Fluke 971 Application Note 2429548

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Investigating residential
humidity complaints
Application Note

Testing
Functions
Case
Study

Tools featured:
Temperature humidity meter, pipe
clamp probe, IR thermometer, DMM

Measurements taken:
Relative humidity, wet bulb
temperature, dew point, superheat

Scenario: Single stage residential cooling equipment. Homeowner complains that humidity
is too high at their desired thermostat setting.
Observations: Ranch house.
Air handler and ductwork in
attic. Slab construction.
Carpeted floors. Thermostat
setting 74 °F. System installed
last summer and load calculations were based on ACCA best
practices for “mixed/humid”
temperature zone.
Actions: You think this is too
easy. Probably a system-related
problem (dirty filter, low charge,
dirty condenser; the usual stuff.)
But you also realize that it’s
been a little cooler than most
summers. You measure the
indoor temperature and relative
humidity and find that it is
74.5 °F, 68 % RH, 66 °F WB,
and 63.3 °F DP.*
You are concerned because
with a cooler than normal
summer, operating times are
less (less moisture removal),
and carpet over concrete slab
can hide moisture problems and
create an environment for
fungal growth. You wonder if
there is a moisture-holding
sand layer between the vapor
barrier and the slab, which
would exacerbate moisture
problems under the carpet.
With your Fluke 60 series
infrared thermometer pointed at
the carpeted floor, you measure
68 °F. This is above dew point
and your psychrometric chart
tells you the vapor pressure is
greater in the carpet than in the
air, so moisture will travel from

Indoor ambient (dry bulb) temperature on the
Fluke 971.

Indoor wet bulb temperature.

Indoor dewpoint temperature.

the carpet to the air. This is
good for the carpet, but
contributes to higher RH in the
occupied space. This is good
because it’s easier to deal with
RH issues than floor moisture
issues.

From the Fluke Digital Library @ www.fluke.com/library

You ask the homeowner if
any remodeling changes have
occurred to the house. They
say, “No. Wait. We had a new
roof installed. Does that count?”
You ask, “Same composition
roof?” They say, “Yes, but we
changed the color from dark
green to white.” You say,
“Thanks. I also noticed that you
run your fan continuously. Is
there a reason for that?” They
say, “We think it keeps the air
moving and makes it feel more
comfortable.” You think you’ll
comment on that later after you
have finished gathering data.
Outdoors, your Fluke 971
Relative Humidity Meter tells
you it is 80 °F, 76 % RH,
74 °F WB, 71.8 °F DP. The dew

Outdoor ambient (dry bulb) temperature on
the Fluke 971.

Outdoor wet bulb temperature.

point has you worried as you
think about the attic. You check
the condensing unit. It’s a
model you respect and is operating properly (based on the
indoor 66 °F WB and outdoor
80 °F DP, your fixed restrictor
charging chart tells you the
suction line superheat should
be 18 °F. You verify this with
your Fluke 80PK-8 pipe clamp
probe).
You move back indoors. As
you climb into the attic to check
the air handler (80 AFUE gas
furnace), you’re thinking about
the reduced attic temperature
and heat gain from a light
colored roof compared to a dark
one. This not only will
contribute to reduced operating
times, but cooler attic temperatures increase the chances of
condensation on the supply
duct and air handler, especially
with a 72 °F DP. The attic is
108 °F. Good. Not too cool. You
look at the ductwork, furnace
and horizontal evaporator. You
scan the foil faced duct wrap
with your infrared thermometer.
You are relieved to see it is an
average of 76 °F. Wait. About 6
feet from the air handler your
infrared measures 72 °F and on
closer inspection, it feels damp.
You cut open the insulation
and find one duct joint wasn’t
properly sealed. This explains
the reduced temperature at this
location. You seal the duct, rewrap the insulation and re-seal
the vapor barrier. Now that this
small problem was corrected
before it could turn into a
bigger issue, you are confident
that the foil facing will remain
above dew point and turn your
attention back to the indoor RH
complaint.
The filter is relatively clean.
You replace it and inspect the
evaporator. It too is clean, but is
not level and water is higher on

Outdoor dewpoint temperature.

2 Fluke Corporation

Investigating residential humidity complaints

the backside of the pan than
the front where the drain is
located. You’re in luck. The
furnace and evaporator were
hung using turn buckles. You
adjust the turn buckles for best
drainage. That will help reduce
re-absorption of moisture into
the air stream during the off
cycle, but you wish the evaporator used a TXV rather than a
fixed restrictor metering device
because you know a TXV will
offer better performance at part
load conditions.

Options:
Now you are ready to decide
what the options are to
improve performance at part
load conditions.
1. First improvement already
accomplished by re-leveling
condensate pan for better
drainage.
2. Advise homeowners to operate fan in “auto” to reduce
moisture regain during off
cycles
3. You noticed they got a best
quality electronic thermostat
that allows adjustment of the
differential setting. You
decide to change it from a
1 °F to a 1.5 °F differential
for longer on times resulting
in more steady state moisture
removal time.
4. Delta-T across the evaporator
was 18 °F that you measured
with your Fluke DMM and 2
80PK-26 general purpose
temperature probes. You
could lower the blower
speed, but not sure of result
of duct surface temperature
at other operating conditions.
5. Replace fixed restrictor with
TXV
6. Replace ripple edged fin
evaporator with straight fins
and wider fin spacing
because ripple edged fins
tend to hold more water, and
closer spaced fins hold more
water.

Options 1, 2, and 3 are implemented. Since the equipment is
not oversized, you think it will
result in the desired effect.
Option 4 makes you nervous, so
you decide if further improvement is needed later, you can
try it then. You don’t want to
bring up options 5 and 6 due to
the inevitable cost discussion.

So now it’s time for
your presentation to the
homeowner...
“The high humidity you have
been experiencing indoors is a
result of lower outdoor temperatures that result in less operating time. Less operating time
means less moisture removal.
Running the fan continuously
allows moisture remaining on
the evaporator and in the drain
pan to be re-absorbed into the
air stream and keeps the
humidity above comfort levels.”

“I strongly recommend a
thermostat ‘Fan Auto’ setting for
overall comfort. I was able to
make an adjustment to the
condensate pan for better
drainage, which will help. And
I’ve adjusted your thermostat for
better moisture control. For
increased comfort at night
when there is no solar heat
gain, I recommend a thermostat
setting 1 to 2 degrees lower
than during the daytime.”
“There are additional things
that can be done, but I don’t
think the expense of these
modifications would be cost
effective for you at this time. I
believe the low cost changes I
have made for you will result in
the humidity levels you are
seeking. If time should prove
me wrong, then we can
address the other options at
that time, but again, I believe
you are going to be happy with
the adjustments I have made.”
*WB stands for wet bulb temperature, DP for
dew point, and RH for relative humidity.

Fluke. Keeping your world
up and running.

Fluke Corporation
PO Box 9090, Everett, WA USA 98206
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
In the U.S.A. (800) 443-5853 or
Fax (425) 446-5116
In Europe/M-East/Africa (31 40) 2 675 200 or
Fax (31 40) 2 675 222
In Canada (800)-36-FLUKE or
Fax (905) 890-6866
From other countries +1 (425) 446-5500 or
Fax +1 (425) 446-5116
Web access: http://www.fluke.com/
©2005 Fluke Corporation. All rights reserved.
Printed in U.S.A. 8/2005 2524516 A-EN-N Rev B

3 Fluke Corporation

Investigating residential humidity complaints



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