Whole-House Humidity Control Reference

Why this matters

Indoor humidity affects comfort, health, building integrity, + energy use. Too dry in winter; too humid in summer. Customers often complain about humidity-related issues without realizing the cause. The tech who diagnoses + addresses humidity issues solves problems customers didn't know they had. This is the field card.

Target humidity ranges

Ideal range: 30-50% RH (relative humidity)

Below 30%:

• Dry skin / nasal passages
• Static electricity
• Wood floor gaps + cabinet shrinkage
• Sinus / respiratory issues
• Furniture damage

Above 50%:

• Dust mites thrive
• Mold growth
• Wood floors cup
• Condensation on windows
• Musty smell
• Insect activity

Above 60%:

• Health concerns (mold, allergies)
• Building damage

Why humidity changes seasonally

Winter:

• Cold outside air holds little moisture
• Heat lowers indoor RH significantly
• Drier outside air infiltrates
• Tendency: 20% RH common without humidifier

Summer:

• Warm air holds more moisture
• AC removes some (latent capacity)
• Some climates very humid (Gulf Coast)
• Tendency: too humid in some regions

Measuring humidity

Hygrometer: instrument that reads RH

• Digital (most common)
• Hair / bimetal (less common, older)
• Inexpensive
• Carry one for diagnosis

Smart thermostats: many include humidity sensor + display.

Inspect ranges across home:

• Different rooms vary
• Cold corners higher RH
• Near showers / kitchens higher

Winter (adding humidity)

Sources of dryness:

• Heat
• Cold outdoor air infiltration
• Air conditioning (some modes; less common winter)

Humidifier types:

Whole-house bypass humidifier:

• Connects to HVAC ductwork
• Water source + drain
• Operates when furnace runs
• Bypass air flows through wet pad → adds humidity → enters supply duct
• 30-60% efficient
• Common; affordable

Whole-house fan-powered humidifier:

• Similar to bypass but with own fan
• Doesn't require furnace to run
• More flexible
• Higher cost

Steam humidifier:

• Generates steam electrically OR via burner
• Most precise output
• Higher cost (electricity / energy)
• Best for large homes / very dry climates

Portable humidifier:

• Single-room
• Refilled by user
• Doesn't address whole home
• Useful for problem rooms

Humidifier installation

For whole-house unit:

• Mount on supply duct (after furnace)
• Water supply (1/4" line from cold water)
• Drain (to nearest drain OR floor drain)
• Power connection
• Humidistat (controls operation)
• Wiring to furnace (sometimes)

Common issues:

• Clogged drain
• Mineral buildup on pad
• Water line leaks
• Humidistat calibration

Humidifier maintenance

Annual:

• Replace pad / wick
• Clean water tank / pan
• Flush mineral deposits
• Check water line / shutoff
• Verify humidistat operation

Without maintenance: efficiency drops + bacterial growth possible.

Summer (removing humidity)

Sources of humidity:

• Outdoor air infiltration
• Cooking + bathing (large amounts)
• People (perspiration)
• Plants
• Pet activity
• Drying laundry indoors
• Plumbing leaks (hidden)

Dehumidifier types:

AC handles much:

• Cooling coil condenses moisture
• Drains away
• 60-80% of typical humidity removal
• Sized correctly = automatic

Standalone dehumidifier:

• Single-room OR whole-house
• Pulls moisture; collects OR drains
• Variable capacity (pints/day)
• Pads OR refrigerant-cycle

Whole-house dehumidifier:

• Ducted into HVAC
• Independent operation
• 90+ pints/day capacity typical
• For homes with humidity issues despite AC

Heat-pump-with-dedicated-dehum:

• Modern systems can run "dry mode"
• AC running for dehumidification specifically
• Without cooling overload

When AC isn't enough

Some homes need dedicated dehumidification:

• Coastal humidity
• AC oversized (short cycles; less moisture removal)
• Tight homes with poor ventilation
• High occupancy

Signs:

• AC running but humidity high
• Mold concerns
• Discomfort despite cool temperature
• Building damage

AC short-cycling effect

Oversized AC = shorts:

• Doesn't run long enough to dehumidify
• House cool but humid
• "Cool + clammy"
• Common in mild weather

Solution:

• Variable-speed AC
• Smaller AC
• Add dehumidifier

Humidity + comfort

Same temperature feels different at different humidity:

• 72°F at 40% RH = comfortable
• 72°F at 60% RH = sticky
• 75°F at 30% RH = OK
• 75°F at 60% RH = uncomfortable

Customer can save on cooling by reducing humidity instead of lowering temperature.

Humidity + building issues

Wood damage:

• Floors: cup (high humidity), gap (low humidity)
• Furniture: warp, crack
• Doors / drawers: stick / loose

Wall damage:

• Moisture under wallpaper
• Paint failure
• Mold growth
• Drywall warping

Insulation:

• Wet insulation loses R-value
• Mold growth
• Settling

HVAC:

• Cool walls + warm humid air = condensation
• Duct condensation
• Pan overflow

Humidity + health

Respiratory:

• Low: dry passages, irritation
• High: mold spores, allergens, viruses thrive

Skin:

• Low: dry, cracking
• High: too sticky for normal moisturizing

Allergies:

• Dust mites thrive at high humidity
• Mold thrives high
• Optimal range minimizes both

Diagnosing humidity issues

When customer reports:

"Air is so dry":

• Measure RH; typically <30%
• Address with humidifier
• Winter season

"It's so humid in here":

• Measure RH; >55%
• Address with dehumidification (AC running, dedicated unit, or both)

"Some rooms feel different than others":

• Measure multiple rooms
• Difference > 10% RH = ventilation issue
• Address airflow + balance

"Windows are wet":

• Indoor humidity too high for window temperature
• Either reduce humidity OR upgrade windows
• More important in cold weather

Specific scenarios

Customer with new tight-construction home + dry winter:

• Modern tight homes lose less moisture
• But ventilation = drier in winter than older drafty home
• Humidifier needed

Customer with old leaky home + winter:

• More natural humidity loss
• Humidifier required typically

Coastal humidity:

• Year-round high humidity outdoor
• AC works hard
• May need dedicated dehumidification

Basement humidity:

• Often higher than upstairs
• Cool walls + humid air = condensation
• Dehumidifier in basement common

Crawlspace:

• High humidity affects whole house
• Vapor barrier + drainage
• Sometimes encapsulation + dehumidifier

Sizing equipment

Humidifier:

• Based on home volume + air changes
• 15-25 gallons/day typical for 2,500 sq ft
• Steam: 18 gallons/day with modest daily operating cost

Dehumidifier:

• Pints/day capacity vs home size + humidity load
• 70-90 pints for typical home
• 50-pint for smaller

Manufacturer sizing guides; ACCA Manual J for HVAC.

Operation

References

• ASHRAE 55 (Thermal Environmental Conditions)
• ASHRAE 62.2 (Ventilation + Indoor Air Quality)
• AHRI Standards for humidifiers + dehumidifiers
• DOE humidity recommendations
• Manuall internal: Indoor Air Quality Testing, Mold Remediation Basics