Jacob,
The key here is that in order for one zone to get cooled at night, its fan
must operate. The fan serves ALL of the zones, however, so ALL zones get
air (and cooling and heating)
Most VAV systems require a non-zero minimum airflow to maintain proper
outdoor air ventilation and maintain that scheme / minimum at all times -
even at night.
It's possible to control VAV boxes for zero minimum airflow in unoccupied
periods, but it would be unusual.
In most cases, the fan itself must also have a minimum airflow for stable
operation and to prevent variable speed drive overheating ... but that's
another problem....
VAV Operation Question
Dec 15, 2014
Hello all -
Full disclosure I'm an architecture intern, but I had a general question about how VAV systems are controlled.
If you have one central VAV system with a boiler and a chiller, and lets say its serving 10 zones. At night with the proper night cycle control, during unoccupied hours if only one of those zones floats above the setback and calls for cooling - do all of the vav terminals come on as well? Or does the entire system energize and deliver cold air just to that one VAV terminal?
The reason I ask, is that I'm modeling a large school with a VAV system in EnergyPlus. When cycling at night, it looks like all of the terminals come on, even if they don't need the cooling.
If anyone can elaborate on how EnergyPlus (or other modeling programs) controls this aspect of VAV night cycling, and also how it works in the real world, that would be much appreciated.
Cheers,
Jacob Dunn LEED AP BD+C
ESKEW+DUMEZ+RIPPLE, APC
2014 AIA National Architecture Firm Award
365 Canal Street Suite 3150
New Orleans LA 70130
504.561.8686
eskewdumezripple.com
Jacob,
Jim is correct. There are a lot of issues that have to be dealt with with
a VAV system in setback. First I'd suggest you read through an ASHRAE
Journal article from October 2011 titled "High Performance VAV Systems" and
then take a look at the Advanced Variable Air Volume System Design Guide
published by Pacific Gas & Electric in March 2007 before you model your
system. If you have a single zone bringing the system on at night - ask
yourself why? Maybe there is an issue with that zone or room in that zone
(heat loss or heat gain) that has a 24 hour load and would be better
handled with a standalone system apart from your VAV system. A few more
things:
1. Model both supply air temperature reset and duct static pressure
reset to minimize reheat in the cooling mode if possible. In reality the
two sequences are a little difficult to set up together in the field and
will require some tweaking.
2. Schedule the OA dampers off or closed in the unoccupied mode when it
comes on just to satisfy a setback or setup condition - especially during
heating because you can definitely subcool the rest of your building to
the point it will never recover at morning warmup if the central system
runs very long with cold air going to boxes with extreme/low setback
temperatures (low 60's and mid 50's).
3. If it is a small VAV system verify that you can keep minimum flow
through your chiller and ask the manufacturer's rep how low the chiller
would turn down and how often it may cycle or be timed out after it cycles.
Most of these conditions are design and operational issues - but they
should be modeled if possible to get an accurate picture of how your system
may actually work.
Dennis
PS: If you cannot find the documents I suggested, write to me offline and
I'll point you in the right direction to get copies of the information.
Dennis and Jim, thank you for the information and advice.
I?m familiar with the ASHRAE article that you spoke of, but not the PG&E guide. I?ll definitely have to take a look.
My previously mentioned large, overly complex school model does indeed model all of the points you outlined. Also, given that it is the LEED baseline, I?m trying to reduce complexity and lump as much together as possible according to the proposed design?s thermal/system zoning. But if there is one zone that is varying wildly different from the others, a separate system may be justified. In some cases the cycling at night overcools the space to the point where it needs reheat, which seems unrealistic. I think, however, I may be experiencing some loads issues and this is not necessarily a problem of the HVAC control.
Thanks again for the background information.
Cheers,
Jacob Dunn LEED AP BD+C
ESKEW+DUMEZ+RIPPLE, APC
2014 AIA National Architecture Firm Award
365 Canal Street Suite 3150
New Orleans LA 70130
504.561.8686
eskewdumezripple.com