Application Note

4 Fluke Corporation Evaluating relative humidity: Key factors and measurements
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Printed in U.S.A. 8/2005 2522930 A-EN-N Rev B
Fluke. Keeping your world
up and running.
•
Sensible vs. latent cooling
and S/T ratio
can be found
by plotting conditions on a
psychrometric chart or from a
psychrometric calculator.
•
Dew point is critical in both
summer and winter evalua-
tions. Duct surface temperature
must be maintained above dew
point to prevent condensation
whether inside or outside of
the conditioned space.
•
Winter indoor relative humid-
ity
must be kept low enough
to ensure inside wall and
window surface temperatures
do not approach dew point.
If condensation appears on
window or wall surfaces,
condensation hidden within
envelope walls will be likely.
Addressing comfort
related complaints
With equipment that does not
have capacity control, or is
staged, most humidity-related
comfort complaints occur at part
load conditions when run times
based on thermostat dry bulb
temperatures are shorter. Less
operating time means less mois-
ture removal. Oversized
equipment will only exacerbate
this as well as increasing occur-
rences of detrimental coincidental
conditions. Changing from a fixed
restrictor metering device to a
thermal expansion valve will
ensure maximum evaporator
capacity at part load conditions
and utilize more c
oil surfac
e for
moisture removal.
Most cooling equipment
can tolerate reduc
ed air volumes
of about 20 %. If evaporator
air volumes are reduced from
400 cfm/ton dow
n to around
325 cfm/ton, the evaporator
temperature will fall further
b
elow dew point and remove
more moisture from the air. This
change will also reduce duct
surface temperature and register
temperature in the direction of
dew point temperature and regis-
ter throw, affecting air patterns in
occupied spaces.
A dehumidistat can lower air
volumes at increased humidity
levels. Another alternative is to
use a Timed-On-Control device,
to provide reduced cfm for the
first 5-10 minutes of cooling
demand, and then switch to the
design cfm to finish the cooling
cycle. A portable dehumidifier
can b
e located in areas of high
humidity, such as a basement,
reducing humidity, increasing
heat gain, and forcing longer
cooling cycles. Make sure rooms
with intermittent high moisture
gain, such as bathrooms,
kitchens, and laundry areas are
ventilated to the outdoors (not
the attic or crawlspace).
Addressing dew point
and/or fungus related
complaints
Ducts in unconditioned spaces
carrying cool, humid air must be
sealed airtight using an NFPA
approved duct mastic. Any air
leaks in a duct will render the
insulation useless at that point
and condensation is likely to
occur. Duct wrap insulation must
not be compressed by hangars.
Hangars must be placed under-
neath duct wrap insulation. Duct
wrap insulation barriers must be
unbroken and sealed at the
seams.
In unconditioned attics,
increasing attic temperature may
increase heat gain on ceilings
below, but will reduce the occur-
rence of condensation on ducts.
Attics in homes of newer con-
struction techniques may result in
lower attic temperatures, but this
increases the chance of conden-
sation on duct or air handler
surfaces. Sealing attic vents and
adding humidistat controlled
flood lights to increase attic tem-
perature can compensate for this.
Crawlspac
es present unique
opportunities. Typical crawlspace
vent sizing is inadequate for con-
trolling moisture by ventilation.
100 % ground cover vapor barrier
up the inside wall to a height
equal to the outside ground level,
sealing the vents, insulating the
perimeter walls, and treating it as
a conditioned space is a preferred
method of moisture control, often
requiring additional supplemental
dehumidification. Air handling
equipment in a crawlspace must
have excellent particulate filtra-
tion in place with no return side
air leaks to reduce microbes and
their food sources in the evapora-
tor and supply duct. Humidity
levels in basements must b
e reg-
ulated to less than 60 % RH to
discourage microbial growth.
Painting the surfaces of hydro-
scopic masonry (cinder blocks,
brick, mortar) will reduce mois-
ture retention, discouraging
microbes.
Resources
If the c
omplex subjects briefly
treated here pique interest for
further study, additional resources
are available through ASHRAE, at
www.ashrae.org. The ASHRAE
handbooks and monthly journal
are an exceptional vehicle for
discovery. Psychrometric charts
are now available inside software
programs that make easy work of
the calculations. Other HVAC
organizations include:
•
ACCA (Air Conditioning
Contractors of America)
(
www.acca.org),
•
PHCC (Plumbing Heating
Cooling Contractors)
(
www.phccweb.org),
•
SMACNA (Sheet Metal and Air
Conditioning Contractors’
National Association)
(
www.smacna.org), and
•
RSES (Refrigeration Service
Engineers Society)
(
www.rses.org).