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Ridiculously high unmet hours on VRF systems - Openstudio

asked 2018-08-18 03:46:14 -0500

Tim.H gravatar image

updated 2018-08-26 19:40:21 -0500

Does anyone know if there are issues with the way VRF systems are modelled? And do I need to know some magic tricks to get them to work?

I'm modelling a long (about 7m W east west by 50m L north south) low set of offices set into the east facing side of a hill, with a massive glazed facade. The internal/Western offices have a full height retaining wall into the hill, and the Eastern area is an open office area of cubicles with pretty much full height glazing to north, south and east. Roof over is a concrete slab with parking.

The HVAC system is to be VRF split into North and south systems with heat recovery - i.e. simultaneous heating and cooling available.

I'm getting roughly 400 unmet occupied hours in all zones, and it doesn't seem to change anything when I change the thermostat priority settings in the VRF system. I have tried rezoning multiple ways, including just having identical areas on a single system, but this still comes out with roughly the same unmet hours. I have also done a check run with SZHP's which comes out with zero unmet hours.

Please Help!

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For future generations, what software are you using?

__AmirRoth__ gravatar image__AmirRoth__ ( 2018-08-25 11:11:31 -0500 )edit

Openstudio, sorry I spotted that I hadn't tagged that, but was having trouble changing it

Tim.H gravatar imageTim.H ( 2018-08-26 19:40:07 -0500 )edit

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answered 2018-08-21 02:28:39 -0500

Tim.H gravatar image

Fixed it! I managed to significantly reduce my unmet hours to reasonable numbers. The issue in my case - which may not apply elsewhere - is that unmet hours for JV3 in Australia is defined by whether the AC is operating or not - i.e. air conditioning schedule. However, the occupancy schedule that we are required to use has occupants still in the building - albeit at a low occupancy - outside the air conditioning schedule. Which results in an "occupied" hour when the AC isn't allowed to run and can't control the temperature, resulting in an unmet occupied hour if the load is sufficient to bump it outside setpoint - not an unmet operating hour.

I fixed it by having a huge jump in my cooling and heating setpoints when the AC comes on and switches off. Thought this might help someone else down the track

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Asked: 2018-08-18 03:46:14 -0500

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Last updated: Aug 26 '18