Honeywell 927 'overshooting'

[Johnytuono]

Hi
I have a Honeywell 927 controlling my c/h. When I set the target temp. to, say, 21c , after a while it reads .5 over. I have been into the user settings and altered the prop. bandwidth a few points at a time and have eventually reached its max. of 3.0. Still its showing .5 over at room temp. Now, my boiler stat. is set to max., so should I now keep the prop. bandwidth at 3.0 and experiment by altering the boiler stat. down a mark at a time?
Ta in advance
JohnyT

 

[bengasman]

The display is 2.5 digit; the actual temperature is not necessarily the reading.
Apart from that, it really does not matter.

 

[Johnytuono]

The display is 2.5 digit; the actual temperature is not necessarily the reading.
Apart from that, it really does not matter.

Thanks but what does ".....the display is 2.5 digit...... mean"?
JohnyT

 

[bengasman]

Two and a half digit; last one is binary, not decimal.

 

[D_Hailsham]

The display is 2.5 digit; the actual temperature is not necessarily the reading. Apart from that, it really does not matter.

Thanks but what does ".....the display is 2.5 digit...... mean"?

Bengasman means that the figure after the decimal point is either a 0 or a 5, i.e. it cannot show 0.1, 0,2 etc. At what actual temperature it changes is unknown. I would guess that between 19.7°C and 20.2°C it would show 20°C and between 20.3°C and 20.7°C it would show 20.5°C.

The thermostat is designed to maintain the temperature within ±0.5°C of the set temperature. If you want to ensure the temperature does not exceed 21°C, set it to 20.5°C.

Changing the prop bandwidth will not have any effect; set it back to the normal 1.5°C. Changing the boiler temperature will have no effect either as the CM927 has overall control and will turn the boiler off when it decides to, which could be before the boiler thermostat. You will probably find that, when the house has reached temperature, the water temperature is not up to the boiler thermostat's setting.