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VRF vs PTHP System

asked 2015-03-11 11:07:54 -0500

tinadarko's avatar

updated 2015-04-13 18:18:23 -0500


I'm trying to compare the energy use between a VRF system and a PTHP system in EnergyPlus. Both systems have a DOAS to supply outdoor air (efficiencies of the DOAS are the same, OA rates are the same), and all other parameters are the same. The VRF and PTHP system have the same rated efficiencies: cooling COP 3.3, heating COP 3.5. Capacities and supply air flow are being autosized. Both systems' fan mode is cycling. Other parameters are using the default values in OpenStudio.

I'm getting higher energy use in the VRF model, and I'm not sure what the reason is to that... any insight would be greatly appreciated.

Thanks Tina

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Might be good to see end use breakdown to see where that increase is coming from. Another first step if you haven't done it is to look at the warnings in the eplusout.err for anything suspicious.

David Goldwasser's avatar David Goldwasser  ( 2015-03-11 11:11:35 -0500 )edit

Thanks David, I looked in the error file and one warning I'm getting for the VRF is:

Warning ** AirConditioner:VariableRefrigerantFlow "VRF SYSTEM" -- Exceeded VRF Heat Recovery min/max outdoor temperature limit error. VRF Heat Pump Min/Max Outdoor Temperature in Heat Recovery Mode Limits have been exceeded and VRF heat recovery is disabled.

I've tried expanding the range of the min/max outdoor temperature for the heat recovery mode but I'm still getting this error.

I'm having an increase of energy mainly in heating, a little bit in cooling and some in fan energy.

tinadarko's avatar tinadarko  ( 2015-03-11 19:12:26 -0500 )edit

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answered 2015-03-11 11:17:59 -0500

scottb's avatar

Do you have the same unmet hours in both cases?

Check your HTML output for 'Time Setpoint Not Met During Occupied Heating' and 'Time Setpoint Not Met During Occupied Cooling'

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Thanks Scott, I checked the unmet hours for both cases and they are more or less the same:

22 hours in heating, 150 hours in cooling for PTHP; 25 hours in heating, 167 hours in cooling for VRF.

tinadarko's avatar tinadarko  ( 2015-03-11 19:14:32 -0500 )edit

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Asked: 2015-03-11 11:07:54 -0500

Seen: 926 times

Last updated: Mar 11 '15