Damper tests provide valuable insights, Part 6; Minimum Outdoor Air Requirements and Building Leakage

After reading the previous post, some of you may be wondering
how Abdi and Brent went about estimating the minimum outdoor air
requirement for the Pacific Energy
air handling unit they were working with. After thinking
through the building and system, Abdi and Brent concluded that the
minimum outdoor air requirement for the facility fell into three
components; air required for ventilation purposes, air required to
make up for the kitchen hood exhaust, and air required to ensure
positive pressurization in the entry lobby.

Ventilation Air

One of the important realizations that Brent and Abdi had was
that the occupant load in the PEC can vary significantly. On days
with no classes, there may only be 20 or so people in the facility.
But, if all of the class and meeting rooms are full, the population
can easily expand to 100 or more people. As a result, they
anticipated that the ventilation air required might vary over a
range of values and estimated this range based on the minimum and
maximum occupant count multiplied by 15 cfm per person. The 15 cfm
per person number is a common rule of thumb that can be found in
many places including Title 24 (the California Energy
and different editions of
ASHRAE 62 (the ASHRAE Ventilation Standard)
. A survey of the
toilet exhaust fans in the facility revealed that the air that was
brought in for ventilation was generally taken out of the facility
via the restrooms.

Kitchen Hood Make-up Air

Given the number of events that occur at the PEC, the kitchen
located in the facility is used daily by the catering company that
provides the food service for the various events. The kitchen hood
represented a significant exhaust flow rate (nominally 1,000 cfm).
Since there is no dedicated make-up air system for the kitchen,
Brent and Abdi anticipated that the necessary make up air would
need to be introduced via the AHU to ensure that it was properly
conditioned. Otherwise, the operation of the hood could result in
infiltration in the immediate vicinity of the kitchen.

Pressurization Air

The front door of the facility opens directly into a lobby area
with educational displays and a reception desk. Brent and Abdi
concluded that maintaining the lobby at a positive pressure would
be desirable to minimize drafts and related discomfort for the
receptionist during cool weather. Most of the time, this is
accomplished at the PEC via the configuration of its economizer and
relief system. Specifically, the AHU is provided with an economizer
cycle, and barometric relief dampers and relief fans are provided
to remove this extra air from the facility when the economizer is
active. The dampers will not open until the building is positive
and the relief fans are cycled based on lobby pressure. But, on a
very cold day, with a low load, Brent and Abdi reasoned that the
economizer might drive to minimum outdoor air and thus, the minimum
outdoor air setting needed to reflect the pressurization
requirements. They estimated the air required by using the
“crack method”, a technique that relates infiltration
to the length of crack around doors and windows. In formation about
this approach and several other quick infiltration estimating
techniques can be found in Part 8 – Infiltration Rules of
b of Arthur Bell’s book
titled HVAC Equations, Data and Rules of Thumb
, a really
hand reference to have around if you are out in the field a lot or
doing schematic design work. 

Total Ventilation Requirement

The table below presents the results of Abdi and Brent’s

As a side note, a building pressurization test we performed at the
PEC as a part of a different class revealed that the facility leaks
quite a bit more than the “crack method”, or any method
for that matter, would lead you to believe. The graph below depicts
the results of a test we did at the facility. To perform it, we
closed all of the doors, blocked the relief openings and shut down
the relief fans. Then we used the AHU in a 100% outdoor air mode to
incrementally pressurize the building; sort of a primitive blower
door test.

I can explore the pressurization test and related resources on
building leakage in a different string of posts if there is
interest in it. I only mention it now because it relates to the
pressurization estimate the class did using a classic technique and
shows how easy it is to underestimate building leakage.

Based on their observations and estimates, Brent and Abdi
developed a test that placed the AHU on minimum outdoor air and
then simulated different supply flow conditions by driving VAV
boxes into full cooling. For each condition, they planned to
document the outdoor air flow, supply flow, and return flow. The
goals of their test were to:

Identify if the desired minimum outdoor air flow rate was
provided under all operating modes.

Identify if the operation of the kitchen hood impacted the
amount of outdoor air that came in through the fixed minimum
outdoor air damper position.

Determine if some sort of revised minimum outdoor air
control strategy that could adjust for occupancy and kitchen hood
operation merited consideration.

While Brent and Abdi did not get to run the full version of
their test due to time constraints, they did obtain enough data to
conclude that:

The current minimum outdoor air setting probably provided
sufficient flow for the design condition, but over-ventilated
during periods of low occupancy

Given the mild San Francisco climate, and the relatively low
number of hours the system spent on minimum outdoor air, it would
be difficult to justify a full blown, demand controlled ventilation
strategy.  However, an approach that provided different fixed
damper settings based on class room schedules may merit

As a result, they recommended additional testing and analysis
using a procedure similar to the one they developed. Their
recommendation became an action item for the class that followed,
and their test became the foundation for the test Wayne and I ran.
Come back in couple of days and we will take a look at the test
procedure and begin to look at the results it generated.

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One Response to Damper tests provide valuable insights, Part 6; Minimum Outdoor Air Requirements and Building Leakage

  1. Thanks for the articles. I think there was a little error on the
    O.A. calc. For O.A. at the AHU I would add the ventilation and
    kitchen exhaust, but not pressurization air. The ventilation air
    will do double duty as both ventilation and pressurization. I use
    pressurization air to estimate how much to reduce the return air.
    Even the kitchen exhaust can be ignored if it can use tranfer air.
    All of this assumes the ventilation needed exceeds the exhaust
    and/or pressurization. I am have been to your presentations and
    really enjoy them.

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