We have a new house with a CM907 controlled heating system. The installers have been back a number of time but the boiler still turns on and off repeatedly. The CM907 even says that the room temperature is 18.5 degrees and the target temperature is 19.0 degrees but it still switched the boiler off. A few minutes later the boiler goes on again. Any help would be appreciated. Cheers.
New heating system 'hunting' and cannot seem to stop this
- Thread starter helpmeout123
- Start date
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That makes perfect sense although it turns the boiler on again a few minutes after turning the boiler off? I would have thought that once it says 18.5 it would leave it like that until it drops back below 18.5. Must be being thick? Cheers.
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The 907 is clicking on and off with the little flame symbol appearing which turns the boiler on and then off again. I am worried all this is going to harm the boiler? Cheers again for the help.
There is a setting within the 907 that allows the Proportional Band Width to be changed from between 0 to 3 degrees? What exactly does this do? It is currently set at 1.5 so does this mean it will turn the heating on within 1.5 degrees of the required temperature? Cheers.
Not quite.
When the temp is more than 1.5deg from the set point, the boiler will run continuously. once it reaches the 1.5deg point it starts to run in proportion to the temp difference. So if it is 1.2 deg from the set point it will run 80% of the time. The time used is determined by the cycle time, which is set to 6 per hour for a gas boiler; this is ten minutes. So 80% will be 8 minutes on 2 minutes off. As the temp gets closer to the set point the on time will reduce and the off time increase. The whole effect is to keep the room temp virtually constant.
According to Honeywell literature, the Proportional bandwidth setting is for use in "well insulated" houses. Presumably you increase the bandwidth if you have double glazing, cavity and loft insulation. The aim (so I have read somewhere) is to stop the system "hunting", i.e fluctuating either side of the set point.
The can't be any harm in experimenting with the PB to see what effect it has.
What a great help, thanks very much. You have given me more help than all the plumbers we have seen so far.
What if I set the setting to ZERO (0)?
Would this be bad then?
The thermostat is in a bedroom as it runs the heating for the bedrooms only so my theory was that if this setting was zero it would keep heating until it reached the exact temperature and then switch off? Would this in our case stop the boiler going on and off? Cheers again.
You can't set the Proportional bandwidth to zero; 1.5deg is the lowest.
Let's assume that you could set it to zero. The boiler would run at full tilt until the stat read 19°C and then switch off. But the radiators would carry on giving out heat as they cooled down, so the temperature would rise above 19°C to say 20/21°C. The room would then start to cool down until it reached the stat turn on of 18°C and the cycle would start again. You then have a temperature swing of 2 or 3 degrees. The proportional bandwidth "slows the boiler down" as it approaches the desired temp to minimise the overshoot.
You don't say what type of boiler you have, but there are boilers and programmers now available which work together to control the gas flow according to requirements. Most of these systems are proprietory as there is no agreed standard for communicating between boiler and programmer. The OpenTherm standard is used by some manufacturers, but it does not have any official backing.
Back to your "problem"
According to the CM907 spec the temperature control accuracy is ±0.5°C, which is considerably better than mechanical thermostats with anticipators, where you could get temperature swings of several degrees, which is noticeable. The display only changes in 0.5°C steps, so the fact that you have it set to 19°C and it says actual is 18.5°C means that the actual is some where between 18°C and 19°C.
This is unusual, to say the least! Is there any particular reason for this such as someone bed-ridden etc or is it how you found it when you moved in?
What controls the temp for the rest of the house? Do you have TRVs on all the other rads?
Provided the regular on and off of the boiler does not disturb you, I would ignore it. The programmer is doing "what it says on the tin". But, as I said, there is no harm in experimenting with the Proportional Bandwidth. Set it to the max of 3°C and see what happens. The real criteria is how much the temperature fluctuates about the set point.
Really grateful for all your time and help on this.
The heating upstairs has been done separately due to the floor space of the house apparently and we have a separate thermostat for the 'living' area which is downstairs and the landing. Well this is what we were told although the radiator on the landing is actually controlled by the upstairs thermostat!!! The thermostat for the upstairs heating is actually in a bedroom which I can sort of understand although losing the plot with all this to be honest. Not sure this makes any more sense to you anyway? We do indeed have trv's on ALL radiators apart from a couple upstairs and one downstairs which is apparently a safety mechanism to allow excess heat to be lost. Cheers again.
So it's a fairly large house with separate heating zones upstairs and down. This is the recommended layout if the total floor area is over 150m².
I do hope you do not have a TRV on the rad in the rooms where the wall thermostat is located. If you do, you will get problems as the TRV and the wall stat will be vying for control.
Rads as a safety mechanism? Is that what your heating engineer told you?
True in the old days of coke fired boilers where you had to allow for the heat to dissipate when you damped the boiler down for the night
(Those were the days ) But gas fired boilers? Not really necessary, although some boilers will run the pump for a few minutes after going out to reduce the temperature in the heat exchanger. But the heat will get lost in the general pipework or a bypass if necessary.Just came across this post while looking into some problems I'm having with my boiler. In parallel, I'd contacted Honeywell who sent me a very nice FAQ document. It may be of use to some people here so have posted it in my album.
Can't add anything to the existing useful technical explanations but can confirm my cm927 (wireless version), on a single zone GCH, does the same thing.
It does seem to be quite effective at maintaining room temperature.
However, I admit, I also thought it a bit odd when set temperature is only slightly higher than room temperature and the bolier just comes on for 5 minutes then shuts down and then might come back in again in 10 minutes time.
Happened this morning at 5.30am when room temp was 18c and programmed set temp changes to 19c for example.
I'd try to let it work for a while and see if you can accept the functionality and take it from there.
It does seem to be quite effective at maintaining room temperature.
However, I admit, I also thought it a bit odd when set temperature is only slightly higher than room temperature and the bolier just comes on for 5 minutes then shuts down and then might come back in again in 10 minutes time.
Happened this morning at 5.30am when room temp was 18c and programmed set temp changes to 19c for example.
I'd try to let it work for a while and see if you can accept the functionality and take it from there.
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