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Just Released: HVAC Sizing Methodology for Insulated Concrete Homes

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Insulated concrete walls are increasingly being used as
an alternative to wood frame walls in residential
construction. The walls are both disaster resistant and
energy efficient, in that the inherent thermal mass, high
levels of insulation, and low air infiltration of
insulated concrete walls increase their energy efficiency
while enhancing structural soundness.

Thus, heating, ventilating, and air-conditioning (HVAC)
equipment can be downsized by as much as 15 to 40 percent
in concrete homes, in comparison to identical wood framed
homes. Yet to date, there has not been a proper method
for sizing HVAC equipment that takes into account all the
elements of insulated concrete walls. The result is often
an inefficient, oversized HVAC system.

'Rightsizing' HVAC systems is a vital component of
thoughtful, 'whole-building' design, in that specifying a
system that's too small or too large can have lasting
detrimental effects. Too small, and the system will run
almost continuously, and may never achieve a comfortable
indoor environment. Too large, and the system will cycle
on and off too frequently, and may never remove
sufficient moisture from the air. This in turn can lead
to mold and other moisture-related problems, which can be
far more serious in the long run than the unnecessary
increase in construction and operating costs occasioned
by an oversized system.

A new report commissioned by HUD's Office of Policy
Development and Research, titled "HVAC Sizing Methodology
for Insulated Concrete Homes," establishes guidelines and
a methodology for properly sizing HVAC equipment in these
types of homes. There are three components to the report:

o A literature review on thermal performance of insulated
concrete walls and on sizing HVAC equipment in
residential construction;
o A summary of thermal mass in concrete and masonry; and
o An Excel-based program developed by the authors and
intended for use by residential contractors when
estimating the required heating and cooling system
capacity for single-family concrete homes.

Properties of the exterior mass walls that affect the
energy use of the house include the type and thickness of
insulation, thermal mass, and air infiltration. Wood and
metal frame walls are considered low-mass walls. Heat
loss through a frame wall is dependent on the amount of
insulation and air infiltration. More insulation
typically means less heat loss and less energy required
for heating and cooling. Walls with high thermal mass,
such as concrete walls, have the ability to store and
later release heat. This ability tends to moderate indoor
air temperatures, and reduces energy associated with
heating and cooling. The report gives an example of the
difference in heating and cooling costs for a mass wall
versus wood-framed house in Colorado. The cost is
approximately 20 percent less for the insulated concrete
home.

The present approach for sizing methodology incorporates
the DOE2.1E calculation engine directly into the Excel
program. Users input thermostat set points, house
dimensions, construction materials, and location. Excel
macros pass the user inputs to the DOE2.1E modules. The
modules calculate the heating and cooling loads of the
space for each hour of a year and simulate operation and
response of the equipment and systems that control
temperature and humidity, as well as those which
distribute conditioned air to the building. The modeling
produces a good estimation of HVAC needs, with the
limitation being variables introduced by the number of
occupants and the ways they use the house, and
differences between the actual construction of the house
and the construction as modeled.

The software developed for this report is available from
the Portland Cement Association. The software is in their
Residential Catalog as CD044 HVAC Sizing software for
Concrete Homes and is available for download at
http://www.concretehomes.com/ or http://www.cement.org/.


"HVAC Sizing Methodology for Insulated Concrete Homes "
is available on the web at
https://www.huduser.gov/portal/publications/destech/HVACSizing.html
or in printed form for a nominal charge by calling HUD
USER at 1-800-245-2691.

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HUD USER
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1-800-245-2691
1-800-927-7589 (TDD)
202-708-9981 (fax)
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