How to set up a (simple) H/V system with night-time cooling

1 post / 0 new

I am trying to figure out how to best simulate the intended behavior of 100% OA VAV heating/ventilating system with night time precooling of thermal mass. At the moment, I don't care much about accuracy of the energy performance. I am more concerned with forcing the system to behave in specific ways, and observing the impact that this has on temperatures.

I'm currently using the system type UVT, since it seemed simplest, though the HVSYS might be more appropriate.

During the day, I want the system to supply air as follows:

1) If the outdoor temp is less than the zone temp, supply up to a set CFM/SF (which I will vary over several runs) to utilize free cooling

2) If the zone is occupied, supply 15 CFM/person (or control by DCV, but if that's more complex to set up I may not bother for now.)

3) If the zone is unoccupied, supply 0.15 CFM/SF
If #1 is hard to implement, I could live with just #2 and #3 for now.

During the night, I want the system to supply a flow rate that I specify, preferably on a square foot basis. (I plan to vary this from 0.5 CFM/SF to 2.0 CFM/SF to observe how this impact daytime temperatures, to figure out the right night time cooling rate and time period.) I want it to provide this airflow regardless of the OA temp or zone temp - I'll control it with a schedule to prevent night time cooling in winter. Mostly, I want to make sure that I'm getting as much air to the zones as I think I am - I don't want the air volume to be overridden by e.g. the zone thermostat.

I can turn the fan on and off via a schedule. The two things I'm not sure how to do are:

1) Disable all economizer activity. This is a 100% OA system. I can set MIN-OUTSIDE-AIR (At the system level) to 1.0, but is that enough? I don't want it overridden by zone settings.

2) Control the airflow on a CFM/SF basis at night, while avoiding any override by the zones or other inputs. When the fans run, I want them to supply the airflow I specify, period.

Thanks for any suggestions you can offer.


Brent Eubanks's picture
Joined: 2011-09-30
Reputation: 0