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How does "Temperature Difference Between Cutout And Setpoint" affect Unmet Hours?

asked 2023-11-08 04:22:26 -0500

Keigo's avatar

updated 2023-11-08 07:06:52 -0500

For ZoneControl:Thermostat, I/O Reference warns against the use of Temperature Difference Between Cutout And Setpoint in as follows:


Note that this option should be used in caution. Following recommendations may be used to input the value of Temperature Difference Between Cutout And Setpoint:

1. The heating and cooling setpoints must be separated by at least 2 times the Temperature Difference Between Cutout And Setpoint or there will be a fatal error.

2. The thermostat setpoint not met hours are incorrect with this option.

3. This option will cause excess operation of heat pump system supplemental heating coils.


Could someone explain more about the recommendation No. 2 above? Why are Unmet Hours incorrect when we use Temperature Difference Between Cutout And Setpoint? How does Temperature Difference Between Cutout And Setpoint affect Unmet Hours? Does this mean that Temperature Difference Between Cutout And Setpoint cannot be used for ASHRAE90.1 Appendix G compliance because the Unmet Hours requirement (< 300hours) cannot be evaluated correctly?

In my experience, Unmet Hours decrease a bit and cooling/heating end use increases when I use Temperature Difference Between Cutout And Setpoint. It is because HVAC systems overcool/overheat zones. I have not found Unmet Hours to be unnaturally large or small with Temperature Difference Between Cutout And Setpoint.

Below is my understanding of how cooling/heating turns on/off and how indoor air temperature changes by Temperature Difference Between Cutout And Setpoint. "0deltaºC" means Temperature Difference Between Cutout And Setpoint is not used. I don't think Unmet Hours are incorrect with Temperature Difference Between Cutout And Setpoint if HVAC systems have sufficient cooling/heating capacity and the opration schedule is appropriate.

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answered 2023-11-10 12:25:30 -0500

updated 2023-11-10 12:26:16 -0500

In my understanding, Temperature Difference Between Cutout And Setpoint is implemented by changing thermostat setpoints, eg. for heating if the setpoint is 70F and the difference is 2F, it changes the thermostat setpoint to be 72F when HVAC is operating, then back to 70F when zone temperature reaches cutout temperature.

You can output these two variables to see its behaviors:

Output:Variable,,Zone Thermostat Heating Setpoint Temperature,hourly; !- HVAC Average [C] Output:Variable,,Zone Thermostat Cooling Setpoint Temperature,hourly; !- HVAC Average [C]

Due to this implementation, my guess is that the unmet hour is calculated based on the thermostat setpoint temperatures which didn't consider deadband operations, and that's why "The thermostat setpoint not met hours are incorrect with this option." is called out. Hope this helps.

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In your example, Setpoint temperature is 70F, and Cutout temperature is 72F. EnergyPlus tries to heat the zone so that the zone temperature doesn't drop below 70F. If Temperature Difference Between Cutout And Setpoint is not implemented, Setpoint temperature is 70F, and Cutout temperature is 70F. EnergyPlus tries to heat the zone so that the zone temperature doesn't drop below 70F. They are the same in terms of heating unmet hours. I'm still not convinced why the former is considered "incorrect". In reality, all HVAC systems operate with deadband. The former is rather more accurate.

Keigo's avatar Keigo  ( 2023-11-10 18:15:59 -0500 )edit

Do you think we should NOT use Temperature Difference Between Cutout And Setpoint for ASHRAE90.1 Appendix G compliance?

FYI, ASHRAE90.1-2016 Performance Rating Method Reference Manual Section 3.5.1 asks us to model Building Zone Thermostat Throttling Range as designed with a default of 2°F.

Keigo's avatar Keigo  ( 2023-11-10 18:27:22 -0500 )edit

If you look at your plots for zone temperature vs setpoint for 0 delta C and 0.5 delta C, you can see that when the heating coil or cooling coil is on and delta is 0 C, there are no unmet hours during that time period, because ZMAT = setpoint. When the equipment is cycling (0.5 delta C) you could have unmet hours because ASHRAE 90.1 defines an unmet load hour as "an hour in which one or more zones is outside of setpoint + OR minus 1/2 temperature control throttling range." Does ASHRAE 90.1 make the distinction of heating vs cooling unmet hours?

sashadf1's avatar sashadf1  ( 2023-11-13 16:27:10 -0500 )edit

@sashadf1

When the equipment is cycling (0.5 delta C) you could have unmet hours

No, we don't have unmet hours. I have already visualised the relationship between ZMAT and unmet hours in my question. We have unmet hours when ZMAT (the thick black line) is above the orange line during cooling or below the orange line during heating.

You misunderstand the definition of unmet load hours. It is an hour in which one or more zones is outside of setpoint plus 1/2 temperature control throttling range during cooling OR minus 1/2 temperature control throttling range during heating.

Keigo's avatar Keigo  ( 2023-11-13 18:27:32 -0500 )edit

Ok, that definition of unmet load hours is more specific than ASHRAE 90.1, makes sense. In that case, you don't have any unmet hours when the coil is on, regardless of what Temperature Difference Between Cutout And Setpoint is set to.

sashadf1's avatar sashadf1  ( 2023-11-13 18:34:18 -0500 )edit

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Asked: 2023-11-08 04:22:26 -0500

Seen: 1,434 times

Last updated: Nov 10 '23