Honeywell CM927 - Boiler cycles every 10 mins

[Nerdio]

I have just installed, well a heating engineer has, a Honeywell CM927 programmable thermostat. I have read that these thermostats will cycle the boiler regularly to maintain the programmed temperature. My boiler/pump seems to be cycling every 10 minutes, which is the configured cycle rate. This seems a lot to me, because the temperature on the thermostat seems solid at the programmed value.

At the recommendation of the installer I run the heating all the time now, and use the thermostat to control things. I am concerned though that this is going to be an expensive strategy.

I have read, and seen denied, that the thermostat is a 'learning thermostat' and will learn what is required. In fact the man at Honeywell eluded to this.

Does anyone have any experience of using this device and getting good results? (Should I run the heating all day, or set the programmer to knock the heating off during the day)?

Thanks in anticipation

 

[mikely]

I installed a CM927 about 12 months and am very pleased with the results. The cycling you refer to is known as time proportional control. As the thermostat approaches the set point temperature, it starts to reduce the heating system output by turning the heating demand on and off over a cycle period. So if the heat is on for 5 minutes then off for 15 minutes, the system is effectively supplying ¼ of the maximum heat output of the boiler. It adjusts the On/ Off cycle so that the heat output is just enough to reach and then maintain the set-point temperature.

I leave the heating on all the time with a night temperature set at 14C, daytime at 16C and morning and evening at 19C. During the recent cold spell this has helped keep the house more comfortable because it does not cool down so much. If you set the optimisation feature ON, the stat will learn how long it takes to warm up the house and start the heating earlier or later depending on the temperature rise required. You tell the stat that you want the house to be 18C at 7 am and it works out when to start the heating. Brilliant!

 

[Nerdio]

Thanks mikely for the reply. You've given me some confidence that the unit is the right thing to have.

What I have noticed is that my pump is running for some period, then shuts off. Within minutes though it is back on again. For example, Pump turned off at 14:00. Was back on at 14:04 and ran again until 14:10. Then a few mins later it was on again. I know the boiler wasn't running all this time, but it would appear that the boiler is on for a reasonable period of time every hour. More than I would expect. This all seems to be in accordance with the cycle rate parameter (2:Cr), which is what I think is controlling what you were talking about. I would have thought that my house would have been reasonably thermally stable, and that it wasn't permanently trying to reach the set-point. From what you say the boiler should only cycle whilst it tries to reach the set point, then to maintain it (which I thought would be less cycling). Maybe my thermostat is in a slight draught, which cools it...

Did you find that after you had run with the thermostat for a while it stabilised a bit, as it 'learnt'? (Or do I need to turn on optimisation to enable the learning ability)?

I would appreciate any further comments / thoughts.

 

[D_Hailsham]

I have just installed, well a heating engineer has, a Honeywell CM927 programmable thermostat. I have read that these thermostats will cycle the boiler regularly to maintain the programmed temperature. My boiler/pump seems to be cycling every 10 minutes, which is the configured cycle rate. This seems a lot to me, because the temperature on the thermostat seems solid at the programmed value.

If the temperature is remaining constant it means the thermostat is doing its job. How many minutes out of the ten is the boiler on?

At the recommendation of the installer I run the heating all the time now, and use the thermostat to control things. I am concerned though that this is going to be an expensive strategy.

Are you saying that you keep the house at the same temperature for all 24 hours of the day? Don't you have a lower night time temperature?

I have read, and seen denied, that the thermostat is a 'learning thermostat' and will learn what is required. In fact the man at Honeywell eluded to this.

The learning aspect is mainly to do with the optimization facility, where you tell the thermostat what time you want the house to be up to temperature in the morning. It takes a few days to learn the rate at which the house cools down when the heating is turned off. once it has this information, the thermostat can then calculate when the heating needs to go on to ensure the house is up to temperature by the required time.

Does anyone have any experience of using this device and getting good results? (Should I run the heating all day, or set the programmer to knock the heating off during the day)?

I have had a CM927 for over three years and wouldn't be without it. It keeps the house at a constant temperature and the gas usage is reasonable, even if the bills are not.

As for keeping the heat on all day, that depends on two things: your lifestyle - how often you are home, and how quickly the house loses and gains heat. The sensible thing to do would be to experiment. Run it for one day with the heating on all day and for a second day with it off. Take gas meter readings (including the red figures or end dials) on each day and compare results.

With six time/temperature settings you can set whatever time-temperature profile you want. High in the morning when you get up, lower when you are at work, back to high in the evening and lower at night.

 

[Nerdio]

Thanks for the reply.

It is only my first day with the thermostat installed, I guess I was just surprised at what happened.

The temperature is stable, at least according to the CM927, so I have no doubt it works in that respect. I am going to monitor the boilers duty cycle this eve. as I am aware that the pump does overrun when the boiler goes off.

I do have a temperature profile set over the day, and drops off at night. (I work at home though, so I do run the heating a little more than most).

I am going to monitor my gas usage as you suggest, I have an idea of what my daily usage was over the past 2 weeks, and they were cold, so I expect less than that.

Is there any merit in reducing the cycles per hour, or does this just mean that the boiler just has to work harder when it does come on?

I'll endeavour to keep this post updated with my findings. Thanks for all the contributions.

 

[Nerdio]

Just a quick update.

The system is cycling pretty much every 10 minutes, but this seems to die down a bit. What I think I have noticed most of all is the pump running for longer than I expect. The boiler itself only runs for 2 - 3 minutes in any one cycle.

As a whole, gas usage seems unchanged. BUT.... The house seems to have a more constant temperature throughout, previously our downstairs toilet would be very cold once the heating had gone off, now it isn't so cold.

I have turned optimisation on, which seems to work. The house is up to set temperature by the time I get out of bed. Well almost, its still learning.

The biggest result of all was that at 5:30 yesterday my wife complained of being hot! That has never happened before, so must be doing something right.

 

[D_Hailsham]

The biggest result of all was that at 5:30 yesterday my wife complained of being hot! That has never happened before, so must be doing something right.

You will find that, because the thermostat maintains a more constant temperature, you can run it at a lower temperature. Try dropping it by 0.5C during the day and see if you get any reaction. If you don't drop it again until SWMBO complains.

One trick to try, if the house is occupied all day, is to make use of all six time periods. no 1 is wake up and No 6 is night time. 2, 3, 4 and 5 should be spread evenly between 1 and 6 and set to the same temperature as 1. If some one puts the temperature up manually it will get reset automatically at the next time period.