Setting a temperature dead band in OpenStudio

asked 2024-07-25 10:52:35 -0600

Olamide
57 ●1 ●7

updated 2024-08-06 13:19:25 -0600

Is there a way to set a temperature dead band in OpenStudio?

I am trying to set up my heating and cooling setpoints to have a dead band range so that the HVAC system can potentially be turned on and off as needed, instead of being constantly on. However, when setting up a heating or cooling schedule, it appears that I can only set one static value.

For example, in the screenshot below, I set a cooling setpoint of 23.9C. I want a dead band of 1.1C, so I set the upper and lower limits as 22.8C and 25C. However, those limit settings seem to be only for display purposes. When I run the simulation, the zone temperature is still constant. Is there a way around this?

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UPDATE: Temperature fluctuation is noticed when the building model is changed from three thermal zones to one. My original building model is made up of an unconditioned crawlspace, a conditioned main floor, and an unconditioned attic. The secondary model is made up of just the conditioned main floor. Both have exactly the same construction materials, but the indoor temperature (in blue) is almost steady in one but fluctuates between the setpoints in another.

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UPDATE 2: The cooling coil (red) responds to outdoor temperature in both cases. In the RHS case, it goes off completely at night and the heating coil (dark green) comes on to raise the temperature back up to the set point. Both models use the "PTAC DX Clg Elec Htg" HVAC system.

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There is an advanced feature in EnergyPlus that allows it, but I doubt it's available in the OpenStudio application. See here.

shorowit ( 2024-07-25 18:39:41 -0600 )edit

Thank you @shorowit. All answers I found are only applicable to EnergyPlus, so I fear you might be right.

Does this mean that an OpenStudio simulation with a static setpoint would result in much higher energy consumption since the HVAC system would always be on? Or can I assume that it would be roughly the same as if my deadband was 2F (1.1C)?

Olamide ( 2024-07-26 10:24:20 -0600 )edit

The EnergyPlus feature is more about capturing short-term transient effects of cycling. Energy use during the timestep shouldn't be much different between the two (as the impacts of cycling are addressed through the PLR curves), which is why it's not a commonly used feature.

shorowit ( 2024-07-26 10:31:48 -0600 )edit

Thanks a lot for the explanation @shorowit.

Olamide ( 2024-07-26 10:37:54 -0600 )edit

@Olamide are you trying to define a deadband difference between heating & cooling thermostat setpoints, or are you trying to define a cutout temperature difference from the thermostat setpoint?

Aaron Boranian ( 2024-08-01 10:30:48 -0600 )edit

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answered 2024-07-31 19:43:33 -0600

mattkoch
1081 ●10 ●9

Hello Olamide, I may be completely misinterpreting your question, but I believe what you are looking for can be accomplished in the Thermal Zone tab of OpenStudioApp 1.7.0. Simply define a cooling temperature setpoint schedule and a heating temperature setpoint schedule and assign these to the Cooling and Heating Thermostat Setpoint fields in the image below, respectively.

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The setpoint schedules could also have some setbacks, say an unoccupied period from midnight to 6 AM and from 8 PM to midnight, where temperature could be, say 80 F for cooling and 65 F for heating, and then an occupied period from 6 AM to 8 PM, where temperature could be 75 F for cooling and 70 oF for heating. All this also makes sure there is no overlap between cooling and heating, that is to say your unoccupied deadband is 80 - 65 = 15 F and the occupied deadband is 75 - 70 = 5 F.

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Hello @mattkoch. I defined static cooling and heating setpoints in the Thermal Zone tab: cooling setpoint = 75F, heating setpoint = 69F. I ran the simulation for a few days in the summer, and the temperature remained steady throughout. My original building model is made up of an unconditioned crawlspace, conditioned main floor, & unconditioned attic.

But I noticed that there was a temperature fluctuation when I reduced the building model to just the conditioned main floor. Both have exactly the same construction materials so I'm not sure why this is happening. The post is updated to show this

Olamide ( 2024-08-01 11:01:35 -0600 )edit

I suppose your building is on the southern half, with high temperatures in January? The profile on the right might be expected with a 75/69 heat/cool combination when the climate zone is such that there is enough cooling/heating load during the day/night to require cooling/heating. HOWEVER, your nighttime OA only drops to 24 C, and the main floor should not drop below that. You could plot the cooling and heating rate profiles on your cooling and heating coils - they should support the temperature profile.

mattkoch ( 2024-08-01 21:21:23 -0600 )edit

The profile on the left may be more realistic in that regard - there are in fact tiny drops on 1/3 and 1/6, but other than that, cooling is likely required day and night to maintain 24 C, though at lesser cooling power at night than during the day. You might want to run a few days in June to verify that temperature indeed drops to 20.5 C and heating is needed most of the time. Now, back to the right, is it possible that your cooling coil is on at full cooling power 24/7. Again, plot your cooling and heating coil rates (or air flow rates) to verify proper modulation.

mattkoch ( 2024-08-01 21:29:33 -0600 )edit

@mattkoch the building is in Texas and the simulation period is actually a week in August, but DViewer, always defaults to the beginning of the year. The heating setpoint is 20.6C, which is the minimum temperature on the right. The cooling power mimics outdoor temperature and heating is flat. I will re-run the simulation and post screenshots. Thank you.

Olamide ( 2024-08-02 11:04:56 -0600 )edit

@mattkoch I plotted the heating and cooling rates (see original post). Even though both cases have the same HVAC system and the same materials, the model on the right seems to have a less efficient HVAC system as the heating coil comes on sometimes. Peak cooling load is 2.2kW on the LHS and 3.2kW on the RHS. I would have expected it to be less with a smaller geometry. Do you know why this may be happening?

Olamide ( 2024-08-06 13:26:38 -0600 )edit

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