Thermostat enforces a 5 minute minimum cooling on time into a DR cooling off event

I'm sending a demand response "cooling off" event to a Smart thermostat ("isCoolOff": true) and normally it works as I would expect. But if the thermostat had called for cooling beginning less than 5 minutes before the DR event started, cooling remains on into the "cooling off" event until cooling has been on for 5 minutes.
What is the reason for imposing a minimum cooling on time, and is there a parameter that I can set to reduce the minimum cooling on time going into a DR event?
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  • MarkK (API Architect) November 10, 2014 18:54
    Hi Allen,

    There is a minimum cycle time on HVAC equipment. This exists so as not to damage the equipment by repeatedly turning it on and off (by intention or bug). Our default is 5 minutes and the equipment itself may have its own setting (depends on equipment).

    The thermostat setting can be changed on the device itself. On the Si and Smart Models it is in Settings -> Installation Settings -> Thresholds -> Advanced Settings, the values are Cool Min On Time and Heat Min On Time. On ecobee3 they are one level up in the Thresholds menu (same navigation).

    The setting is global and affects all calls for heat, cool and off.
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  • Mark:
    Thank you for pointing me to the parameter. Setting the parameter to a lower value solves my current problem, but still leaves me with a question:
    You say "The setting is global and affects all calls for heat, cool and off.", but If I set the requested temperature on the thermostat down until the A/C starts, then immediately set the requested temperature above room temperature, the A/C cycles off within a few seconds; Cool min on time seems not to apply in this (very common) case.
    Are there rules as to when it does apply, or is this a bug?
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  • MarkK (API Architect) November 10, 2014 20:23
    According to our embedded developers, the min on time is for turning on the equipment and not turning it off. The equipment will be turned off according to the new preference. However, once off/not running, the system will enforce the min on time before turning on again. This exists to allow the compressor to depressurize before activating it again.
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  • Mark:
    Can the "cool min on time" setting be read over the API? If so, what object is it in, and what is it called?

    Thanks,
    Allen Downs
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  • MarkK (API Architect) November 10, 2014 20:47
    We have not exposed those because they affect the equipment directly. They can only be set through the device at this time (being advanced settings).
    • Mark:
      I understand the need for a minimum off time to allow pressure to dissipate so that the compressor does not have to start under full load, but I do not understand the need for a minimum on time and it seems the thermostat enforces minimum on time in some cases but not in others.
      I believe the parameter that controls minimum off time is settings object->compressorProtectionMinTime and on the thermostat is Settings -> Installation Settings -> Thresholds -> compressor settings -> Min Cycle Off Time.
      You say "once off/not running, the system will enforce the min on time before turning on again." . Aren't you talking about minimum OFF time?
      I'm not looking to set the cooling minimum on time through the API, but I am looking to be able to read it. It affects our DR strategy and I want to be able to verify remotely that it is set correctly.

      Thanks,
      Allen Downs
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  • Hello Allen,

    There seems to be some confusion of the two parameters. The Min Cycle Off Time (located in Settings->Installation Settings ->Thresholds->Compressor Settings) is a strictly enforced time that the thermostat will always honour. It controls how long the thermostat will wait before turning back on the compressor, after its last on/off cycle. This, as you correctly pointed out above, is done to protect the compressor motor and this is why it is always strictly enforced all the time.

    The Min Cycle On Time (located in Settings->Installation Settings->Thresholds->Advanced Settings -> Compressor Settings) is a parameter that is a suggestion that the thermostat applies selectively. The purpose of this parameter is to prevent compressor short cycling, and not having the thermostat bounce around the deadband by short cycling the compressor and thus incurring all the start up losses and costing the home owner a lot in wasted energy. The thermostat will, for the most part, always use the min cycle on time to force the compressor to run for that amount of time even if it results in overshooting the deadband. There are a few rare occasions when the thermostat will not honour it, and one of them is if the set point is changed (e.g. the user applies a hold). The reason why the thermostat does not force the min on time in this scenario is because a user is likely at the thermostat trying to raise the set point to stop the A/C. So it will be discomforting for the user if the thermostat is not responding to the request, and likely lead to the perception the the thermostat is not working.

    The min cycle on time is not a parameter that is exposed by our API. The default value is 5 minutes, and for the majority of our users this value is not typically changed.

    Hope this info helps clarify things.
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  • Hesham:
    Thank you for explaining the reason for selective enforcement of a minimum run time; now I understand why the thermostat is acting as it does.
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  • I hate to revive an older thread, but what is the maximum value for Min Cycle Off Time? My older gas pack HVAC system needs greater than five minutes of Off time between cycles and I have yet to find a thermostat that allows this. I am concerned far more with preventing short cycling than precision temp control. Thank you!
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  • ScottF (Official Rep) July 25, 2016 19:04
    Hi Francis,
    Looking at the settings on the thermostat, the max value for that field is 900s (15 minutes).

    Hope that helps,
    Scott.
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  • That helps immensely and I thank you sincerely for the info, Scott!

    Francis
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  • hello I need for the min cycle off time to be 180s not 240s My ac is in a legal medical grow in California and 4 mins off causes the temp to rise to much would there be any work around to make this happen? Thank you
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