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

Equipment sized right and installed right still fails the customer if nobody operates it correctly. Most humidity callbacks are operation, not hardware.

Winter humidifier setpoint follows the outdoor temperature. The correct indoor RH in January is lower than the correct RH in November, because the coldest surface in the house (the window glass) gets colder. Set it as high as you can go without condensation on the windows, and back it down as the outdoor temperature falls. An outdoor-reset humidistat does this automatically off an outdoor sensor and is worth the upgrade on any house with a customer who will not touch the dial. Manual controls need a conversation: window fogging in the morning means turn it down, static shocks and gapping floors mean turn it up.

Seasonal changeover is a real service item. On a bypass humidifier, the damper flips to the summer position and the water supply gets shut off in spring, then reversed in fall. Left in winter position through cooling season, the bypass duct short-circuits conditioned supply air back to the return and the customer's cooling bill goes up for no reason. A humidifier left fed with water all summer is a leak waiting for nobody to be home.

Fan on AUTO during cooling season. This is the highest-value thing you can tell a homeowner about summer humidity. With the fan set to ON, the blower keeps running after the compressor stops and re-evaporates the water sitting on the wet coil straight back into the house. Same equipment, same setpoint, noticeably higher indoor RH. If the customer wants continuous circulation for air quality, that is what a variable-speed blower at low continuous speed is for, not the ON switch.

Dehumidifier setpoint and drainage. Set standalone and ducted dehumidifiers around the middle of the target band rather than at the bottom; chasing a very low number runs the unit constantly for little comfort gain. Drain to a floor drain or a condensate pump, never to a bucket the customer has to empty, because the bucket is what turns into a unit that has been off for three weeks. In a crawlspace or unheated basement, protect the drain line from freezing and pitch it properly.

Thermostat dehumidify modes. Many modern thermostats paired with variable-capacity equipment will overcool slightly or drop blower speed to pull more latent load. Enable it, then verify it actually engages, and tell the customer the house may run a degree or two below setpoint on humid days by design so they do not call it a malfunction.

Verify, do not assume. Put a hygrometer in a couple of rooms, not just at the thermostat, and check it against the thermostat's own reading. Thermostat humidity sensors drift, and a reading taken in a hallway does not describe a basement or a north bedroom.

Set a reminder for the customer. Two dates: changeover in spring, changeover in fall, each paired with the maintenance items above. A humidifier still running in May and a dehumidifier still running in December are the two most common findings on a maintenance visit.

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