I am currently building a Baseline Model for system 8 with PFP in IESVE
for compliance with ASHRAE 90.1 2004 Appendix G. I am not familiar with
the VAV PFP or ASHRAE 90.1 and this is my first attempt to model it. I
am modeling an office building which has significant glazed facades in
climate zone 4a.

1. Can anybody tell me does the design air flow rate temperature
difference of 20 F apply for heating as well as cooling? (G3.1.2.8
2. If it applies for heating also what happens when the parallel fan
is sized at 50% of the peak design airflow and is insufficient to
provide heating to the space at the temperature difference of 15C, can
the parallel fan airflow rate be increased from 50% to take account for
the additional heating requirement? (G3.1.3.14)
3. If the heating loads are larger (and therefore the airflow rates)
then the cooling loads then, do I still size the parallel fan at 50% of
the cooling load or do I size based on 50% of the heating load, or do I
increase this fan volume and the electrical reheat to provide the
required capacity for the larger heating loads? Or is the primary
airflow increased at elevated temperatures to make up the difference.

I am confused so any help on this would be appreciated

Regards

Michael Keohane

An example of sizing Fan Terminal Units (FTU)....

I have a 1000 cfm zone (determined from peak load conditions) to be
served by a FTU, therefore the parallel fan in my box is sized for 500
cfm (50% of peak flow). Now typically in heating is when the primary
air is going to go to an absolute minimum. This must be 300 cfm or the
total volume of OSA that I need for the zone, whichever is larger. In
theory, the fan could provide all 500 cfm of its air when the primary
air is at a minimum if that meets the load. The fan will typically
always be the first stage of heating. After that, if setpoint isn't
met, the reheat coil comes on. I would say that the design condition,
depending on climate of course, will be cooling dominated 7 times out of
10.

Also, as a note, the discussion of minimum flow setpoint is an area of
weakness for a VAV system. If the zone was designed for 1000 cfm in
cooling mode and the minimum outside air was determined to be 300 cfm,
you will almost never reach this value when the VAV is operating at part
load. There is a good article on this in the January issue of
Engineered Systems magazine.

Hope this helps.

Regards,

Andrew Craig, EIT,LEED AP