Confirmation for finding the Baseline Energy Model Supply Airfllow

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We are having a discussion at the office about how to define the supply
airflow and energy model baseline. I pick the baseline system type per
Appendix G (packaged single zone, packaged VAV, etc., set the heating
sizing ratio to 1.25 and the cooling sizing ratio to 1.15, leave the
heating and cooling capacities blank so the model will auto-size, and
then set the default airflow to 0.5 cfm/sqft and let eQuest run. I then
check to see if the baseline energy model unmet hours are less than 300
and are within 50 hours of the proposed model. If the baseline energy
model is above 300 unmet hours or isn't within 50 hours of the proposed
model, I will then start tweaking the airflow in the baseline energy
model thermal blocks that are causing the unmet hour issues to solve the

Is this what everyone else is doing or do leave the airflow with the
default values it calculated with the 0,5 cfm/sqft limit and enter
capacity values in the previously blank HVAC cooling and heating
capacities slots so the baseline energy model stops auto-sizing?


Kathryn Kerns

Kathryn Kerns's picture
Joined: 2011-09-30
Reputation: 0


I think that .5 cfm/sf is too low. I never saw it used when I was designing
HVAC systems except for an occasionally used interior space. I immediately
change it to1.0 cfm/sf which I feel is a good place to start. For highly
loaded rooms with lots of internal gain I sometimes up that amount after I
have checked the other possible inputs.


cmg750's picture
Joined: 2010-10-05
Reputation: 0

Yes, Carol I know the default airflow value is very low. Usually the
eQuest airflow auto sizing routine will select larger values than the
default. The problem is: what if the eQuest baseline energy model
selects airflows larger than the default cfm/sqft airflow values but the
baseline energy model still ends up with greater than 300 unmet hours or
the baseline energy model has more than 50 unmet hours greater than the
proposed energy model?

Do you start increasing the baseline energy model airflows above the
auto sized airflow value in the thermal blocks that are having unmet
hours issues, or do you retain the auto sized airflow value and start
manually increasing the baseline heating and cooling capacities that
were also auto sized by eQuest using the 1.15 and 1.25 oversized

Kathryn Kerns

Kathryn Kerns's picture
Joined: 2011-09-30
Reputation: 0

You must use the 1.15 and 1.25 multipliers to meet LEED requirements. I
would look at the throttling range that eQ is defaulting to. if it's .5 F,
which is I think what it is for PSZ, I would raise that to1 or 2 F, which is
reasonable. Then I would look at the space loads, equipment especially and
make sure all that looks good, then I'd look at the schedules, make sure
they are good, and then I'd look at the cfm/sf. It's not unreasonable to
match it to your proposed building and I don't know of any issue LEED has
with doing that.


cmg750's picture
Joined: 2010-10-05
Reputation: 0

Appendix ASHRAE 90.1 2007 Appendix G3.1.3.13 VAV Minimum Flow Setpoints
(System 5 and 7) states:

"Minimum volume setpoints for VAV reheat boxes shall be 0.4 cfm/sf of
floor area served or the minimum ventilation rate, whichever is larger."

For systems 5 & 7, I will set the minimum flow rate to 0.4 CFM/SF. If I
have high unmet hours due to the small amount of CFM, I will increase
the value on a room by room basis. The baseline's flow rates are
typically 30% less than my proposed model. The fans typically run more
often in the baseline model.

Does anyone else have a different way to model the flow rates for the
LEED Baseline model?

Otto Schwieterman's picture
Joined: 2011-09-30
Reputation: 200

That sounds to me exactly like what you should do. The baseline fans running
more often signals they are too small to meet the load. As you increase the
cfm/sf I would guess that eQUEST will respond accordingly and the fans will
get bigger and life will be beautiful all the time. Trees, flowers, chirping
birds....Oops, I forgot this is a professional listserv.


cmg750's picture
Joined: 2010-10-05
Reputation: 0

Wait a minute! I think we are mixing very different keywords and functions here, both in the modeling world and in reality. This is fundamental to understanding VAV systems.

Kathryn and Carol were discussing FLOW/AREA or the minimum design airflow. This is the zone design airflow (or VAV max airflow) that eQUEST starts with as a minimum when autosizing. If you are letting eQUEST autosize the zone design airflow, then the program will increase the airflow from that starting point to meet the load. Most zone loads will require more airflow than this so this value is not really that important, until you start setting it too high. Typical office interior zones are usually less than 1 cfm/sf so FLOW/AREA=1 will probably oversize interior zones. To the original question, in my experience, insufficient zone airflow is seldom the cause for unmet hours. Neither are system airflow or primary coil capacity, especially if you are letting eQUEST autosize these. Appendix G (G3.1.2.1) gives bad advice on this situation. You have to dig a little deeper in the reports (like SS-R) to understand the cause of the unmet hour. If it's unmet heating hours, I would try looking at the REHEAT-DELTA-T or MAX-SUPPLY-T or HEAT-SET-T depending on your system type. If you are VAV, you need HEAT-SET-T defined in order to have a heating coil in your AHU. Also, I think a lot of people confuse the COOL-TEMP-SCH and DESIGN-COOL-T (and same for heating side). The schedule describes the zone temperature setpoint at each hour, not DESIGN-COOL-T. There are tons of discussions in the archives about how to deal with unmet hours. Although it can be very effective at reducing unmet hours, I think increasing the zone THROTTLING-RANGE should be among the very last options to fix unmet hours, when nothing else fixes the problem.

Otto, you are describing MIN-FLOW/AREA or the VAV zone minimum airflow. This is the minimum airflow that is required when the zone is either in deadband, or deadband and heating, depending on when you have a reverse-acting or proportional thermostat. This keyword should have no impact on whether the zone meets cooling loads. If you have a proportional t-stat, then a low MIN-FLOW/AREA could impact whether you have enough reheat capacity. If you have a reverse-acting thermostat, this shouldn't really have any impact on unmet hours, unless you have a zone with very low internal loads and insufficient reheat capacity. In that case, you should probably be fixing the reheat capacity, not the zone minimum.

Although the keywords and names are very similar and easy to confuse, the eQUEST dictionary help files describe all of these variables very clearly.

Date: Wed, 8 Sep 2010 09:47:17 -0400

Hwakong Cheng's picture
Joined: 2011-09-30
Reputation: 0

You are correct, I was describing the MIN-FLOW/AREA (VAV zone minimum

I allow eQUEST to auto-size the design airflow rates based on G3.1.2.8.
( a supply-air-to-room-air temperature difference of 20 F). If the
heating room temperature is 72, the supply air temperature is set to 92
F. If the cooling room temperature is 75 F, the supply air temperature
is set to 55 F. I allow eQUEST to size the reheat capacity. In many
cases, eQUEST sizes the airflow for small rooms such as individual
restrooms and storage rooms at the minimum zone airflow / area which
results in high unmet heating hours. This is when I increase the
MIN-FLOW/AREA for that specific room. In this case, the FLOW/AREA and
the MIN-FLOW/AREA are related to each other.

As described in the archives, it is important to determine why each room
has unmet hours. Sometimes the problem is the same for every room (like
a scheduling problem) and other times it is specific to an individual
room (not enough airflow for small rooms with little or no internal

Otto Schwieterman's picture
Joined: 2011-09-30
Reputation: 200