It’s Not a Dry Heat: How to Install a Humidistat

Depending on where you live, you may find yourself sweating even though the HVAC is on or experiencing dry nose irritation due to the lack of moisture in the air. Both conditions are caused by home humidity, or the amount of moisture in the air.

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Although the HVAC thermostat controls the temperature in your home, it can't control humidity. This is where a humidistat comes in. It measures the moisture level in the air and sends a signal to the HVAC, dehumidifier or humidifier to automatically adjust the amount of moisture in the air.

Do I Really Need a Humidistat?

Humidity is the amount of water vapor in the air. It's measured as the relative humidity (RH), which represents the amount of moisture in the air compared to the maximum amount of moisture the air can hold. In Florida, the maximum RH is typically between 85% and 90%, while in Arizona, the air is much drier, so the RH may be as low as 20%.

According to the EPA, a comfortable humidity level is between 30% and 50%. If the RH is below 30%, the air is too dry and feels uncomfortable. Your skin and nose can become itchy and easily irritated. If the humidity is too high, you sweat easily, and it encourages mold, condensation and mildew.

With a humidistat, in conjunction with a humidifier or dehumidifier, you can measure and control the amount of humidity in the home by adjusting the humidistat setting. Humidity control allows you to adjust the humidity to a comfortable level.

What Does a Humidistat Connect To?

A humidistat on its own only measures the humidity in the air. To control the humidity level in your home, you connect the humidistat to the HVAC system or a humidifier or dehumidifier.

Dehumidifier

Dehumidifiers remove moisture from the air. They're usually freestanding, but some types can be installed inside your return air ducts or with their own ducts.

There are two basic types of dehumidifiers: one that uses the refrigeration cycle to remove moisture and one that uses a desiccant. The refrigeration type is like a small air conditioner that cools the air to a low temperature so the excess moisture in the air condenses and is removed. A desiccant dryer is filled with a water-absorbing substance that dries the air.

Humidifier

A humidifier adds moisture to the air. It may be freestanding or fitted to your central HVAC system. Humidifiers have a water supply and a mechanism for spraying water onto a pad or filter.

There are two types of humidifiers. A bypass humidifier draws pressurized air from the air circulation fan on your HVAC system. It feeds this air through the humidifier and back into the low-pressure return air side of the ducting before the fan. By contrast, pressurized humidifiers incorporate a fan that blows moist air into the HVAC ducting and doesn't need additional ducting.

Direct to HVAC

Your air conditioner also removes moisture from the air. In some climate zones, this is sufficient to reduce the humidity in the home to a comfortable level. In this case, the humidistat works in conjunction with the thermostat to control the humidity.

Where Should a Humidistat Control Be Installed?

A humidistat has two components: a sensor and a controller. Depending on the model, these may be separate or combined into one unit. The correct placement of the humidistat sensor is crucial. To control the indoor air humidity in one room, place the sensor on an interior wall away from windows or doors.

For whole home humidity control, the best place for the sensor is in the return air duct after your air circulating fan. Some models use an outdoor thermometer mounted on an outside wall of the home to more accurately measure humidity.

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Step-by-Step Guide on How to Install a Humidistat

Here is a step-by-step guide for installing an AprilAire humidistat. Other makes may differ slightly. Always take precautions when working on electric appliances, and disconnect the power before proceeding. Unless you're qualified, installing a humidistat is best left to a professional.

• Mount the humidifier. Using the template provided, cut a hole in the supply air plenum above the AC coil. Fit the humidifier and seal around the edges using sealing tape or caulk.

• Run the water supply. Run a water supply line to the humidifier.

• Mount the humidistat. Drill a hole in the return air duct before the air filter for the humidistat sensor. Fit the humidistat backplate and sensor.

• Connect the wiring: Connect the humidistat to the HVAC and humidifier.

• Connect the humidifier. Connect the two control wires from the humidifier to the HH terminals on the humidistat in series with a 24-volt transformer.

• Connect the power supply. Connect the HVAC 24-volt AC output to the humidistat using terminals R and C.

• Connect the heat signal. Connect the heat signal from the HVAC unit to terminal W.

• Outdoor thermometer. Connect the outdoor thermometer sensor to the OUT terminals.

• Connect the fan running signal. Connect the wire from the thermostat fan controller to terminal G.

• Fan start signal. Connect the Gf terminal to instruct the HVAC fan to start.

• Connect the AC supply to the humidifier. Connect the 120-volt AC supply cable to the humidifier.

Let the humidifier or dehumidifier unit run for a few hours, and use a portable hygrometer to check that it's controlling correctly.

If you have difficulty controlling the humidity in your home, installing a humidistat together with a humidifier or dehumidifier in your HVAC system will do the trick.