Fluke 971 Application Note 2429548

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Application Note

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

Tools featured:

Temperature humidity meter, pipe

clamp probe, IR thermometer, DMM

Measurements taken:

Relative humidity, wet bulb

temperature, dew point, superheat

Testing

Functions

Case

Study

Investigating residential

humidity complaints

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.

Scenario: Single stage residen-

tial cooling equipment. Home-

owner complains that humidity

is too high at their desired ther-

mostat setting.

Observations: Ranch house.

Air handler and ductwork in

attic. Slab construction.

Carpeted floors. Thermostat

setting 74 °F. System installed

last summer and load calcula-

tions 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.

2 Fluke Corporation Investigating residential humidity complaints

point has you worried as you

think about 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

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

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

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

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.

Outdoor ambient (dry bulb) temperature on

the Fluke 971.

Outdoor wet bulb temperature.

Outdoor dewpoint temperature.

Fluke. Keeping your world

up and running.

Fluke Corporation

PO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.

PO Box 1

186, 5602 BD

Eindhoven, The Netherlands

For more information call:

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Fax (425) 446-5116

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From other countries +1 (425) 446-5500 or

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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

Options 1, 2, and 3 are imple-

mented. 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 improve-

ment 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 temper-

atures that result in less operat-

ing 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.”

3 Fluke Corporation Investigating residential humidity complaints

“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.