Whole-Home Dehumidifiers in Florida: When You Need One, and What to Ask

Start with a diagnostic measurement, not a feeling

Relative humidity (RH) compares the water vapor in the air with the most the air can hold at that temperature. The EPA advises keeping indoor humidity below 60 percent. Ideally it should be between 30 and 50 percent. A moisture or humidity meter can measure it, the EPA says. A National Renewable Energy Laboratory computer simulation of hot, humid homes found, in its thermal comfort analysis, that occupants are unlikely to notice indoor humidity problems. Our reading: trust a meter reading over how the air feels.

The 2022 Building America county guide lists Hillsborough, Pasco and Pinellas counties as Hot-Humid, zone 2A. The National Weather Service's Tampa Bay office compares two days. A 90-degree day at 50 percent relative humidity feels more humid than a 50-degree day at 100 percent. Our reading: write down the room temperature with every humidity reading.

Moisture sources and maintenance come first

ENERGY STAR says reducing sources of moisture can reduce your need for dehumidification. Its examples are vent fans in bathrooms and kitchens. It also names clothes dryers that are properly vented to the outdoors.

A National Renewable Energy Laboratory simulation found a major culprit of high indoor humidity in hot, humid homes. It was mechanical ventilation combined with air leaking into the house. ENERGY STAR also says home sealing and duct sealing improve the performance of a whole-home dehumidifier. Duct sealing vs duct repair explains the difference. To prevent future problems, ENERGY STAR says to consider regular maintenance of your heating and cooling system. Our reading: a dehumidifier that offsets a fixable moisture source treats the symptom. Find the source first.

Visible mold, a musty smell that keeps returning, or standing water call for a professional. Mold in HVAC systems in Florida covers when it needs professional attention.

When a dedicated dehumidifier is warranted

Florida Solar Energy Center researchers wrote that air conditioning alone cannot keep indoor humidity below 60 percent in every hour of the year. They point to low cooling-load periods, when the air conditioner runs less. Without supplemental (extra) dehumidification, humidity can climb then.

For a humid climate with central air conditioning, ENERGY STAR says a certified whole-home dehumidifier may be the right choice. It points to relatively well sealed homes where humidity readings are consistently above 55 percent throughout the house. A 2002 Building America report monitored 20 homes in Houston, Texas. It evaluated six dehumidification and ventilation systems. In its energy-efficient houses with controlled mechanical ventilation (fan-driven fresh air), supplemental dehumidification was required to keep humidity below 60 percent throughout the year.

A Department of Energy case study monitored a retrofit home in Windermere, Florida, with air conditioning plus a whole-house dehumidifier, for six months. The study reports that the system held moisture below 60 percent relative humidity for 99.86 percent of the monitoring period. It says a combined system can give better comfort than air conditioning alone. The combined system was set to 55 percent humidity and 78°F, and the air conditioning alone to 75°F. The gain is greatest in spring and fall, when cooling may not be needed but dehumidification is. That is one home, so treat it as an example, not a rule. Our reading: the pattern in these sources is a tight or lightly cooled house. There, the air conditioner has little reason to run while the air stays wet.

When you may not need one

The same 2002 report had three reference houses. They met only code minimums for energy efficiency and had no mechanical ventilation. From those houses, the report concluded that dehumidification separate from cooling may not be necessary to keep humidity mostly below 60 percent. That holds in homes where all three of these are true:

• Clear windows and code-minimum insulation.
• Relatively low cooling set points, so the cooling system operates often. A set point is the temperature the thermostat is told to hold.
• Low occupant density and relatively little interior moisture compared with the size of the house.

The report warns that a house with acceptable humidity control in Houston may have unacceptable humidity control in central and south Florida. Our reading: this is a 2002 study of new Houston homes. It is a reason to measure your own house, not a rule for Tampa Bay. If your house does not meet all three conditions, or you cannot tell, measure it.

What whole-home means, and how a unit works

ENERGY STAR defines a whole-home dehumidifier by its ducting. The unit uses ducts to bring house air to its inlet. It also uses ducts to send dried air from its outlet to one or more places in the house. A portable unit works inside the space it dries, without added ducting. ENERGY STAR's definition allows an optional duct attachment.

ENERGY STAR's definition lists the parts inside a dehumidifier. They are a refrigerated surface that condenses moisture, a refrigeration system and a fan. The fourth part is a way to collect or dispose of the water, called condensate. A Department of Energy case study says an air conditioner and a whole-house dehumidifier use the same kind of refrigeration cycle. The difference is where the heat goes. The air conditioner rejects its condenser heat outdoors. The dehumidifier releases heated, dried air indoors.

ENERGY STAR notes that most whole-home units have only a drain. An HVAC professional routes it to a floor drain or condensate pump during installation. Our reading: a portable unit suits one damp room. A ducted unit suits a house that reads humid throughout. Follow the manufacturer's instructions for any portable unit.

How the unit connects to the ducts matters

Two Florida Solar Energy Center lab reports, from 2018 and 2019, tested how ducted dehumidifiers are connected. In a lab building, they compared three common connections:

• To and from the main body of the building.
• To and from the return side of the central air conditioner. That is the duct that carries house air back toward the cooling coil.
• To and from the supply side. That is the duct that carries cooled air out to the rooms.

They found that connecting to the return duct ahead of the cooling coil is not recommended. Some manufacturers show it as an option anyway. In that setup, the air conditioner's moisture removal decreased by 28 percent when both machines ran together. When the air conditioner cycled off, the dehumidifier also evaporated water back off the central cooling coil. The rate was 2 to 3 pounds per hour. The main-body and supply-side connections showed very little difference in predicted annual energy use. The 2018 report estimates that a unit and its ducts in an attic likely lose some capacity. A garage would have a smaller impact.

These were lab results from one test building. The cooling set point was 76°F and the humidity set point was 50 percent. The test used one unit rated at 70 pints per day. The 2019 report says that unit was about the smallest and most efficient ducted model on the market. On the return-duct connection, a larger or less efficient unit would have done worse. The report did not test other set points. It says higher set points generally lower the impact. Our reading: ask which connection is planned. Avoid the return duct ahead of the coil unless the contractor explains in writing why it suits your house.

How to read the pints-per-day number

Capacity is how much moisture a dehumidifier removes. It is measured in pints collected per 24 hours under specified conditions. Federal test rules rate a whole-home unit at 73°F. They rate a portable unit at 65°F. Both ratings use 60 percent relative humidity.

Our reading: a pints-per-day figure compares models under set lab conditions. It does not predict how much water a unit will remove in your house. Portable and whole-home ratings also use different temperatures. ENERGY STAR says the capacity you need depends on two things. They are the size of the space and the conditions in it without dehumidification. Florida Solar Energy Center researchers wrote that metrics to help professionals and consumers pick the right capacity are sorely missing. So this page gives no pints-per-square-foot rule. Ask the contractor to show how the size was chosen.

Where the sensor sits, and the set point

A humidistat is a device that lets you set the relative humidity you want. In the 2002 Houston study, one dehumidifier ran about 75 percent of the time. The cause was its humidistat, which sat inside the metal cabinet instead of in the living space. The researchers noted that the humidistats were sensitive to temperature as well as relative humidity. They called for more testing with the humidistat placed in the living space.

The National Renewable Energy Laboratory simulation compared set points and technologies. Whole-house energy use was much more sensitive to the humidity set point than to the technology chosen. The Florida Solar Energy Center reports add that dehumidifiers have the potential to use a lot of energy. Our reading: ask where the sensor will be. Ask what set point the contractor proposes, and how you can see and change it.

What to ask a contractor, in writing

These seven questions turn the points above into things a contractor can answer on paper.

1. What humidity readings, taken where and when, show that this house needs a dehumidifier? What should they read afterward?
2. Have the moisture sources, the duct leaks and the air conditioner's own settings been checked first?
3. Is the unit ducted? Where does it connect: to the main body of the house, to the supply side, or to the return before the coil?
4. Where will the unit sit: conditioned space, an attic or a garage? How will its ducts be insulated?
5. Where is the humidity sensor? What set point will you use, and how can I see or change it?
6. What is the unit's rated capacity, and at which test conditions was it rated? How did you choose that size for this house?
7. Where does the condensate drain go, and what happens if it clogs?

Ask for the answers in writing, in the proposal itself. A whole-home unit holds sealed refrigerant and runs on household electrical power. So leave the cabinet closed, and leave its connection, wiring and drain to a licensed professional.