Application Note
3 Fluke Corporation Evaluating relative humidity: Key factors and measurements
Condensation
Conditions that allow condensa-
tion to form on surfaces are more
obvious, so action can be taken
immediately. When a surface
temperature is at or below the
dew point temperature, conden-
sation will form. Likely places for
this to occur are on basement
surfaces, crawlspace surfaces,
cold water pipes, on air handling
equipment and duct work, and
unseen within envelope walls.
Basements typically require
supplemental dehumidification
equipment, since comfort cooling
equipment can’t control humidity
in basements with minimal heat
gain. Crawlspaces are particularly
difficult and expensive to deal
with, but sealing them with
vapor barriers up to outside
ground level, as well as insulat-
ing, and incorporating them into
the conditioned space and
adding additional means of dehu-
midification can control many
crawlspace moisture problems,
provided standing water or
excessive ground moisture is not
present (this assumes free crawl-
space ventilation air is not
required for fossil fuel burning
equipment). Water pipes can be
insulated. Air handling equip-
ment and ductwork must be
sealed air tight and insulated
with no breaks in the vapor bar-
rier especially when located
outside of the c
onditioned enve-
lope. Ductwork in all walls must
be sealed to reduce unseen mois-
ture migration due to air pressure
differentials.
In cooling systems, relative
humidity in supply ducts can be
95 % or higher, and evaporators
and condensate pans will be
wet. So, since moisture control is
not feasible, c
ontrol of airborne
spores and food (dust and air-
borne particles) with good, tight
fitting filtration systems in plac
e
is essential to control fungus
growth. If evaporator components
are resistant to UV radiation, a
UVC “germicidal” light that can
see the entire evaporator surface
can kill mold and microbes. UVC
lights should be selected that do
not emit ozone, which is an irri-
tant. Oversized equipment will
experience reduced operating
times resulting in less condensate
production which may actually
increase the microbial coloniza-
tion on the fin surfaces.
Temp-humidity meters
From dry bulb temperature and
relative humidity measurements,
temperature-humidity meters such
as the Fluke 971 can calculate wet
bulb temperature and dew point
temperature, psychrometric points
that are essential for HVAC evalu-
ations and diagnostics.
•
Wet bulb is very closely related
to enthalpy, or the total heat in
the air (dry bulb and wet bulb).
In a psychrometric chart, the
wet bulb lines are nearly paral-
lel the enthalpy scale values.
Return wet bulb temperature is
mandatory for accurately
charging a cooling system that
incorporates a fixed restrictor
metering device.
•
Supply and return wet bulb
temperatures
across an evapo-
rator can be used with a
psychrometric chart or enthalpy
table to calculate total cooling
capacity, sensible and latent
capacity, and S/T ratio.
•
Total heat may be found by
multiplying cfm x 4.5 x
enthalpy difference across
evaporator (Q
t
= cfm x 4.5 x
∆
h).
Legend
Severe-Cold: A severe-cold climate is defined as a region with approximately
8,000 heating degree days or greater.
Cold: A cold climate is defined as a region with approximately 4,500 heating
degree days or greater and less than approximately 8,000 heating degree days.
Mixed-Humid: A mixed-humid climate is defined as a region that receives more
than 20 inches of annual precipitation, has approximately 4,500 heating degree
days or greater or less and where the monthly average outdoor temperature
drops below 45 °F during the winter months.
Hot-Humid: A hot-humid climate is defined as a region that receives greater
than 20 inches of annual precipitation and where the monthly average outdoor
temperature remains above 45 °F throughout the year*.
Hot-dry/Mixed-Dry: A hot-dry climate is defined as a region that receives less
than 20 inches of annual precipitation and where the monthly average outdoor
temperature remains above 45 °F throughout the year;
A mixed-dry climate is defined as a region that receives less than 20 inches of
annual precipitation, has approximately 4,500 heating degree days or less and
where the monthly average outdoor temperature drops below 45 °F during the
winter months.
*This definition characterized a region that is almost identical to the ASHRAE definition of hot-humid
where one or b
oth of the follow
ing oc
cur:
•
a 67 °F or higher wet bulb temperature for 3,000 or more hours during the warmest six consecutive
months of the year; or
•
a 73 °F or higher wet bulb temperature for 1,500 or more hours during the warmest six consecutive
months of the year
.