[Bldg-sim] ASHRAE 90.1 Appendix G3.1.2.2

Nick Caton ncaton at smithboucher.com
Tue Mar 29 08:59:28 PDT 2011


Sorry, I thought I had written a response to this query but can't seem
to find it!

 

On a general note, anyone seeking to better understand the intent behind
various parts of 90.1 Appendix G would be well advised to borrow or
purchase the 2007 User's manual.

 

Here are my condensed thoughts - the user's manual supports some of this
directly:

1.       The oversizing factors appear analogous to "safety factors" in
real-world design, but they exist in 90.1 primarily because it's
recognized the capacity of the baseline systems is an easy means of
cheating the system.  One could very easily decrease relative baseline
performance by undersizing or oversizing the baseline equipment.

2.       With that in mind, and because the loads handled by the
baseline systems are arbitrarily by a unique load profile, 90.1 has to
set something as the sizing factor.  To be fair to real-world systems
which are often oversized, both for "safety factors" and because various
equipment operates more efficiently at <100% capacity, oversizing
factors above 1.0 have been set. 

3.       I think oversizing factor for heating is larger than cooling
primarily because that reflects real-world design for packaged equipment
selection.  After calculating desired loads (and applying design safety
factors), packaged equipment is often selected first on the desired
cooling capacity, and then heating options are considered secondarily.
Where heating capacity options are finite, the result is often heating
elements which are oversized some degree beyond the design heating load.

4.       I would discourage the mindset that oversizing factors are
necessary to prevent unmet heating/cooling hours.  They may mitigate
unmet hours, particularly those occurring as thermostats are coming out
of setback periods (morning warmup), but baseline system capacity is
rarely the root problem in my experience.  In my mind, if a baseline is
set up correctly, oversizing factors of 1.0 ought to result in
sufficient auto-sized capacity.

 

There are inherent "uncertainties" to energy modeling, just as with any
HVAC load calculation, water or electrical service size calculation,
lighting maintained illuminance calcs, LRFD structural design loads and
so on.  Very little in world of building design does not have some sort
of assumed future uncertainty worked into the design via "safety
factors," both prescribed by code and self-imposed as standard practice.
In that context, I don't believe the 90.1 oversizing factors speak to
any uncertainty that's unique to energy modeling.

 

~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 varun
kulkarni
Sent: Tuesday, March 29, 2011 6:57 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] ASHRAE 90.1 Appendix G3.1.2.2

 

 

 

Milda,

I think if you are performorming energy simulations for the baseline, it
doesnt matter what is this 25% and 15% upsizing is used for, as it used
only as much capacity it needs to satisify the loads and thermostat. And
your system size should be based on the design results and not
simulation results (that also proposed model).

But nevertheless I always interpret this as you said in your first
point, to cover uncertainity in building simulation. 

I also think it helps baseline to reduce its unmet load hours.

 

Hope this helps.

On Tue, Mar 29, 2011 at 3:27 AM, milda margarin <empire999 at hotmail.com>
wrote:

	hi , I already submitted this question but there were no
responses so I'd like to try again: 

	 could somebody share some insight to the ASHRAE 90.1 Appendix
G3.1.2.2 point regarding heating and cooling equipment sizing based on
building energy simulations. It is stated that the equipment should be
oversized by a factor of 1.25 for heating and 1.15 for cooling. Are
these "safety factors" based on the uncertainty associated with building
energy simulations or a requirement that makes sure that the system can
respond quickly enough during startup? Why are there different factors
applied for heating and cooling?

	thanks!

	Milda

	 

	 

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-- 

Thanks and Best Regards,

 

Sincerely,

Varun Kulkarni , M.S. , EIT

RMF Engineering

5520, Research Park Drive, Suite 300

Baltimore, MD - 21228

 

Office : 410-576-0505 ext. 5352

Mobile: (+1)405-385-1928 <tel:%28%2B1%29405-385-1928> 

 

Note: Please do not print, unless required.

 

 





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