Whole-Home Humidity Control
Why this matters
The ASHRAE 55 thermal-comfort envelope ranges from 30 to 60 percent relative humidity at typical conditioned-space temperatures. AC alone controls humidity only as a byproduct of cooling; in shoulder seasons (April, May, September, October across much of the US) the cooling load is low while the moisture load is high, and AC short-cycles before it can pull humidity down. The customer feels clammy at 74 F and complains the system is broken. The system is fine; it cannot do the job alone in that condition. Whole-home humidity control is the upsell that turns a "they couldn't fix it" callback into a "they understood the actual problem" repeat customer.
Humidity targets by season and climate
| Indoor condition | Target RH | Why this range |
|---|---|---|
| Summer (cooling season) | 45 to 55 percent | Comfort range; below 45 percent is overdrying AC, above 55 percent is mold/dust mite range |
| Winter (heating season, mild climate) | 35 to 45 percent | Comfort range with low envelope condensation risk |
| Winter (heating season, cold climate, below 20 F outdoor) | 25 to 35 percent | Window condensation limits humidity ceiling |
| Health-sensitive (asthma, allergy) | 40 to 50 percent year-round | Dust-mite minimum at 50 percent; mold minimum at 60 percent |
| Wine cellar / cigar storage | 55 to 70 percent | Specific product requirements |
The window-condensation ceiling in winter is the under-appreciated constraint. Humidifying a home to 50 percent RH at 70 F indoor with 0 F outdoor produces window condensation on every single-pane window and most double-pane windows. The water runs down, rots the sill, and the customer blames the humidifier.
| Outdoor temp (F) | Maximum indoor RH at 70 F before single-pane window condensation |
|---|---|
| 40 | 65 percent |
| 20 | 45 percent |
| 0 | 25 percent |
| -20 | 12 percent |
A humidistat set to "summer comfort" 45 percent in a cold-climate winter actively damages the home. Use an outdoor-temperature-compensating humidistat.
Sources of indoor moisture (the actual load)
The moisture load that must be removed (cooling/dehumidification season) or added (heating season):
- Occupant respiration and perspiration: roughly 0.2 lb water per person per hour at rest
- Cooking and showering: 1 to 4 lb per day depending on family habits
- Houseplants: 0.3 to 1 lb per plant per day for medium-large plants
- Crawlspace or basement moisture migration: 5 to 20 lb per day for uncovered earth crawls
- Ventilation infiltration (humid outdoor air, cooling season): 5 to 30 lb per day depending on tightness
A single uncovered crawlspace can be the dominant moisture source in a southern home. Encapsulation (vapor barrier across floor + walls, sealed vents) is often the right first step before any equipment is sized.
Dehumidifier sizing
For dedicated whole-home dehumidifiers (Aprilaire, Ultra-Aire, Honeywell, Santa Fe), capacity is rated in pints per day at AHAM standard test conditions (80 F, 60 percent RH inlet).
| Home size | Typical capacity needed | Notes |
|---|---|---|
| Under 1,500 sq ft | 70 pints/day | Single-zone home, moderate moisture load |
| 1,500 to 2,500 sq ft | 90 to 130 pints/day | Most common residential sizing |
| 2,500 to 4,000 sq ft | 130 to 180 pints/day | Larger home or higher load |
| Over 4,000 sq ft | Multiple units or commercial-grade | Often two units zoned |
| Crawlspace dedicated | 65 to 90 pints/day | Lower temperature operation required |
The DOE 2019 test procedure changed dehumidifier ratings (now using IEF, Integrated Energy Factor, and a slightly different test condition). A unit rated 70 pints under old DOE ratings may rate 50 pints under new ratings. Match the rating system when comparing.
Crawlspace and basement dehumidifiers must be rated for low-temperature operation. Standard home dehumidifiers ice up below 55 F ambient. Sentinel HD, Santa Fe Compact, and Aprilaire 1830 are rated for low-temperature work.
Humidifier sizing
For winter humidification, capacity is rated in gallons per day at standard test conditions.
| Home size, tight envelope | Capacity (gallons/day) |
|---|---|
| Under 1,500 sq ft | 6 to 10 |
| 1,500 to 2,500 sq ft | 10 to 17 |
| 2,500 to 4,000 sq ft | 17 to 24 |
| Over 4,000 sq ft | Multiple units |
Leaky homes need more capacity because infiltration of cold dry outdoor air continuously dilutes the moisture added.
Equipment types and characteristics
Whole-home dehumidifiers (ducted)
Aprilaire E080, E100, E130, E150; Ultra-Aire MD33, XT155, SD12; Honeywell DR65, DR90, DR120; Santa Fe Ultra98.
- Installed in line with the air-handler return or as a stand-alone duct system
- Run independent of cooling; cycle on humidity setpoint
- Filtered; quieter than portable
- Drain to gravity floor drain or to condensate pump
- Annual filter change, condensate pan inspection
Bypass humidifiers
Aprilaire 600, Honeywell HE220, HE240.
- Mounted on supply or return plenum
- Use furnace airflow to evaporate water from a wetted pad
- Water-panel pad replacement annual
- Low cost, lower capacity than power humidifiers
- Require active heat for evaporation (do not work outside heating call)
Power humidifiers
Aprilaire 700, Honeywell HE280, HE360.
- Built-in fan; do not require furnace airflow
- Higher capacity than bypass
- Can run during fan-only call (some models)
- Water-panel pad replacement annual
Steam humidifiers
Aprilaire 800, Honeywell HM509, Nortec residential.
- Boil water to produce steam, inject into supply duct
- Highest capacity; works regardless of furnace operation
- More expensive equipment and operation (electric resistance heating)
- Cylinder replacement annual (or every 1,000 to 5,000 hours)
- No mineral concerns; mineral collects in disposable cylinder
Portable units
Removable consumer-grade units. Useful for spot rooms, not whole-home.
Controls
A humidistat is required regardless of equipment type. The four common approaches:
References
- ASHRAE Standard 55-2020 (Thermal Environmental Conditions for Human Occupancy).
- ASHRAE Standard 62.2-2022 (Ventilation and Acceptable IAQ in Low-Rise Residential Buildings).
- ASHRAE Standard 160-2021 (Criteria for Moisture-Control Design Analysis in Buildings).
- AHAM DH-1 (dehumidifier performance) and current DOE test procedure 10 CFR 430 Subpart B Appendix X1.
- EPA Indoor airPLUS Construction Specifications (current revision).
- Manufacturer literature: Aprilaire, Ultra-Aire, Honeywell, Santa Fe, Nortec, Skuttle, Carrier, Trane.
- ACCA Manual J (residential load calculation, latent-load component).
- Manuall internal: HVAC Indoor Air Quality Services, HVAC IAQ Add-Ons.