How to Troubleshoot Humidistats

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A characteristic that affects our surrounding environment is its humidity content or relative humidity (RH). During a heavy storm, for instance, when the rain is just pouring down, the RH of the air outside can quickly reach its dewpoint or 100% RH.

It can even surpass the dewpoint or pass 100% RH, where it is said to be supersaturated. But as air can absorb moisture, so do hygroscopic objects, having an affinity for moisture.

Through hygroscopy, a difference in humidity level between a hygroscopic body (able to absorb moisture) and its surrounding environment creates a differential force that will transfer the excess moisture to whichever is drier.

Therefore, when the humidity level is high, the moisture will tend to saturate your clothing, leaving chilled water molecules against the skin, leaving you to feel colder—The surrounding air always feels colder when the air cools down, and the humidity goes up.

The clothing you wear provides you with layers of trapped air containing water molecules. Since it takes more body heat to warm water than to warm air, the damp surrounding air is absorbed by your layers of clothing by affinity, thus absorbing more body heat energy trying to reach a temperature equilibrium between the air and the body, leaving you feeling chilled.

A comfortable RH level inside a house or a building for optimal well-being is considered to be between 30% to 50%—anything above 50% RH could promote bacteria growth, and in colder climates, an RH above 40% could cause condensation on the windows.

As damp air can cause wood floor boards to cup, and windows and doors to jam or stick, dry air can cause drywall, wood furniture, and leather to crack.

On the other hand, low indoor humidity below 30% RH could be detrimental to you and to your family’s comfort and also healthwise.

Air that is too dry will accentuate health issues such as flaky and dry skin, chapped lips, eczema, asthma, bronchitis, and of course, allergies—these last effects from pulmonary dysfunctions caused by the dryness that dried up mold formations, mildew, and dust allergens causing them to become airborne and breathed in.

Temperature Levels vs Humidity Levels

We all know that the temperature of the air is quite easily measured with a thermometer. But how do you measure the relative humidity of the air?

This is where an instrument called a “hygrometer” comes in—a very simple instrument designed to measure two very different temperature readings of the same surrounding environment simultaneously.

The hygrometer, in its simplest form, has a “dry” thermometer that measures the surrounding air as it is, and a “wet” thermometer with its bulb (at the bottom) wrapped up inside a water-saturated cloth that measures the same air as it is saturated with moisture at 100% RH.

The cloth called the “wick” is an absorbent sleeve covering up the bulb of the wet thermometer as it is dipped into a glass container filled with water.

The two thermometers are secured side by side on a plaque with the glass container underneath with an opening directly under the wet thermometer from which the wick is attached at one end while the other end is conveniently placed inside the water-filled glass container saturating the full-length of it right up to the bulb.

This gives you a wet reading that is lower than the dry thermometer reading. The differential is used to calculate the actual level of relative humidity or %RH.

Nowadays, however, electronic sensors have replaced thermometers and wet cloths with electronic hygrometers. They’re designed to measure either the “capacitance” or the “resistance” of the air and come up with a % RH from its calculations.

The capacitance is basically the ability of the air gap between two metal plates to store a static electrical charge proportional to the amount of moisture between the plates giving the capacitive sensor a value to calculate the %RH.

A resistive sensor, on the other hand, measures the current flowing through a ceramic body exposed to the air, as the moisture in the air directly affects the resistance of the sensor. The amount of current that flows through the ceramic can provide an accurate measurement of the air’s relative humidity.

The Humidistat (Also Known as Hygrostat)

The humidistat or hygrostat is a device equipped with an electronic hygrometer that can use either the capacitive or the resistive reading from the humidity level to generate a signal that is sent to a control board within a humidifier or dehumidifier.

It will then power up the unit, which will process the moisture and maintain the RH percentage at the level the humidistat is set at.

The humidistat for a home central heating and cooling unit is usually installed on the wall at a critical location in the house while portable humidifiers and dehumidifiers are equipped with a wired-in humidistat that can be set from the unit’s control panel.

“Preset” humidistats are also used in several appliances, such as refrigerators and microwave ovens (with smart cooking) at a manufacturer’s fixed setting.

There are also some other extensive uses for humidistat, such as for controlling the moisture inside greenhouses and also for climate-controlled warehouses. So to sum it up, the hygrometer is a measuring instrument, and the hygrostat or humidistat is the switch.

Understanding Hygrometric Equipment

The first component in your humidifier and your dehumidifier that’s affected by the RH or the amount of moisture in the air is the moisture sensor (or hygrometric sensor) that picks up the water vapor present in the surrounding air and passes it between the capacitive plates or through a ceramic component to create an electrical current that is then fed to the control board or module integrated within the humidistat of the unit.

The signal is then amplified and used to activate a power relay should the % RH value of the air become less than the humidistat setting for a humidifier or exceed the humidistat setting for a dehumidifier.


In the humidifier, the now closed relay would simply switch on the circulation fan motor and the rotating drum motor to initiate water evaporation through the air.


For the dehumidifier, the closed relay would start up the circulating fan and the compressor to begin condensation and removal of water droplets from the air.

Troubleshooting Humidistats

Although unlikely, it is possible for the hygroscopic sensor to go bad, but also for other components of the unit. The following steps will guide you through the proper process to determine exactly where the problem lies in your unit.

Step 1

Plug the unit in and switch it on.

Step 2

Make sure that the pilot light on the control panel of the unit is on. If it is on, go to Step 5.

Step 3

If it doesn’t light up or the pilot stays off, check your outlet with an outlet tester or try plugging it into a different outlet. You should also check the circuit breaker panel for a tripped breaker and reset it.

Step 4

If you have power at the outlet, check for a fuse on the unit to see if it is burnt out by testing for continuity with a multimeter. Replace the fuse if it is burnt out.

Step 5

With the power on, if your unit doesn’t come on, adjust the integrated humidistat for the maximum humidity setting for a humidifier or for a minimum setting for a dehumidifier. If it still doesn’t come on, unplug the unit from its outlet.

Step 6

Get access to the float switch and unplug both wires from their connectors. Short the two wires together to close the circuit and plug the unit back in. If it still doesn’t come on, unplug the unit from its outlet. Leave the float switch wires together.

Step 7

Remove the necessary covers from the unit to gain access to the humidistat controller.

Safety Warning! - Do not attempt to troubleshoot with the unit plugged in if you’re not a certified technician or an expert DIYer and are used to doing it. Wear insulated gloves while taking electrical readings.

Step 8

With your multimeter set for at least 120 AC volts, plug the humidifier back in, touch the ground probe against the grounded metal chassis and the red probe on each of the two connections with the humidity set on maximum for the humidifier and on the minimum setting for the dehumidifier.

Either one of the readings should be around 120 VAC. If one of the connections has no voltage present, you can assume that the humidistat is defective and replace it with a new one.

Step 9

For a humidifier, if the readings are good, replace the fan motor. For a portable dehumidifier, proceed to Step 10.

Step 10

With voltage present at the humidistat, if the compressor doesn’t run at all, it will likely be caused by a faulty compressor itself and it should be brought to a certified repair shop for confirmation and replacement since this will mean removing the refrigeration gas and unsoldering the copper connections tying it to the evaporator and the condenser coils.

Step 11

If, however, the compressor motor can be heard running, but no refrigeration is happening on the condenser, it leaves you with two possibilities—a refrigerant leak or a defective “time-delay relay.”

This relay lets the gas warm up for a few minutes before engaging the compressor, which can be easily heard when it kicks in. Test the time-delay relay by shorting its two leads across with a jumper wire for just a few seconds, and you should hear the compressor coming on.

Step 12

Plug the two wires removed from the float switch in Step 6 back into their respective terminals.

This is as far as it can be tested by a DIYer without proper testing equipment and training, and you should take it to a certified refrigeration repair shop.

Furnace and HVAC Humidistats

Most homes are usually heated with a furnace or a central air system (HVAC), circulating heated or cooled air through your ductwork to provide your entire home with comfortable, warm conditions.

But since the warm air flow and the weather conditions tend to vary the RH levels, it is especially important to inject or remove the moisture inside your home in order to maintain a comfortable balance between heat and RH.

Although it may not apply to all central air systems, a lot of them are equipped with a central humidifier. The temperature and the relative humidity of the home can then be controlled from a wall-mounted combined climate control.

The wall control is set at a specific temperature and at a comfortable RH at which the heating unit or the air conditioner is set when either setting reaches its mark.

While the thermostat measures and controls the temperature inside the home, the humidistat does the same but for its relative humidity. If, for instance, the temperature is low and the RH is at a low concentration, the humidistat will trigger the humidifier to inject moisture into the air as long as the circulating fan is running.

With the same low temperature and an elevated RH level, the condensation coil of the air conditioner will remove the moisture from the circulating air.

When the air gets warmer in the house, the thermostat triggers the air conditioning unit, removing moisture at the same time when the RH is high or starting up the humidifying unit if the RH is low.

Since it wouldn’t be very practical to fill the water pan of a humidifier unit every time it runs out, the ductwork is usually complemented with one of 3 different types of humidifiers installed within your heating or cooling system.

Steam Humidifiers

The “steam humidifier” produces a water mist and therefore delivers the most moisture throughout the home. They are usually the easiest to maintain.

Flow Humidifiers

The “flow through humidifier” exposes a humidifier pad soaked with water to the warm air passing through the air duct and evaporating the water picked up by the airflow. They are also low maintenance.

Drum Humidifiers

The “drum humidifier” has a pan inside the air duct that is filled from a water line to a pre-determined level, controlled by a float switch.

A drum-shaped cylinder wrapped with an absorbent foam sleeve is set up so that the bottom section is submerged in the water inside the pan, and when the humidistat activates the motor, it slowly rotates the drum bringing the soaked area up inside the air duct where the air flow picks up the moisture and distribute it around the house.

This type of humidifier does, however, require more care and regular cleaning as the standing water inside the pan could become an issue for mold formation.

Possible Malfunctions

The humidistat section of the climate control will therefore send a signal to the control board, which will either activate a “solenoid valve” that can inject a moisture spray or power up a motor to turn the humidifier drum.

So if the humidifier doesn’t operate properly after finding that the humidistat is fully operational, you’re left with 5 possibilities:

  • Disabled water feed
  • Faulty solenoid valve
  • Faulty drum motor
  • Faulty humidistat or climate control
  • Faulty control board

The next section describes the procedure to follow for isolating the actual cause of a faulty humidifier unit.

Troubleshooting a Flow-through Humidifier and a Drum Humidifier

Step 1

Make sure that the water line delivers its water into the pan by pushing down on the float to open the valve.

Step 2

If you don’t get water flow, check along the water line and make sure all the valves are open.

Troubleshooting a Steam Humidifier

Step 1

Turn off the humidistat. Verify if the injector’s nozzle tip is not plugged with some dirt or debris by unscrewing and removing it. Have someone turn the humidistat to its maximum setting briefly until water starts coming through.

If you get water coming through, replace the spray nozzle with a new one. If you don’t get water from the nozzle, screw the nozzle tip back on and check the solenoid valve.

Step 2

If you follow the water line back from the spray nozzle, you will get to the solenoid valve, which receives a 24 volts electrical signal from the control board to open and send water to the spray nozzle whenever the humidistat goes on.

The solenoid valve is an electro-mechanical device that is connected to a water line and opens the water flow when voltage is applied to an electromagnet and pulls the shut-off trap in its open position.

Remove the water line from the injector’s side of the valve and have someone briefly turn the humidistat to its maximum to get water flowing.

Step 3

If you don’t get any water flow, remove the cover from the solenoid to expose the electrical connections, and with the humidistat turned on maximum, use a multimeter to check if you get 24 volts across the electromagnet.

If the meter registers 24 volts, the problem is either a stuck valve in the closed position or a burnt-out coil. Either way, you’ll need to replace the solenoid valve.

Step 4

If you don’t get any voltage at the solenoid valve with the humidistat on its maximum setting, you’ll need to open up the humidistat and disconnect the wires going to the control board.

With your multimeter set on continuity, place the probes on the humidifier’s terminals and turn the humidity level to its maximum setting. If you can’t get any reading, you’ll have to replace the humidistat or the climate control.

Step 5

If you did get continuity at Step 4, it’s likely that you have a defective control board and need to replace it with a new one.

For more information on this subject or for other closely related articles, check out our pieces on "Best Home Humidity Levels For Air Conditioning", "2 Factors That Affect The Humidity Level in a House", and "How Do I Test My Home Humidity Level."