CM927 Thermostat Accuracy

[jclark]

Hi,

First post and I hope its not a stupid question, we have a Baxi 105e combi connected up to a Honeywell CM927 and BRD91 which has been running quite happily for the past 10 years (with the exception of having to heat up the LCD over the toaster every few months as the LCD failed a couple of years back!), I've never paid much attention to any of the installer parameters and have just left it as is.

Our house isn't very well insulated by modern standards as it was built in 1902 , we have more than the required recommendation of insulation in the loft and new double glazing throughout, but we have single brick walls that don't have a cavity so no option for CWI.

I had someone round to give us some ideas for installing either external or internal wall insulation and whilst he was measuring up I was taking some reading with his thermal leak detector, I was surprised to find that there was about a -1.5 temperature difference as an average in the lounge compared to the stat reading on the CM927 when the CH had been on for a few hours.

So I purchased a 2 types of digital thermometers and put these is in the lounge right next to the CM927, from a cold starting position all 3 were virtually identical, the CM927 read 16.0, 1st thermo read 16.1 and the 2nd one read 16.2, I then turned the CH on and set the room temp to 19, after approx. 1 hour the CM927 read 19, and both digital thermos read 17.5, I also borrowed a old style mercury thermometer and again it differs from the CM927 by about a degree less.

Is this expected of the CM927, that when its running the CH that the temperature difference is offset like this?.

I know that the CM927 is supposed to be fairly accurate but I was surprised that there was such a difference, I know there is a installer parameter whereby you can set the temp offset, so that was going to be my next task to set the CM927 first down by -0.5 and then see what happens.

Am I overthinking this!

Cheers


John

 

[John.g]

I have a simple FantiniCosmi digital thermostat and it lags the other check digital thermometer (both the same initially) by about 0.2/0,3C, but catches up after a few minutes it has the hysteresis set to 0.3C and controls very well. I have asked this question elsewhere, how do these stats sense the temperature as they are almost hermetically sealed with no slots for air sensing/circulation, is it through the display window itself?.

 

[Chuck]

In a room containing a warm radiator, the radiant temperature is normally higher than than the air temperature. My guess is that a CM927, which has the sensor inside a closed matt plastic case, measures closer to the former than the other thermometers which are closer to the latter.

 

[jclark]

In a room containing a warm radiator, the radiant temperature is normally higher than than the air temperature. My guess is that a CM927, which has the sensor inside a closed matt plastic case, measures closer to the former than the other thermometers which are closer to the latter.

true, according to the documentation it measures the ambient temp I think, but then I would of thought that a IR sensor would of been closer in the range as that is reading the thermal temp, as John G alluded to not quite sure how it reads the temperature inside the box, I forgot that I had a second CM927 from where the display has failed completely, after swapping over the displays i used the other CM927 but it was the same so I think that it must be somewhere within the display as I vaguely recall that when I heated up the LCD once to try and revive the display when I reassembled the device the temperature was at the Max limit, but they may of been because it was still warm and the sensor on the PCB was picking this up.