[Bldg-sim] pro-rating fan power to supply/return fans per 90.1-2007?

[Nick Caton]

James,

 

I do appreciate the heads up - I've been reading the ongoing discussion
with interest!

 

In the particular case I referenced, it didn't seem a big deal to me at
the time as to my understanding my baseline supply and return fans were
operating in tandem, but as I think it through now, I see this might
skew the results somewhat with regard to fan energies in the airstreams.


 

This has got me wondering:  What, if anything, does 90.1 or similar have
to say regarding fan heat in the baseline airstreams?  

 

I've developed a practice in eQuest of intentionally specifying baseline
fan motors as outside the airstreams.  I've done this specifically to
not have their heat incident on the airstreams, because the procedure I
follow for calculating baseline efficiencies (backing out the calculated
fan energy, Pfan, from the prescribed efficiencies), can become a
potentially cumbersome/iterative process otherwise.  For example, a
simulated baseline sizing run gives an airflow/capacity, from which I
calculate Pfan, which once entered and simulated affects the calculated
capacity, which affects the airflow, so I have to recalculate Pfan...
and so forth until the reported figures match my calculations.  For one
system this is cake, but when I've got a bunch of baseline systems it's
quite a pain.

 

I recognize some others follow a baseline efficiency procedure that
don't involve the Pfan for baseline heating/cooling efficiency
calculations, for whom baseline fan heat energy is a moot concern, but
for the moment that's really another can of worms I'd rather not open...
I'd specifically like to hear if anyone knows of what guidance is or
isn't out there for whether and  to what extent baseline fan heat should
be incident on baseline airstreams.  

 

Thanks again to everyone for providing such continuously supportive and
productive discussions - it makes me proud to be a part of this
community!

 

~Nick

 

 

 

 

NICK CATON, E.I.T.

PROJECT ENGINEER

Smith & Boucher Engineers

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

www.smithboucher.com 

 

From: James V Dirkes II, PE [mailto:jvd2pe at tds.net] 
Sent: Tuesday, March 15, 2011 7:43 PM
To: Nick Caton
Subject: RE: [Bldg-sim] pro-rating fan power to supply/return fans per
90.1-2007?

 

Dear Nick,

 

It's not a big deal, but the physics are a little different if you put
some energy from the fan in the Return air stream and some in the supply
air.  Some of the return air energy is normally exhausted and the coil
never sees it.  Probably preaching to the choir here, but can't help
mentioning it.

 

The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
Sent: Tuesday, March 15, 2011 5:00 PM
To: James Hansen; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] pro-rating fan power to supply/return fans per
90.1-2007?

 

Last time I did this, I entered 0.00 kW/CFM for the return fans, and all
of the calculated kW/CFM for the supply fans - better to keep it in one
place imho to avoid the headache of explaining it.  Just got news that
project wrapped up review with a silver rating this week =).

 

If you wanted to distribute the power regardless, I'd do it by weighting
with the airflow amounts.  Generally, I don't think it matters, so long
as you can backtrack your steps if asked to explain yourself ;).

 

~Nick

 



 

NICK CATON, E.I.T.

PROJECT ENGINEER

Smith & Boucher Engineers

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

www.smithboucher.com 

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of James
Hansen
Sent: Tuesday, March 15, 2011 2:05 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] pro-rating fan power to supply/return fans per
90.1-2007?

 

App G says that if the proposed building has return fans, the baseline
building is to be modeled with them as well, sized either at the minimum
required to ensure ventilation, or 90% of the supply volume, whichever
is greater.  And the fan power calculations show that the KW allowance
is for all supply, return, exhaust and relief fans combined.  

 

How do you generally pro-rate the fan power between supply and return
fans in the baseline model?  Ultimately it probably doesn't make too
much difference, but is there an actual ruling that tells you how to do
this?

 

Thanks,

 

GHT Limited
James Hansen, PE, LEED AP

Senior Associate

1010 N. Glebe Rd, Suite 200

Arlington, VA  22201-4749

703-338-5754 (Cell)

703-243-1200 (Office)

703-276-1376 (Fax)

www.ghtltd.com <http://www.ghtltd.com/> 

 

 

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