# Theromstat Spec

Thanks to advice from this group I have successfully fitted a room thermostat to my central heating system.
I noticed however on reading the spec sheet that came with the unit (Drayton Digistat 1) that there is a spec for "Differential <0.6 degrees c at 4 degrees c per hour" just curious to know what this means if anyone knows?
Thanks
Marc
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My guess is that the hysteresis is set for 0.6 degrees, provided that the temperature changes at or below 1C per 15 minutes. I may be wrong.
Christian.
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I can only guess! The differential is the amount of in-built hysteresis - i.e. the difference in temperature between opening and closing. This is less than 0.6 deg C. For example, if the stat opens at 20 degC, the room would have to cool to something approaching 19.4 degC before it would close again.
The 4 degC per hour bit seems to be referring to the rate at which the room temperature is changing. It seems to suggest that the differential is <0.6 at a rate of 4 deg/Hr - but may be different at a different room temperature change rate (for reasons which are not immediately apparent to me!).
Maybe someone who knows about these things rather than guessing can explain this better?
Roger
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They're telling you the conditions of the test: The chamber in which the thermostat was tested was heated at a rate of 4 degrees per hour until the thermostat switched (off) and the temperature was recorded, the room was (perhaps heated further) then cooled at a rate of 4 degrees per hour until the thermostat switched again (on) and the temperature was recorded. The difference between the two temperatures was less than 0.6 degrees.
At a higher or lower rate of change temperature the thermostat may behave differently, eg. at a high rate of change the differential may be lower, or at a lower rate of change, the differential may be higher - or vice versa.
HTH
--
fred

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Marc Jennings wrote:

A thermostat with a very low hysteresis and a quick response can cause the boiler to short cycle.
In the one I have there is a switch inside to limit the number of boiler cycles per hour..
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