Dear all,
We are submitting a LEED project and have a query regarding ASHRAE 90.1 (2004) section G.3.1.2.8 - Design Air flow rates. It states that the baseline building should be modelled with fans serving the 'same functions and sized for the baseline system supply fan air quantity less the minimum outdoor air, or 90% of the supply fan air quantity, whichever is larger.'
We figure there are 3 different ways to interpret this sentence:
1) Model the baseline building with supply fans as normal, plus return fans with either 90% supply or supply minus outside air.
2) Only model supply fans but change the supply flow rate to 90% or supply minus outside air.
3) Does 90% mean supply minus 90% or supply minus 10%?
If someone could provide an example that would be very useful.
How do we incorporate the return fan power when reporting total fan power allowances, i.e. do we subtract return fans from the total?
Thanks in advance
Kat
Katherine Roberts
Hi Katherine,
Your quote looks incomplete - be sure you're reading the whole section
to avoid unnecessary confusion ;)
1) Baseline fans are auto-sized. Return fans (when required) must
be specified with a CFM capacity equal to either 0.90 x (the auto-sized
CFM of the SA fan), or (the auto-sized CFM of the SA fan) minus the OA
for the same system, whichever is greater. This is relatively simple if
you let excel do the legwork.
2) No. Whether you model return fans in addition to your SA fans
in your baseline model systems is stipulated, not a choice. Re-read
that section.
3) No and Yes. "90% of SA" = 0.90xSA = SA - (0.10xSA)
4) Baseline RA fan power is calculated following the same equations
as SA fans. See following section G3.1.2.9.
NICK CATON, E.I.T.