Commercial-grade crawl space dehumidifier, properly sized.

Sizing depends on the crawl-space cubic footage and the moisture load. For most East TN homes with 1,500–2,500 sq ft footprint and 24–36 inch crawl clearance, a 70-pint-per-day commercial unit (Aprilaire 1820 or Santa Fe Compact 70) is right. Larger homes or higher-moisture-load crawls (any home in Sequoyah Hills or Bearden near the river, for example) typically need a 90-pint unit. Going smaller than 70 pints/day in East TN summer humidity is a false economy — the unit runs near 100% duty cycle, fails sooner, and never quite catches up. We size to the actual cubic footage and the moisture-load assessment from the inspection, not to a blanket “one size fits all” recommendation.

For sealed crawl spaces, inside the crawl. Period. The dehumidifier needs to remove moisture from the air it’s regulating, and that air lives in the crawl. Some installers try to save labor by ducting humid air from the crawl into a basement or utility-room dehumidifier; this works occasionally but introduces a host of new failure modes (duct leaks, condensation in the supply line, fan reliability under continuous duty). Commercial-grade crawl-space dehumidifiers are designed for the in-crawl environment — they handle the higher dust load, the cooler ambient temperature in winter, and the duty cycle. A consumer-grade unit moved into a crawl typically fails in 18–24 months.

A properly sized 70-pint commercial unit pulls about 600–700 watts when running. In East TN summer conditions it runs 30–50% duty cycle to hold a 50% RH setpoint, which works out to roughly $15–25 per month in electricity at current KUB rates. In winter the duty cycle drops to 5–15% because the cool outdoor air carries less moisture; the dehumidifier mostly sits idle. Annual operating cost for most East TN encapsulated crawls works out to $120–200. Compared to the structural and indoor-air-quality damage an uncontrolled crawl causes, the electric bill is minor.

Three big differences. First: duty cycle rating. Consumer units (Frigidaire, GE, etc. sold at Lowes/Home Depot) are rated for intermittent residential use, typically 4–6 hours per day. A crawl-space dehumidifier runs 8–16 hours per day in summer. Consumer units literally aren’t designed for this load and fail accordingly. Second: humidistat accuracy and minimum setpoint. Consumer units typically only go down to 40% RH minimum setpoint with rough ±5% accuracy; commercial units can hit 35% RH with ±2% accuracy, which matters when you’re trying to stay below 55%. Third: condensate handling. Consumer units have a bucket you empty manually or a small built-in pump rated for low head. Commercial units have integrated condensate pumps or gravity-drain ports designed for continuous operation. A consumer unit in a sealed crawl will fill its bucket in 4–6 hours, then shut off, then humidity climbs, then mold starts.

Three options, in order of preference: (1) gravity drain to an existing crawl-space drain or a nearby plumbing drain — simplest and most reliable; (2) condensate pump pumping to a higher discharge point — works when no gravity drain is accessible, requires the pump to keep working forever, occasional float-switch maintenance; (3) discharge to a French drain or daylight exit through the foundation wall — we use this when neither plumbing nor a condensate pump location is practical. We figure out which option is right at the inspection and quote accordingly. We don’t install dehumidifiers that drain into buckets — that’s not a permanent system.