ASHRAE 90.1 Appendix G3.1.2.2

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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

milda margarin's picture
Joined: 2011-10-02
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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 wrote:


Thanks and Best Regards,


Varun Kulkarni , M.S. , EIT

varun kulkarni's picture
Joined: 2011-09-30
Reputation: 1

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

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-Caton's picture
Joined: 2011-09-30
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