Application Note

2 Fluke Corporation Investigating residential humidity complaints
point has you worried as you
think ab
out the attic. You check
the condensing unit. It’s a
model you respect and is oper-
ating 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 tempera-
tures 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, re-
wrap the insulation and re-seal
the vapor barrier. Now that this
small problem was corrected
b
efore it c
ould tur
n into a
bigger issue, you are confident
that the foil facing will remain
ab
ove dew point and tur
n your
attention back to the indoor RH
c
omplaint
.
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
You ask the homeowner if
any remodeling changes ha
ve
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 9
71
Relative Humidity Meter tells
you it is 80 °F, 76 % RH,
74 °F WB, 71.8 °F DP. The dew
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 evapo-
rator 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 oper-
ate 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
w
ith your Fluke D
M
M and 2
80PK-26 general purpose
temperature probes. You
c
ould lower the blower
speed, but not sure of result
of duct surface temperature
at other operating conditions.
5. Replace fixed restrictor with
TXV
6.
Replac
e ripple edged fin
evaporator with straight fins
and wider fin spacing
b
ecause ripple edged fins
tend to hold more water, and
closer spac
ed fins hold more
water.
Outdoor ambient (dry bulb) temperature on
the Fluke 971.
Outdoor wet bulb temperature.
Outdoor dewpoint temperature.