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"Ideal Air Loads" and Packaged Rooftop Heat Pump with COP of 1

asked 2018-05-15 02:36:51 -0500

Constance's avatar

updated 2018-05-16 07:40:41 -0500

I created a multi-zone single detached dwelling model in OpenStudio. I tried to run the model by using Ideal Air Loads and it gave total district heating loads of 21GJ. However, if I use a Packaged Rooftop Heat Pump for each zone and set the COP for Coil:Cooling:DX and Coil:Heating :DX to 1, the heating electricity usage will be 32GJ.

Based on the result of the post:

I tried to create a reasonable on/off schedule for Coil:Heating:Electric and Fan based on cooling and heating schedules, but the results didn't change.

Does anyone know why Packaged Rooftop Heat Pump with COP of 1 gives such a high heating and cooling electricity usage compared to district heating and cooling?

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Did you properly assign the "Control Zone" for the SepointManager:SIngleZone:Reheat of each airLoopHVAC? Also, COP is only the rated COP, there are modifier curves that are used that will lead to different performance at different operating conditions, so you're not at a perfect 1 all the time (actually you're probably almost never at 1).

Julien Marrec's avatar Julien Marrec  ( 2018-05-15 02:57:07 -0500 )edit

Also, your packaged rooftop heat pump will use fan energy too, whereas the ideal load doesn't have them.

Julien Marrec's avatar Julien Marrec  ( 2018-05-15 03:00:25 -0500 )edit

I only considered the heating/cooling electricity instead of the fan electricity. I checked the "Control Zone" for each airLoop HVAC are properly assigned. I changed the curve coefficients manually based on the EnergyPlus ExampleFiles of HeatPump. The results seem little bit better now, the heating electricity is 28.5GJ and cooling electricity is 5.8GJ (the district heating is 21.38GJ; district cooling is 4.28GJ). The climate condition is in Melbourne Australia which has a mild temperate weather. Do you think the results are reasonable compared to the district heating and cooling?

Constance's avatar Constance  ( 2018-05-16 07:39:21 -0500 )edit

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answered 2018-05-15 15:41:18 -0500

Packaged rooftop energy use will include the energy needed to heat the outdoor air. The default ideal air load does not include outdoor air unless you have specified it in the IdealLoadsAirSystem object.

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This is a rather incorrect statement as far as OpenStudio is concerned (the question is tagged with 'openstudio'): as long as your spaces have a DSOA (either hardcoded or entered at the spaceType level), your ideal loads air system created by OpenStudio will have outside air.

Julien Marrec's avatar Julien Marrec  ( 2018-05-16 02:36:46 -0500 )edit

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Asked: 2018-05-15 02:36:51 -0500

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Last updated: May 16 '18