Retrotec Residential Pressure & Air Leakage - Manual (Page 73)

Page 73 of 75
©Retrotec Inc. 2012
Glossary
Term
Definition
ACH50
or ACH @ 50 Pa
Designation for “Air Changes at 50 Pa”. Can be calculated by taking CFM50 x 60 minutes/ hour,
divided by the house volume.
Air Changes per Hour The number or times per hour that the volume of air in the enclosure will flow out of the
enclosure. A flow rate normalized to the volume of the enclosure and allows comparison of
the “leakiness” of larger volumes to the “leakiness” of smaller volumes.
Always expressed in units of /h.
Calculated as:
General: Flow / Volume
Units:
1
h
= CFM ∗ (
60 min
1 h
) ∗ (
1
ft 3
)
= (
m 3
h
) ∗ (
1
m 3
)
= (
m 3
s
) ∗ (
60 s
1 min
) ∗ (
60 min
1 h
) ∗ (
1
m 3
)
= (
l
s
) ∗ (
60 s
1 min
) ∗ (
60 min
1 h
) ∗ (
1 m 3
1000 l
)
airtightness Pertains to how free air leakage may be in an enclosure. In actual fact, measurements can only
be made of air leakage rates not airtightness itself, so one could think of these terms as being
opposites. In spite of the confusion, the terms are used interchangeably. Airtightness is
determined by measuring the air flow rate required to maintain a specific induced test
pressure. (e.g. airtightness can be measured and reported in units of CFM/sq ft of air leakage
at a uniform test pressure of 75 Pa)
ATTMA Airtightness Testing and Measurement Association
Baseline Pressure Pressure that exists when the enclosure has been prepared for the test, but before the fan is
activated. There is always some Baseline pressure due to stack, wind, flues and active HVAC
systems. There are two components of Baseline pressure. A fixed Baseline offset (usually due
to stack or HVAC) and a fluctuating pressure (usually due to wind or elevator operation). A
method determining baseline pressure is by having a digital gauge accumulate readings over an
adjustable time period
(Note: The terms “static pressure”, “bias pressure,” and “zero Fan Pressure difference” are
used interchangeably with the term baseline pressure in other documents/standards used in
the industry.)
Blower The calibrated fan unit that induces pressure differences causing air flow and provides a Fan
Pressure signal from which flow is calculated.
Blower Door See “Door Fan”. A test instrument that fits into an open doorway in order to pressurize or
depressurize an enclosure. It is a calibrated fan capable of measuring air-flow, and is used
while mounting it into an opening in the enclosure under test. A blower most accurately
describes an air moving device of the squirrel cage variety; hence the adjective “blower” does
not normally apply to the bulk of Blower Doors since they do not use a blower.
CFM Cubic feet per minute, the units of volumetric flow
CFM50
or CFM @ 50 Pa
CFM @ 50 Pa is the flow rate, in cubic feet per minute, required to depressurize/pressurize the
building to 50 Pascals.
CGSB Canadian General Standards Board
Conditioned Space An area or volume that is normally air-conditioned or heated (i.e. inside the thermal envelope).
Even though supply ducts may not discharge directly into these spaces, they are considered