Fluke 971 Application Note 2412973

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Application Note

Understanding the relationship between dry bulb

temperature, wet bulb temperature, relative humidity,

and dew point temperature is essential in all facets of

air conditioning. These psychrometric processes play

an especially important role in building and materials

integrity, occupant health and comfort, and overall

indoor air quality.

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

Evaluating relative

humidity: Key factors

and measurements

Relative comfort

ANSI/ASHRAE standard 55-2004,

Thermal Environmental

Conditions for Human Occupancy,

sets an upper limit to absolute

humidity levels (0.012 humidity

ratio, or 0.012 x 7000 = 84 grains

moisture/lb dry air, also equiva-

lent to a dew point (DP) of 62 °F),

above which most occupants

become uncomfortable.

Since all occupants won’t be

satisfied by the same thermal

conditions, especially all at the

same time, the standard attempts

to identify a norm based on a

PMV (Predictive Mean Vote) of

80 % satisfaction. From that, a

PPD (Predicted Percentage

Dissatisfied) of 10 % is calculated

for general thermal comfort

dissatisfaction and 10 % PPD

from local (“my ankles are cold”)

comfort dissatisfaction.

The standard lists six primary

factors that affect thermal com-

fort: metabolic rate, clothing

insulation, air temperature, radi-

ant temperature, air speed,

humidity.

Understanding the combined

affects of these factors can help

technicians configure building

systems appropriately.

The good news, strangely

enough, is that poor humidity and

temperature levels are likely to

cause occupant discomfort.

Occupant complaints open a win-

dow of opportunity for the HVAC

contractor to proactively discover

related undesirable psychrometric

effects on materials integrity and

indoor air quality, including

microbial propagation.

To evaluate relative humidity,

wet bulb temperature, and dew

point, HVAC technicians tradition-

ally used a sling psychrometer

and psychrometric chart.

Now-days they use “humidity”

meters that are accurate, more

convenient, and usable in

confined locations unsuitable for

sling psychrometers.

Standards adoption

Many states have adopted ANSI/

ASHRAE Standards 55-2004 on

humidity and 62-2004 on IAQ

into their building codes. Since

both standards have been newly

updated, the following descrip-

tions may help inspectors and

contractors update practices to

meet new requirements.

The Fluke 971 Temperature Humidity Meter measures tempera-

ture from -20 °C to 60 °C (-4 °F to 140 °F), dewpoint, wetbulb,

and relative humidity from 5 % to 95 %.

Humidity levels

ANSI/ASHRAE Standard 62-2001,

Ventilation for Acceptable Indoor

Air Quality, specifies that

“Relative humidity in habitable

spaces preferably should be

maintained between 30 % and

60 % relative humidity to mini-

mize growth of allergenic or

pathogenic organisms.”

The updated ANSI/ASHRAE

Standard 62.1-2004, Ventilation

for Acceptable Indoor Air Quality,

is more specific. Now, relative

humidity upper limits are based

on peak values. “Occupied space

relative humidity shall be

2 Fluke Corporation Evaluating relative humidity: Key factors and measurements

designed to be limited to 65 % or

less at either of the two following

design conditions:

1. at the peak outdoor dew-point

design conditions and at the

peak indoor design latent load

or

2. at the lowest space sensible

heat ratio expected to occur

and the concurrent (simultane-

ous) outdoor condition.”

Good HVAC equipment selection

practices generally recommend:

•68 °F to 70 °F and 30 % RH

(relative humidity) winter

design,

and

•74 °F to 76 °F and 50 % to

60 % RH summer design

at

•outdoor conditions of 97.5 %

winter and 2.5 % summer dry

bulb (DB).

This means that on average,

2.5 % of the extreme seasonal

temperatures will be beyond

equipment capacity. The equip-

ment will be effectively

undersized during these times.

This is critically important in

equipment selection, since only

30 % of the operating hours of

comfort cooling equipment occur

within 5 % of outdoor design dry

bulb temperature. Summer latent

load control is more difficult to

control at part load conditions,

although most commercial equip-

ment is staged or has some form

of capacity control.

If comfort cooling equipment is

oversized, moisture related com-

plaints and problems will

increase. Residential heat pumps

should be selected according to

the cooling requirements, not the

heating requirements, especially

in geographic areas where “dirty

socks syndrome” is prevalent and

air handling equipment is located

in a crawlspace.

Fungus

With enough knowledge and

measurement, HVAC systems can

be set at the appropriate summer

and winter psychrometric condi-

tions to discourage fungal

growth. Conditions for fungal

growth include spores settling on

a surface, a micro-environment

ensuring oxygen, optimal tem-

peratures, nutrients, and

moisture. Four of these conditions

are found in nearly every envi-

ronment. The most controllable

variant is moisture.

Wet bulb temperature: Represents the

cooling effect of evaporating water, the

temperature air will cool to when water

evaporates into unsaturated air.

Dewpoint temperature: The tempera-

ture under which water will condense

out of the air.

Dry bulb temperature: Air temperature

determined by an ordinary thermome-

ter.

Relative humidity: Ratio of water

vapor pressure (amount currently in the

air) to the saturation vapor pressure (the

amount the air can hold) at a given air

temperature.

Met(abolic) rate: The rate by which

the body transforms chemical energy

into heat and work through activity. In

ASHRAE 55, this rate is measured in

“met units” (18.4 Btu/h*ft2).

Sensible cooling: Factors such as peo-

ple, appliances, solar radiation, and

infiltration create heat gain, each

adding a sensible load to the environ-

ment within a house, office, etc. This

sensible load raises the dry-bulb tem-

perature. The process by which the

sensible, or dry bulb, temperature is

reduced without changing the moisture

content of the air is referred to as a

sensible cooling process.

Latent cooling: An amount of moisture

is added to the inside air by plants,

people, cooking, and other sources. A

latent cooling process involves the con-

densation of moisture out of the air,

reducing the wet bulb, dewpoint, and

humidity levels, but leaving the dry

bulb temperature untouched.

S/T ratio: Sensible to total heat ratio,

or sensible heat factor. Of the total

capacity of a cooling system, there is a

sensible capacity and a latent capacity.

The sensible capacity cools the air by

absorbing heat to lower the dry bulb

temperature. The latent capacity

absorbs the latent heat of vaporization

to remove moisture from the air without

changing the actual dry bulb tempera-

ture. The S/T ratio, when used with a

psychrometric chart, will provide the

temperature at which a cooling coil

must operate in order to support both

sensible and latent heat removal.

“Dirty socks syndrome”: A common

condition during the cooling season that

describes

an odor generated by burning airborne

contaminants on an indoor coil, typi-

cally during the heat pump defrost

cycle.

clo: A unit of measurement to express

the amount of thermal insulation pro-

vided by clothing and other garments.

•Example: An ensemble including

briefs, t-shirt, calf-length socks,

shoes, straight trousers,

long-sleeve dress shirt,

double-breasted (thick) jacket totals

1.14 clo*

•Example: An ensemble including

briefs, short-sleeve knit sport shirt,

walking shorts,

sandals totals .31 clo*

*clo values per ASHRAE 55-2004

Psychrometrics has a language all its own. To better understand

how the various parameters interact to support thermal comfort, here

are some of the more common terms described in this document:

Relative humidity above 60 %

can support fungal growth on

hygroscopic (sorbent) surfaces

and hygroscopic surfaces at 80 %

RH are likely to promote fungal

growth. Nearly all surfaces are, or

can become, sorbent and include

painted surfaces, gypsum dry

wall, carpets, wall coverings, and

masonry products. Even glass

with a dirt film and dust on it can

support fungal growth.

Masonry products such as

brick, cinder block and concrete

are excellent sorbents and can

adsorb vast quantities of moisture

and become an inviting breeding

environment for molds. The vapor

pressure within the pores of

manufactured masonry can be

less than the vapor pressure of

the ambient air which moves

moisture from the air into the

masonry pores. As the pores

become wetted, capillary action

takes over and fills the pores,

thus providing an ideal breeding

ground for fungal proliferation.

This explains why some surfaces

above dew point can become

wetted.

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 conditioned 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, control of airborne

spores and food (dust and air-

borne particles) with good, tight

fitting filtration systems in place

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 (Qt= 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 both of the following occur:

•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.

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 coil surface for

moisture removal.

Most cooling equipment

can tolerate reduced air volumes

of about 20 %. If evaporator

air volumes are reduced from

400 cfm/ton down to around

325 cfm/ton, the evaporator

temperature will fall further

below 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 be 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.

Crawlspaces 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 be 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 complex 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).