Honeywell CM901/907. Cycles per hour

[jpadeutsch]

What does "cycles per hour" mean?

Does it mean that the boiler turns on/off at this rate all the time?

Can I disable this feature?

 

[Charnwood]

I think there 4 options; 3, 6, 9 or 12cph. No ‘off’ option.

 

[Dan Robinson]

Yes and don't think so.

Not sure if having optimisation disabled also disables cycling control.

 

[D_Hailsham]

Not sure if having optimisation disabled also disables cycling control.

Optimization is not related to cycles per hour; they are connected to completely separate features.

Optimization is the feature where you set the first ON time to when you want the house to be up to temperature, e.g when you get up, say 7am. The controller then calculates how long before then it needs to turn on, which will vary according to the weather - i.e the overnight temperature drop.

Cycles per hour is related to the TPI control feature, along with Minimum ON Time and Proportional Bandwidth.

You can read about this is the Honeywell FAQ: TPI

Having the cycles set to 6 per hour, i.e 10 minute periods, does not mean that the boiler will automatically light up every 10 minutes. It means that the controller will calculate every 10 minutes how long the boiler needs to be on during the next 10 minute period. This could be all the time or it could be no time or anything in between. So the boiler could run for several 10 minute periods without coming on.

 

[Dan Robinson]

Hey DH....


On Evo, do you know exactly what the "hot water differential" is for?

I assumed it was an offset as Honeywell said their is a switched activation at 10 degrees fall.


But the NEVER seem sure of themselves? Got mine working a real treat on the thermal store

 

[jpadeutsch]

Thanks so much DH. That is a wonderful, clear explanation. So I guess that overall the pump will not be active for any more time than it would be on a unit without this feature.

What happens if the calculated time for it to be on in the next 10 minutes is less than the minimum on time?

 

[D_Hailsham]

On Evo, do you know exactly what the "hot water differential" is for?

No idea! Didn't even know that Evo could control DHW.

Normal HW cylinder thermostats have a differential of about 10C, so if you set it to 60C the stat will open at that temperature but not close until it has dropped to 50C. All I can assume is that the EVO has a facility to vary this differential, if the application requires closer control of the HW temperature.

So I guess that overall the pump will not be active for any more time than it would be on a unit without this feature.

The boiler should be running for much less time. As for the pump, this may run after the boiler has stopped - it's called pump overrun. The purpose is to cool the boiler. So there's a possibility that the pump might run for longer; but I won't lay any money on it.

What happens if the calculated time for it to be on in the next 10 minutes is less than the minimum on time?

The boiler will light for the minimum of one minute, even if the TPI says it only needs 30 seconds.

However, the 10 minute cycle time is not fixed; it's just a starting point! It can be made longer under the control of the TPI algorithm. This is not made clear in the standard literature; I found it online in a Honeywell Powerpoint presentation introducing the CM900 series.

If you have a very well insulated house, so the temperature drops very slowly, the cycle time could be half an hour or more, with the boiler lighting for only a few minutes.

You will find that, once the house is up to temperature, the radiators will feel relatively cool as they do not have to produce full output to maintain the room temperature, just enough to top it up when the temperature has dropped half a degree.

It's just like driving a car: you need full power to accelerate to 70mph; but you can cruise at that speed with a very light throttle.

 

[Dan Robinson]

Cheers D.... It makes sense, but doesn't always seem to pan out.

I suspect it is a quirk of the software/polling times for the RF. Honeywell Tech are still rather vague on it (and they don't like that I have it running on a Thermal Store.

Got it set to 1 degree at the moment - might try it on 5.

Spec sheet says it goes to ten, and implies it does what you say. Perhaps having it set too low confuses the sensor and RF polling?

Check out the Sundial Pack version - I didn't know it existed until I bought Evo, and then an RF2 Pack 4 and convinced the CS92a to talk to Evo.

Now I have half a Pack 4

Its how I found out the hard way that there are different BDR90's too. Thanks Honeywell

 

[Dan Robinson]

You will find that, once the house is up to temperature, the radiators will feel relatively cool as they do not have to produce full output to maintain the room temperature, just enough to top it up when the temperature has dropped half a degree.

I've lost days of my life trying to explain this to customers.

 

[jpadeutsch]

You will find that, once the house is up to temperature, the radiators will feel relatively cool as they do not have to produce full output to maintain the room temperature, just enough to top it up when the temperature has dropped half a degree.\\

How does this differ from a non-programmable thermostat, switching boiler off when the desired teperature is reached, and on again when temperature has dropped by some predefined amount?

John

 

[Dan Robinson]

Its all done on an algorithm and is what the proportional band is about.

Some non programmable thermostats like the DT9x series can do this but non digital certainly won't.


A programmable thermostat has the target times to work with as well as the temperatures making it more efficient.

Don't confuse TPi with hysteresis.

 

[jpadeutsch]

But doesn't all this mean that the system takes longer (possibly much longer) to reach the desired temperature? If one uses the optimisation mode then the boiler will be fired earlier and may well therefore waste energy.

Also it worries me that the decision about firing and cutting the boiler will only be taken once every 10 minutes (with the shortest possible cycle time). Isn't it possible that during the "off" part of the cycle, the temperature will fall lower than the point at which a conventional thermostat would fire the boiler? For example, when the front door is open there could be a big drop in temperature - and there might be up to 10 minutes before the boiler fires to correct this. Similarly if the TPI system overestimates the time to each the setpoint, the boiler will presumably stay on longer than necessary.

Are there any figures available to verify what the gas saving might be with a TPI thermostat as compared with a non-TPI when operating at the same set temperature? (Particularly when the house is NOT particularly well insulated)

Are there any programmable thermostats available that do not use TPI, or on which TPI can be disabled?

 

[D_Hailsham]

But doesn't all this mean that the system takes longer (possibly much longer) to reach the desired temperature? If one uses the optimisation mode then the boiler will be fired earlier and may well therefore waste energy

Before optimization you had to knows how far the temperature would drop overnight and set the time the boiler came on by guesswork. Optimization removes the guesswork. The stat will take maybe a week to learn the characteristics of your house, but once it has the boiler will light at the right time every day to bring the house up to the required temperature at the specified time.

Some time ago I rigged up system to monitor the turn-on times and found that they varied from 10 minutes to nearly two hours, depending on the time of year.

Also it worries me that the decision about firing and cutting the boiler will only be taken once every 10 minutes (with the shortest possible cycle time). Isn't it possible that during the "off" part of the cycle, the temperature will fall lower than the point at which a conventional thermostat would fire the boiler? For example, when the front door is open there could be a big drop in temperature - and there might be up to 10 minutes before the boiler fires to correct this.

I have never found this to happen, but then I don't tend to keep the front door open for long periods of time, particularly in the winter.

Similarly if the TPI system overestimates the time to each the setpoint, the boiler will presumably stay on longer than necessary.

The TPI system is self monitoring, so it will make the necessary correction if this happens.

Are there any figures available to verify what the gas saving might be with a TPI thermostat as compared with a non-TPI when operating at the same set temperature? (Particularly when the house is NOT particularly well insulated)

There was a cursory report published a few years ago which tended to denigrate TPI systems. But you should bear in mind that TPI controls have been the norm in commercial heating systems for many years and their usefulness, in terms of keeping heating bills as low as possible, is not in doubt.

Are there any programmable thermostats available that do not use TPI, or on which TPI can be disabled?[/quote]

 

[jpadeutsch]

Thanks for another useful reply
John

 

[D_Hailsham]

Are there any programmable thermostats available that do not use TPI, or on which TPI can be disabled?

I missed that bit.

Danfoss make a stat which can have TPI turned on or off.