of the CM series cannot be disabled (completely at least, you can set all relevent values to minimum to reduce it) and, to be honest, if you didn't want it you'd be better off with another stat as TPI is what the CM is all about (and is a key capability that you pay for).

I'd recommend sticking with it - if you try and view it's actions in a positive light you might start to appreciate its accurate temperature maintaining capability a bit more!

Mathew

I am positive about the concept, I've done some work on control techniques before and I know they're normally very smart methods of efficiently controlling a system, but I do think TPI just isn't suitable for a modern house with a very high green rating. Well, not with the current programmer settings.

I think what I'll do (next time I'm home) is monitor each cycle for a couple of hours and see what proportion of cycles are essentially a waste. If on 2 of the 3 per hour the relay is only enabled for the minimum time then I will change the programmer for a traditional one.

 

[MJN]

The default Pb is 1.5 and it can be increased to 3 in 0.1 steps, but doing this would make my problem worse if I understand it right.

Yes, I think it would. It is rather unusual to want to disable TPI hence probably why you cannot reduce the window further than the default. A reduction would probably not suit your needs anyway given that the steady-state temperature is being held within +-0.5C (and reducing the window to 0C would be effectively completely disabling it).

I think what I'll do (next time I'm home) is monitor each cycle for a couple of hours and see what proportion of cycles are essentially a waste. If on 2 of the 3 per hour the relay is only enabled for the minimum time then I will change the programmer for a traditional one.

In determining 'waste' do ensure you factor in the absence of overshoot when compared to a non-TPI controller. TPI is all about accuracy of temperature maintenance and so what you call waste others might call comfort.

If you do decide to replace it put it on eBay - the CM-series are popular stats so you'll easily get £35 or so for it.

Mathew

 

[staz1000]

Yes, I think it would. It is rather unusual to want to disable TPI hence probably why you cannot reduce the window further than the default. A reduction would probably not suit your needs anyway given that the steady-state temperature is being held within +-0.5C (and reducing the window to 0C would be effectively completely disabling it).

Yep I think you're probably right there.

In determining 'waste' do ensure you factor in the absence of overshoot when compared to a non-TPI controller. TPI is all about accuracy of temperature maintenance and so what you call waste others might call comfort.

If you do decide to replace it put it on eBay - the CM-series are popular stats so you'll easily get £35 or so for it.

Mathew

I think it's going to pretty impossible to determine an accurate figure other than spending a week or so using this programmer and then another week using a normal thermostat. At that would then rely on those two weeks having the same weather and the same heating requirements.

I agree that it's going to provide more comfort but if, for example, over the course of a number of hours more than half of the cycles are empty, for want of a better word, I will not be convinced that the benefits outway the losses.

It won't be until I'm home next week but I will report my findings.

Thanks again for your help Mathew.

 

[MJN]

I agree that it's going to provide more comfort but if, for example, over the course of a number of hours more than half of the cycles are empty, for want of a better word, I will not be convinced that the benefits outway the losses.

Any boiler firing, at least of a couple of minutes or more, will be contributing heat to the building and so as long as you're not overshooting (0.5C wouldn't be considered an overshoot as thermistor accuracies in the domestic environment are <1C) then it's really not being wasted. Gas purging and drawing of cool air muddy the waters a little but not enough to offset the general principle.

If you were to omit these occasional firings you'd only have to fire for longer when it finally drops to a level that you're willing to let it fire at. Over a given time period there needs to be a certain amount of kwH energy to maintain the temperature - how this divided up in terms of power vs time is down to you (or rather the stat) but I know which strategy I'd prefer in terms of accuracy and comfort.

Thanks again for your help Mathew.

No worries. I commend you for challenging the function of operation and claimed benefits however I also urge caution in considering ditching what is generally a highly-regarded stat. Sure, it's not perfect, but at this level it's a hard one to beat.

Mathew

 

[staz1000]

Any boiler firing, at least of a couple of minutes or more, will be contributing heat to the building and so as long as you're not overshooting (0.5C wouldn't be considered an overshoot as thermistor accuracies in the domestic environment are <1C) then it's really not being wasted. Gas purging and drawing of cool air muddy the waters a little but not enough to offset the general principle.

If you were to omit these occasional firings you'd only have to fire for longer when it finally drops to a level that you're willing to let it fire at. Over a given time period there needs to be a certain amount of kwH energy to maintain the temperature - how this divided up in terms of power vs time is down to you (or rather the stat) but I know which strategy I'd prefer in terms of accuracy and comfort.

I hear what you're saying but the thing that makes it much less convincing for me is that after changing the cycle rate from 6/hr to 3/hr there are still occurances of these empty cycles, that simply says to me that 6/hr was clearly too much. Also I don't believe that the boiler firing, but only reaching 60 degrees (which takes about 2 minutes) before switching off again, has any real effect on the already-cold radiators.

You have to admit there will be an optimum cycle rate for each and every house (or household), and the more efficient the house (and its green occupants) the lower this rate must be. I believe that my green house, with only me in it who doesn't go in and out all day, needs a lower rate. If after testing I find that my empty-cycle rate is lower than 50% then I may well be satisfied by 3/hr.

 

[MJN]

It might be worth pointing out that the cycle rate setting is a maximum cycle rate - it doesn't mean it will fire up, say, 3 times an hour every hour if it doesn't consider it has to.

If you up the min on/off time then any firing will have a more pronounced effect. This may lead to it being called upon less.

Incidentally, do not take my defence of this stat to be purely for the sake of argument or as a slave to the manufacturer's claims but I truly believe that its TPI control is effective, once configured correctly for the system/house. In my experience the same cannot be said for its optimum start capability which I have observed getting the house upto temperature nearly an hour too early - unfortunately the CM-series is pretty poor in this respect as there is no configuration tweaking possible.

Mathew