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Packaged Rooftop with air economizer and gas heating (question about heating consumption)

asked 2019-01-24 14:54:51 -0500

dahdo1m gravatar image

updated 2019-01-27 09:44:36 -0500

Hi all,

I am solving a research problem that involves running a model at the same region and different economizer controls. I noticed that the heating gas consumption changes from one economizer type to the next, even though I believe from the knowledge I have the economizer control type should not affect heating gas consumption. I'm either wrong about this or there's something I need to change in my idf file.

I appreciate your help.

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answered 2019-01-25 09:04:44 -0500

updated 2019-01-25 09:06:01 -0500

Economizer controls should be optimized to provide maximum free cooling while minimizing the need for heating. Changing the economizer control type will essentially change the way the economizer operates and therefore provide different amounts of outdoor air during the simulation. To see how economizer controls affect heating energy use add report variables and plot them after a simulation. For this test, plot OA volume (or mass) versus heating coil heating energy and compare across the different economizer controls. Replace the asterisk in the reports below with the node name of the OA node in the OA mixer and the name of the heating coil to get just the data needed. See the rdd file to be sure of the actual reports available for each simulation. After running the simulation the rdd file will contain all available report variables for a specific simulation.

Output:Variable,*,System Node Standard Density Volume Flow Rate,hourly; !-HVAC Avg [m3/s]
Output:Variable,*,System Node Mass Flow Rate,hourly; !- HVAC Average [kg/s]

Output:Variable,*,Heating Coil Heating Energy,hourly; !- HVAC Sum [J]
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answered 2019-01-30 17:23:22 -0500

dahdo1m gravatar image

updated 2019-01-31 11:55:30 -0500

I have created the suggested plot. It shows the volumetric air flow on the x-axis and the heating coil energy consumption on the y-axis. The plot is clearly different for each economizer control type (especially dry bulb vs enthalpy types).

image description

My question is, how do I know if my results are acceptable knowing that we were expecting the heating energy consumption to be independent of the control type? Is it unrealistic to expect the heating to be independent of the economizer control type especially that the heating setpoint is the same for all economizer types?

image description

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This plot appears to show that FE and DE use greater heating energy than FDB or DDB. Between 0.35 m3.s OA during economizer operation and 1 m3/s there is essentially no additional heating energy which means you are getting the free cooling most of the time.

This plot also shows that when economizer is not active (at 0.22 m3/s OA flow) that there is a difference in heating energy. I would ask why. I would guess that the supply air flow rate is different between these simulations. You might also plot heating coil energy vs supply flow rate to see if something reveals itself.

rraustad gravatar image rraustad  ( 2019-01-31 07:40:30 -0500 )edit

I checked the supply air flow rate, it was exactly 0.982 m3/s for all economizer settings. I'll add the new plot to the previous answer.

dahdo1m gravatar image dahdo1m  ( 2019-01-31 11:54:56 -0500 )edit

The SA plot shows that there is no difference in flow rate between simulations (which means any difference in heating energy is not due to different supply air flow rates). So the differences in heating energy is due solely to economizer operation. Focusing for a minute on heating differences with no economizer operation (Figure 1, OA flow = 0.22), if SA and OA flow rates are the same then why is there a difference in heating energy? Maybe you should add a table in your question that shows what the magnitude of the differences are so a reviewer has more information on your results.

rraustad gravatar image rraustad  ( 2019-01-31 12:04:09 -0500 )edit

Based on my understanding, heating consumption is going to differ from one timestep to the next based on the dynamic change in inputs and the need for heating at certain times. Would you like me to insert a table showing SA and OA in addition to heating energy? I can also strip Fig. 1 to show the no economizer case alone.

dahdo1m gravatar image dahdo1m  ( 2019-01-31 13:20:09 -0500 )edit

I think all information is useful until the differences are identified. A table showing differences in each simulation would help.

rraustad gravatar image rraustad  ( 2019-01-31 13:55:15 -0500 )edit

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Asked: 2019-01-24 14:54:51 -0500

Seen: 459 times

Last updated: Jan 31 '19