What is a humidistat and how is it used with air conditioning in Florida, including for seasonal residents?

What a humidistat is

One equipment maker defines a humidistat as an electronic device that monitors and controls the level of humidity in your home. It says the device compares the measured RH with a target level that you set.

When RH goes past that target, the same maker says, the humidistat turns a device on or off. Its examples include a dehumidifier or a fan.

Another maker describes a humidistat as a device that works with a home's heating and cooling system. It says the device does this to automatically adjust the amount of moisture in the air.

LCEC is the Lee County Electric Cooperative. Its vacant-home guide says humidistats are also known as de-humidistats.

How a humidistat works with your air conditioner

UF/IFAS, the University of Florida's extension service, says periodic air conditioner operation will remove moisture from your home. The Building America Solution Center, a U.S. Department of Energy resource, warns that in very humid climates the AC alone may not hold RH below 60 percent.

The same resource lists dehumidification controls that can be added to the heating and cooling system. They run the cooling longer at a slower fan speed, giving the coil more time to remove moisture.

LCEC says a humidistat can be wired either parallel to or in series with the thermostat. LCEC says each layout calls for slightly different set-up instructions. Its test uses only settings: humidistat off, mode on cool, fan on auto, thermostat lowered at least 10 degrees below room temperature. If the AC starts, the wiring is parallel; if not, it is series.

LCEC also notes that on new high-efficiency systems, the humidistat may be built into the thermostat. One thermostat maker's manual says its thermostat can control humidity using the cooling system or a whole-house dehumidifier.

That manual says the temperature may drop up to 3 degrees Fahrenheit below your setting until humidity reaches the desired level. It calls this over-cooling. If you do not want it, the manual says to turn dehumidification off in the dehumidification menu.

For why a running AC can still leave a house damp, see high humidity with the AC running.

Why seasonal homes are where this matters in Florida

FSEC's vacant-home study, reported in 2005, put snowbird residents at more than 900,000 in Florida during the winter. LCEC notes that many Southwest Florida homeowners live elsewhere during the summer. UF/IFAS says unoccupied homes are more open to mildew, storm and pest damage.

An FSEC news release calls 70 percent RH a critical level, above which mold growth is likely or probable. LCEC says mold and mildew can grow nearly anywhere if RH is consistently 70 percent or higher. EPA, the Environmental Protection Agency, says to keep indoor humidity below 60 percent if possible.

FSEC found that leaving a house with no AC, which it calls letting the building float, averaged about 70 percent RH in most homes. It called that marginal at best.

For mold inside the equipment itself, see mold in HVAC systems in Florida.

What the Florida research found about thermostat-only settings

FSEC monitored three homes in Brevard County, on Florida's east coast, in a study reported in 2005. The study was sponsored by Florida Power and Light. It tested five approaches for vacant homes, including a thermostat set at 85 or 83 degrees. FSEC found three of the methods effective, and said none stood out as substantially better within its limited sample of homes.

FSEC rated the 85-degree setting as not effective in most homes. On cooler, cloudier days the AC ran little or not at all, so little moisture came out. Even 83 degrees did not ensure good results, especially in the cooler months.

The UF/IFAS guide on closing a seasonal home says to leave the AC set no higher than 85 degrees, so it cycles whenever the home reaches that temperature.

Our reading: 85 degrees is the upper limit UF/IFAS names, not a setting FSEC found reliable.

Air infiltration is the rate at which outside air leaks into the house. FSEC found that it matters. Leakier homes needed more moisture removal to keep RH in check.

Settings the sources give for an empty home

LCEC lists three approaches. It says a humidistat set to cycle the AC whenever indoor humidity exceeds 65 percent uses the least power. It says a timer cycling the AC two hours every day gives the most security against mildew.

LCEC calls a plain 83-degree thermostat setting the least efficient method. It recommends that only for short or infrequent summer trips.

For a vacant home, LCEC's set-up card says to set the humidistat to 65 percent, the fan to auto and the mode to cool. With parallel wiring, it says to set the thermostat to the highest possible temperature. With series wiring, it says 80 degrees.

LCEC's set-up card treats occupied and vacant homes differently. For an occupied home, LCEC says to set a parallel-wired humidistat to off and a series-wired one to on, with the fan on auto.

FSEC's study report says running the cooling system in the early morning hours appears to work well in a majority of homes. An FSEC news release gives the example of running the AC at 74 degrees for two hours, such as 3 to 5 a.m. In homes with high air leakage, the FSEC report says run time may need to be four hours. FSEC says the fan should be on auto whenever the system runs in a vacant home.

FSEC also says that method assumes the AC still produces cold air. Its causes of warmer than normal supply air include improper refrigerant charge. Another is large return duct leakage.

To set a timed morning run, see how to program a thermostat.

Built-in humidistats, dehumidifiers and other controls

FSEC tested a dehumidifier with its built-in humidistat set to 62 percent. It reported erratic and undependable humidistat operation, and called built-in humidistats imprecise.

FSEC added that, used with a more reliable humidistat, setting a dehumidifier at 62 percent and letting it cycle can be quite effective. It found that running a dehumidifier at its lowest RH setting on a timer for set hours each day was effective and reliable. FSEC prefers a timer with battery backup, so it keeps the correct hour.

LCEC says any dehumidifier or moisture-absorbing product left in an empty home needs drainage, such as setting it in a bathtub, basin or sink. UF/IFAS says a dehumidifier with a drain can help, though not as much as air conditioning.

The thermostat maker's manual says it can read humidity at the thermostat or at wireless indoor sensors. It also describes an away mode, set up by the installing pro, that controls temperature and humidity while the home is vacant in the humid season.

UF/IFAS advises, if possible, having someone check your house occasionally for damage and security breaches.

For ducted units that serve the whole house, see our whole-home dehumidifier guide.