Application Note

2 Fluke Corporation Evaluating relative humidity: Key factors and measurements
designed to be limited to 65 % or
less at either of the two following
design conditions:
1. at the peak outdoor dew-point
design conditions and at the
peak indoor design latent load
or
2. at the lowest space sensible
heat ratio expected to occur
and the concurrent (simultane-
ous) outdoor c
ondition.”
Good H
V
AC equipment selection
practic
es generally recommend:
•
68 °F to 70 °F and 30 % RH
(relative humidity) w
inter
desig
n,
and
•
7
4
°
F to 7
6 °F and 50 % to
60 % RH summer design
at
•
outdoor c
onditions of 9
7
.5 %
winter and 2.5 % summer dry
bulb (DB).
This means that on average,
2.5 % of the extreme seasonal
temperatures will be beyond
equipment capacity. The equip-
ment will be effectively
undersized during these times.
This is critically important in
equipment selection, sinc
e only
30 % of the operating hours of
comfort cooling equipment occur
w
ithin 5 % of outdoor desig
n dr
y
bulb temperature. Summer latent
load control is more difficult to
control at part load conditions,
although most commercial equip-
ment is staged or has some form
of capacity control.
If comfort cooling equipment is
oversized, moisture related com-
plaints and problems will
increase. Residential heat pumps
should b
e selected according to
the cooling requirements, not the
heating requirements, especially
in geographic areas where “dirty
socks syndrome” is prevalent and
air handling equipment is located
in a crawlspace.
Fungus
With enough knowledge and
measurement, HVAC systems can
be set at the appropriate summer
and winter psychrometric condi-
tions to discourage fungal
growth. Conditions for fungal
growth include spores settling on
a surface, a micro-environment
ensuring oxygen, optimal tem-
peratures, nutrients, and
moisture. Four of these conditions
are found in nearly every envi-
ronment. The most controllable
variant is moisture.
Wet bulb temperature: Represents the
cooling effect of evaporating water, the
temperature air will cool to when water
evaporates into unsaturated air.
Dewpoint temperature: The tempera-
ture under which water will condense
out of the air.
Dry bulb temperature: Air temperature
determined by an ordinary thermome-
ter.
Relative humidity: Ratio of water
vapor pressure (amount currently in the
air) to the saturation vapor pressure (the
amount the air can hold) at a given air
temperature.
Met(abolic) rate: The rate by which
the body transforms chemical energy
into heat and work through activity. In
ASHRAE 55, this rate is measured in
“met units” (18.4 Btu/h*ft
2
).
Sensible cooling: Factors such as peo-
ple, appliances, solar radiation, and
infiltration create heat gain, each
adding a sensible load to the environ-
ment within a house, office, etc. This
sensible load raises the dry-bulb tem-
perature. The process by which the
sensible, or dry bulb, temperature is
reduced without changing the moisture
content of the air is referred to as a
sensible cooling process.
Latent cooling: An amount of moisture
is added to the inside air by plants,
people, cooking, and other sources. A
latent cooling process involves the con-
densation of moisture out of the air,
reducing the wet bulb, dewpoint, and
humidity levels, but leaving the dry
bulb temperature untouched.
S/T ratio: Sensible to total heat ratio,
or sensible heat factor. Of the total
capacity of a cooling system, there is a
sensible capacity and a latent capacity.
The sensible capacity cools the air by
absorbing heat to lower the dry bulb
temperature. The latent capacity
absorbs the latent heat of vaporization
to remove moisture from the air without
changing the actual dry bulb tempera-
ture. The S/T ratio, when used with a
psychrometric chart, will provide the
temperature at which a cooling coil
must operate in order to support both
sensible and latent heat removal.
“Dirty socks syndrome”: A common
condition during the cooling season that
describes
an odor generated by burning airborne
contaminants on an indoor coil, typi-
cally during the heat pump defrost
cycle.
clo: A unit of measurement to express
the amount of thermal insulation pro-
vided by clothing and other garments.
•
Example: An ensemble including
briefs, t-shirt, calf-length socks,
shoes, straight trousers,
long-sleeve dress shirt,
double-breasted (thick) jacket totals
1.14
clo*
•
Example: An ensemble including
briefs, short-sleeve knit sport shirt,
walking shorts,
sandals totals .31 clo*
*clo values per ASHRAE 55-2004
Psychrometrics has a language all its own. To better understand
how the various parameters interact to support thermal comfort, here
are some of the more common terms described in this document:
Relative humidity above 60 %
can support fungal growth on
hygroscopic (sorbent) surfaces
and hygroscopic surfaces at 80 %
RH are likely to promote fungal
growth. Nearly all surfaces are, or
can become, sorbent and include
painted surfaces, gypsum dry
wall, carpets, wall coverings, and
masonry products. Even glass
w
ith a dirt film and dust on it can
support fungal growth.
Masonry products such as
brick, cinder block and concrete
are excellent sorbents and can
adsorb vast quantities of moisture
and become an inviting breeding
environment for molds. The vapor
pressure within the pores of
manufactured masonry can be
less than the vapor pressure of
the ambient air which moves
moisture from the air into the
masonry pores. As the pores
become wetted, capillary action
takes over and fills the pores,
thus providing an ideal breeding
ground for fungal proliferation.
This explains why some surfaces
above dew point can become
wetted.