In July 2023 I was in a 2,800-square-foot house in Roswell running commissioning checks on a two-stage heat pump installed the previous November. By June the owner was calling because the house felt clammy at 74°F and the basement smelled like a wet towel. I measured the RH on a Thursday afternoon: upstairs 58 percent, main level 62 percent, basement 71 percent. The cooling system was cycling on temperature and stopping before it had time to dehumidify — the classic symptom of a multi-stage system in a humid climate.
Why North Georgia summers are a dehumidification problem, not just a cooling problem
Roswell sits at roughly 1,100 feet elevation in Fulton County, with average July dew points of 68°F to 72°F. When outdoor air is 91°F and 72°F dew point, the absolute moisture content of that air is about 140 grains per pound of dry air. Your cooling system removes moisture by running the evaporator coil below the dew point of the indoor air and collecting the condensed water. The longer the system runs, the more moisture it pulls. An oversized or multi-stage system in mild summer weather satisfies the thermostat in 8 to 12 minutes, shutting off before it has time to dehumidify. The room is cool but the relative humidity stays high.
High indoor RH above 60 percent sustains dust mite activity, holds moisture in organic materials, and creates mold-favorable conditions in basements. The EPA’s guidance on indoor humidity recommends keeping residential RH between 30 and 50 percent year-round.
Whole-house dehumidifier: what it does differently from the AC
A standalone whole-house dehumidifier is sized for latent load — moisture removal — rather than sensible load — temperature. It runs continuously at low speed regardless of the thermostat and processes air through a dedicated coil and heat exchanger. The most common residential units from manufacturers like Aprilaire, Honeywell, and Santa Fe are sized between 70 and 130 pints per day, significantly more than the typical 20 to 40 pints a central AC removes per day in average conditions.
Whole-house units integrate with the existing HVAC ductwork, either in series with the air handler or in a parallel bypass duct. They have a separate humidistat — usually set to 50 to 55 percent — and run their own compressor. In a house that already has adequate sensible cooling, the dehumidifier handles the latent load independently, so the AC is free to cycle as temperature requires without being asked to do double duty.
Sizing: pints per day and what drives the number
ASHRAE’s residential guidelines suggest roughly 0.8 to 1.2 gallons of moisture removal per 100 square feet per day in a high-humidity climate for a reasonably tight house. For a 2,800-square-foot house in Roswell, that translates to roughly 90 to 135 pints per day of dehumidification capacity. A 130-pint whole-house unit costs $1,200 to $1,800 in equipment; installation in a house with accessible ductwork runs $400 to $800, for a total of $1,600 to $2,600.
Houses with crawl spaces add a complication: the crawl space is a major moisture source even when it is sealed and conditioned. A vapor barrier on the crawl floor and walls, properly lapped and sealed, reduces moisture infiltration significantly. Without it, the whole-house dehumidifier is fighting the ground as well as outdoor infiltration, and may need to be oversized by 30 to 50 percent.
What the Roswell house needed
In the 2023 house the cooling system was a two-stage unit designed for a mixed climate — it was running at 30 to 40 percent capacity on most July days, never getting the evaporator cold enough to condense significant moisture in the short cycle times. The fix had two parts: reprogram the system to run the first stage longer before staging up (extending runtime from 9 minutes average to 18 to 22 minutes), and install a 90-pint whole-house dehumidifier in a bypass configuration. Three weeks later, the basement read 54 percent and the main level 48 percent on the same Thursday-afternoon measurement. The smell was gone.
For the installation and system reprogramming, the owner worked with a Roswell HVAC contractor who could access the manufacturer’s commissioning software for the two-stage unit — that step is not something an HVAC technician without that software can do on a modern variable-speed system.
Frequently asked questions
Can my central AC dehumidify without a separate dehumidifier? Yes, but only if it runs long enough to condense moisture on the evaporator coil. Systems that are oversized or run in a milder climate often cycle off on temperature before they can adequately dehumidify. A whole-house dehumidifier is the reliable fix because it runs continuously on its own humidistat regardless of the temperature setpoint.
What RH should I target in a Roswell home in summer? Between 45 and 55 percent relative humidity at normal indoor temperatures. Below 45 percent can cause hardwood floors to gap; above 60 percent creates conditions favorable for dust mites and, in basements, surface mold. A good target is 50 percent as a set point on the dehumidifier humidistat.
How much does a whole-house dehumidifier cost installed? Equipment runs $1,200 to $1,800 for a 90- to 130-pint unit from a major manufacturer. Installation with ductwork modifications typically adds $400 to $800, bringing the total to $1,600 to $2,600. Expect energy costs of $20 to $50 per month during peak summer operation depending on how wet your house runs and local electricity rates.