Simulating VRF With Full Airside Economizer

asked 2015-06-25 16:06:50 -0600

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Has anyone attempted to simulate a VRF system with a full airside economizer and do you have any direction you may give? Ideally the goal is to have an oversized DOAS allowing for full economizer operation when the OA temperature is below the temperature lockout. If the OA temperature is above the lockout then the DOAS just provides room neutral air and the VRF handles any subsequent latent and sensible load at the fan coil.

I've yet to attempt this myself, but thought I would reach out and see if anyone has modeled this system and control before, and see if they had any information to share.

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answered 2015-06-28 07:51:15 -0600

Jim Dirkes
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I have not attempted it and never seen it in practice. It's a bit counter-intentional for VRF, which is primarily a sensible-only load system for multi-zone systems. One of its major benefits is reduction of air transport energy and duct installation cost; you would lose those by installing a full-size duct system (mostly). It seems, though, that if you really want to investigate the energy benefits, it could be done. Without thinking it all the way through, you might adjust the DOAS airflow rate using EMS or have one air loop with two, different capacity fans with complementary availability in parallel.

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At the moment it's mainly for evaluating purposes and I agree with the bulk of what you have said. However, I am aware of a couple of buildings in the PNW that have the DOAS supplying room neutral air or cooler if in economizer mode with condensate lines at each fan coil for any subsequent load.

I was mainly curious to see how people have implemented this from a modeling perspective and to see what sort of results have been observed. I'll probably go the EMS route.

pflaumingo ( 2015-06-29 19:05:19 -0600 )edit

I'm also interested in seeing recommendations on this. In Washington State, though there is an economizer exception for VRF systems, it requires providing heat recovery on the DOAS. Given these trade-offs, some designers are inclined to use the ducted VRF terminals w/ economizers. In this case, the cost trade-off is between a) providing an OA duct sized to 100% OA + economizer vs b) providing a smaller duct sized for vent only + ERV unit. Not saying these configurations are more efficient that non-ducted terminals + DOAS ± ERV, but hence, wanting to model and compare them.