Dehumidifier size calculator for Knoxville homes

Buying a dehumidifier that's too small is one of the most common ways a damp basement stays damp. A unit that can't keep up runs all day, fills its bucket, and still leaves the room above 60 percent humidity, which is where EPA says mold gets its foothold. This calculator takes the size of the space and how damp it feels and returns the capacity, in pints of water removed per day, from the sizing chart ENERGY STAR publishes for dehumidifier shoppers. For a sealed crawl space it also gives the figure the Tennessee residential code uses. Nothing you enter leaves your browser.

The sizing chart behind the calculator

This is the chart on ENERGY STAR's dehumidifier buying page. For a sealed crawl space, Tennessee's 2018 residential code (section R408.3) sets the dehumidifier option at 70 pints a day per 1,000 square feet: a 1,500-square-foot crawl space works out to 105 pints a day.

The pint numbers on the box changed in 2019

If you've owned a dehumidifier before, today's capacity labels can look small. That's because the federal test changed. Under DOE standards that took effect in June 2019, portable units are now rated at 65°F instead of 80°F, and ENERGY STAR says a unit tested the new way will likely report a smaller capacity than it did under the old test. ENERGY STAR publishes a rough translation:

So a "70-pint" unit you remember from years ago is closer to a 40- or 45-pint model on a shelf today. ENERGY STAR notes the shift varies by model, so if a box or listing shows two numbers, check which test each one comes from before comparing.

Say what you found and where, and we will hand you to a mold remediation company that covers your end of the valley. (865) 417-9186

Knoxville's air, in numbers

A dehumidifier in East Tennessee works hardest from late spring into fall, and NOAA's 1991-2020 hourly normals for McGhee Tyson Airport show why. We averaged the hourly figures by month:

The dew point is the column that matters below grade. With July's outdoor dew point near 68°F, any surface cooler than that, such as a basement wall, a cold-water line or a supply duct in the crawl space, collects condensation. Even air that doesn't condense ends up very humid once it cools: we calculate that July's average outdoor air brought down to 70°F sits at about 92 percent relative humidity. That is why a vented crawl space or an open basement window in summer tends to add moisture rather than remove it, and why the "very damp" or "wet" rows of the chart often fit Knoxville basements in July even when they feel fine in January. The National Weather Service office in Morristown puts Knoxville's normal yearly rainfall at 51.93 inches, which keeps the soil around foundations damp as well.

Getting the most from the unit you buy

• Measure first. A basic hygrometer is inexpensive, and readings over a humid week tell you which row of the chart you're really in.
• Drain it. A unit that stops when the bucket fills isn't drying anything. Use a hose to a floor drain or a condensate pump where you can.
• Close the room. Shut basement windows and doors to unconditioned spaces while it runs.
• Look for the label. ENERGY STAR certified dehumidifiers use about 20 percent less energy than standard models for the same water removed, and every certified model has a built-in humidistat.
• Fix the water first. Seepage, gutters dumping at the foundation and a torn crawl space liner are water problems, not humidity problems.

Already seeing growth? A dehumidifier stops new mold but doesn't remove what's there. See crawl space mold, humidity and mold in Knoxville, or professional mold remediation for patches larger than about 10 square feet.

Sources

• ENERGY STAR, Dehumidifiers: the sizing chart, condition descriptions, the 30-50% target, the 55% whole-home threshold, frost below 65°F, the 20% energy figure and built-in humidistats.
• ENERGY STAR, Dehumidifier testing and capacity: the June 2019 DOE standards, the 65°F test temperature and the old-to-new rating table.
• Tennessee Residential Code 2018, section R408.3: 70 pints per day per 1,000 square feet for a dehumidified unvented crawl space.
• NOAA NCEI, 1991-2020 hourly normals, Knoxville McGhee Tyson Airport: monthly dew point and humidity figures (our averages of the hourly values).
• NWS Morristown, Knoxville climate normals, and U.S. EPA, A Brief Guide to Mold, Moisture and Your Home: annual rainfall and the 60 percent humidity ceiling.

FAQ