[bldg-sim] VAV vs. Constant volumn cooling energy

[Craig Gann]

Anna,

I do not know the intricacies of how DOE 2.2 models work, however I have
seen this sort of thing happen with other energy simulation software. The
greater degree of control that you get with a VAV system means that more
zones are controlled to their respective cooling setpoints. Many people
forget this but better zoning and better thermostatic control of a building
usually costs more energy than a system with poor zoning control, like a
SZCV unit for instance. Often a single zone constant volume system will use
the least amount of energy when compared to other systems and will likewise
have the poorest comfort control as it controls to the average temperature
or only to the temperature at the location of the thermostat, while all
other areas are not controlled properly. The savings of VAV generally
relates to fan energy, which is generally a third of the total HVAC energy.
Depending on how much load diversity you have with that building, and the
VAV box turndown ratio, the VAV system may save enough fan energy to
overcome the additional cooling energy required to maintain the thermostat
setpoints in all the VAV zones. VAV systems should be primarily justified on
comfort. Energy is a secondary consideration, in my opinion. 

One final thought: if the VAV box turndown is too low this will likely
reduce the economizer air (free cooling) that you are getting from
ventilation since the ventilation rate is generally proportional to total
supply air quantity. If this turndown is below that required to condition
the building the compressors will have to make up the deficit in the
cooling, resulting in higher energy costs. In some cases, especially in
cooler climates, increasing the ventilation rate has the effect of actually
saving energy as this acts as an economizer.

Regards,

Craig J. Gann, P.E. 

-----Original Message-----
From: postman at gard.com [mailto:postman at gard.com] On Behalf Of Zhou, Anna
Sent: Monday, April 12, 2004 6:34 PM
To: bldg-sim at gard.com
Subject: [bldg-sim] VAV vs. Constant volumn cooling energy


The simulation software was DOE2.2.

-----Original Message-----
From: postman at gard.com [mailto:postman at gard.com]On Behalf Of Zhou, Anna
Sent: Monday, April 12, 2004 3:19 PM
To: bldg-sim at gard.com
Subject: [bldg-sim] VAV vs. Constant volumn cooling energy


Hi,There

When doing simulation, I noticed that the cooling energy consumption from a
variable air volumn system (VAV,PVAVS) is always larger than that of a
constant volume system (PSZ). Did anybody have this problem, too? Can
anybody tell me what's the possible cause of this?

Thanks a lot.

Anna

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