Investigation
of Potential Benefits of Revising Exception 1 under FLORIDA BUILDING CODE,
ENERGY CONSERVATION, 101.4.7.1.1 Duct Sealing upon Equipment Replacement (see full
code section text included at end of proposal)
Janet
McIlvaine
Florida
Solar Energy Center
September
12, 2013
Research Purpose and Goal: This research proposes to
quantify the extent of need, benefit, and cost of sealing building cavities
used as return air pathways that are formed by all or part of an air handler
closet located in the conditioned space. These are arguably excluded from the
sealing requirement of the referenced code section under Exception 1 by virtue
of appearing to be “in conditioned space” when they usually are connected to
unconditioned space. This connection creates an unintended airflow path from adjacent
attic or floor cavities (Figure 1).
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Figure
1. Left: A return plenum formed by unfinished framing under an air handler
support platform is on the other side of this wall mounted return air grille
in a utility room. Right: Infrared image showing hot attic air (see color
scale at bottom of image) being pulled down the interior wall cavity during
air handler run time. |
Definition of the Problem: This code section (101.4.7.1.1) requires
HVAC contractors to seal accessible (a minimum of 30 inches clearance) joints
and seams in the air distribution system when new equipment is installed.
Exception #1 eliminates the requirement for ducts in conditioned space, and by
doing so, effectively eliminates the requirement for building cavities used as
air distribution paths. These building cavities are often not separated from
unconditioned spaces by a complete air barrier.
Building
cavities used for supply distribution are not likely to meet the accessibility
criteria; and therefore would not be subject to the sealing requirement.
However, building cavities used for central return air conveyance often are
accessible. These are usually formed by open wall cavities of adjacent interior
walls or by poorly sealed air handler closets. By modifying Exception 1 to
apply clearly to ducted pathways and not building cavities, this pervasive and apparently
large leakage path could be eliminated in 1,000’s of existing homes, enhancing
performance with very little extra effort or cost.
Building Science Background: Unsealed return plenums are
under significantly greater pressure than other leakage points in the air
distribution system because of proximity to the air handler fan. Return leakage
from unconditioned spaces is likely to be at an extreme temperature and dusty.
It often bypasses the filter, fouling equipment components, degrading equipment
performance, eroding equipment life, comfort, and potentially indoor air
quality.
Magnitude of Opportunity: In an FSEC field study conducted
between 2009 and 2012, researchers observed a large number of unsealed return
plenums and AHU closets in the conditioned space (McIlvaine, et al.). Interior
air handler configurations were found in 40 out of 70 homes ranging in vintage
from the 1950’s to 2006 (Figure 2). These
types of returns are often connected directly to adjacent wall cavities and
attic spaces through missing or compromised air barriers.
Figure
2. In a recent FSEC field study (McIlvaine,
et al. 2012), 40 of the 70 homes had an interior air handler closet with a
platform or full closet return plenum (blue).
These interior
air handler closets generally manifested in the field study in two
configurations: a framed platform supporting an up-flow air handler with
through wall filter-back return grilles (Figure 3) or a metal or frame air handler
stand where the closet functions as the return plenum with return air pathways
through louvered doors or door mounted grilles (Figure 4).
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Figure
3. This air handler in an interior closet (left, top) is served by a return
air plenum directly beneath (left, bottom) formed by the open, unfinished
framing of the closet walls. |
Figure
4. Interior air handler closet with no
dedicated return air pathway. The unfinished closet walls (and poorly sealed
ceiling) function as a return plenum. |
Approach to the Research: In the field study mentioned above
and more recent research, FSEC builder partners and their mechanical
contractors have achieved “substantially leak free” duct systems (Qn,out ≤ 0.03) in 20 HVAC retrofits including many
with the type of returns under discussion. The portion of leakage attributable
to return side pre- and post-retrofit has not been assessed because this level
of investigation was not covered in the funded scope of work. However, the FSEC
research team proposes to leverage partnerships formed during the field study
to identify and investigate 10 mechanical system retrofits involving return air
plenums similar to those shown in Figures 3 and 4 above.
The technical
approach, focused on open frame platform returns and whole closet return
plenums located in the conditioned space, would consist of duct system testing in
10 homes as follows:
• Test
leakage of as-found air distribution system including quantifying the portion
attributable to return leakage and the degree of connection to unconditioned
spaces
• HVAC
contractor will replace AC equipment and seal accessible ducts as required by
code section 101.4.7.1.1 using the contractor’s standard approach with the addition
of sealing the interior return plenum
• Re-test
leakage of air distribution system including quantifying the portion
attributable to return leakage and the degree of connection to unconditioned
spaces
• Compare
pre- and post-retrofit return side leakage to characterize the relative success
of the contractor’s sealing effort including the contribution of return side
improvement to overall system improvement.
Test results
will be used to model the impact of return sealing on heating and cooling annual
energy use for several different levels of whole house efficiency.
Expected Outcome and Impact on the
Code: The
outcome of this research will be a report describing the impact of sealing return
plenums formed by building cavities in the conditioned space and associated
cost. From the results in this report, it is anticipated that the Florida Building
Commission might consider modification of Exception 1 under section 101.4.7.1.1 Duct sealing upon equipment
replacement to limit applicability of Exception 1 to ducted pathways and
not building cavities.
Budget: $29,000
Relevant Code Reference:
2012
SUPPLEMENT TO THE FLORIDA BUILDING CODE, ENERGY CONSERVATION,
Chapter
1 – Administration, 101.4.7.1 Replacement HVAC equipment
101.4.7.1.1
Duct sealing upon equipment replacement (Mandatory). At the time of the total
replacement of HVAC evaporators and condensing units for residential buildings,
all accessible (a minimum of 30 inches clearance) joints and seams in the air
distribution system shall be inspected and sealed where needed using reinforced
mastic or code approved equivalent and shall include a signed certification by
the contractor that is attached to the air handler unit stipulating that this
work has been accomplished.
Exceptions:
1. Ducts in conditioned space.
2. Joints or seams that are
already sealed with fabric and mastic.
3. If system is tested and
repaired as necessary.