How to Maintain a Furnace-Mounted Whole-House Humidifier

Whole house furnace humidifier

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Project Overview
  • Working Time: 30 mins - 1 hr
  • Total Time: 1 hr
  • Skill Level: Beginner

In many parts of the country, excessively dry indoor air is an ongoing problem, especially in the winter when the air tends to be naturally dry. Maintaining proper humidity levels in your home through the use of a humidifier is essential for the protection of wood furniture and wood flooring. It is critical to make sure your humidifier works correctly and is properly maintained.

Increasing indoor humidity can be accomplished with portable humidifiers, or with a central furnace-mounted humidifier—a special piece of HVAC equipment fixed to the furnace that adds water vapor to the air discharged from your furnace. These humidifiers are used primarily in the winter heating season when humidity levels are low—dry winter air is what causes frequent static electricity discharge.

Newer tightly constructed homes have much less air infiltration than older homes. As a result, humidity levels are more easily maintained in these newer homes, reducing the need for a humidifier. 


Repairs and maintenance on gas appliances (including furnaces) are best left to professionals. Amateur repairs can lead to potentially dangerous situations, including gas leaks. If you perform any maintenance yourself, work with caution, and call a professional if anything unexpected occurs.

Types of Furnace-Mounted Humidifiers

There are three main types of central humidifiers you may find in the home: 

Reservoir humidifiers use a rotating drum-styled foam pad or similar media that picks up water for evaporation from a float-controlled reservoir. These types are the most prone to mold problems, due to the reservoir that is constantly full of standing water.

Flow-through drip-style humidifiers use a specially coated metal or plastic screen that has a small amount of water constantly dripping across it while the furnace is running. Since the water supply is constantly flowing, there is a lower likelihood of mold problems.

Steam humidifiers contain self-generating steam heaters and are the most expensive, lowest maintenance, and most efficient design. They are also the least prone to mold problems since there is no standing water supply—the steam is injected directly into the airflow. These humidifiers require that water has a certain range of hardness and quality. Some homes may require special filtration of the water in order to be suitable for a central steam humidifier.

Before You Begin

Turn off power to the furnace. This can be done with the electrical switch mounted on the furnace, or by turning off the circuit breaker that controls the furnace. Next, turn off the water supply tap valve to the humidifier. This is often a small saddle valve mounted on a cold water pipe near the furnace, or it may be a standard fixture shutoff valve.

What You'll Need

Equipment / Tools

  • Adjustable wrench
  • Screwdrivers
  • Chisel (if needed)


  • New evaporator pad (if needed)
  • Household vinegar
  • Multimeter
  • New water filter (if needed)


Reservoir/Drum-Style Humidifiers

The drum-style humidifier uses a float-controlled reservoir pan filled with water that saturates a rotating foam-covered evaporator drum pad. This style of humidifier provides air flow through the rotating evaporator pad, which in turn adds humidity to the air. The unit may have humidity controlled with a humidistat.

This type of humidifier is prone to housing stagnant water, which can become a site of mold or mildew growth. Owners of these units may want to consider purchasing specialty tablets made to discourage these growths. Additionally, the foam evaporator pads must be cleaned monthly and replaced annually, and they can quickly become hardened by sediment and minerals in the water, which limits absorption and effectiveness.

  • Usually the least expensive style

  • Maintenance is easy

  • Can be unreliable

  • Mold could be a problem

How to Maintain a Drum-Style Humidifier

Perform the following maintenance routine every month.

  1. Remove the Humidifier Cover

    Depending on your humidifier’s design, loosen and remove the nuts, or remove the clips securing the humidifier cover. Remove the humidifier cover and set it aside.

  2. Remove the Drum

    Remove the humidifier drum and check the pad. If it is hardened from mineral deposits, try to clean it using the approach described below. If that does not work, then the evaporator pad must be replaced.

  3. Clean the Pad

    Remove the foam pad from the drum shaft by removing the clips or other fastening mechanisms on the center shaft and pulling the two halves of the drum shaft apart.

    Clean the foam pad by using a 1:3 solution of water to vinegar, or use a commercial calcium-removing fluid such as Lime Away. Soak the foam pad until the deposits dissolve. Rinse the pad with clean water and repeat if necessary. If the pad is ripped or does not come fully clean, replace the foam pad.

  4. Reassemble the Drum

    Replace the cleaned or new pad onto the drum, then reinstall the drum into the humidifier housing. Check the owner’s manual for the proper depth if the unit does not have a marking in the reservoir showing the position for the drum. Turn on the water supply to the humidifier and allow the reservoir to fill until the water supply is turned off by the float assembly.

  5. Check the Water Level

    If the drum dips into the water reservoir and is fully wetted when you rotate the drum, the water level is correct. You can raise or lower the water level by adjusting the float arm. There is usually an adjusting screw that attaches the float to the float lever arm. Loosen the screw and raise the float to raise the water level, or lower the float to reduce the water level.

  6. Test the Operation

    Turn the power back on to the furnace. Wait one hour to allow the humidifier to run, then check the water level again and adjust as required. Check to make sure the water supply hose fitting is tight and does not leak; tighten with an adjustable wrench if required.

Flow-Through/Drip-Style Humidifiers

The flow-through drip-style humidifier uses water dripping onto a specially coated rectangular metal or plastic evaporator pad to add humidity to the air. These units control humidity with a humidistat. This type of humidifier requires less maintenance and stays cleaner than drum-style humidifiers; it is not prone to mold and mildew growth.

  • Competitively priced

  • Less maintenance and stays cleaner

  • Reliable operation

  • Consumes more water than other humidifier styles

  • Evaporative screen pads must be replaced annually

How to Maintain a Flow-Through/Drip-Style Humidifier

Perform the following maintenance routine each month.

  1. Remove the Manifold

    Pop open the clips that hold the inlet feed tube and distribution manifold in place at the top of the humidifier. The water inlet feed tube will separate from the distribution manifold. Remove the manifold from the evaporative screen pad and the manifold housing

  2. Clean the Parts

    The water distribution manifold may be plastic or metal, depending on your style of humidifier. If it is metal, you may have to clean mineral deposits away with a chisel or flat-blade screwdriver.

    Check the evaporator screen pad. If it has a scale, clean it using a 1:3 solution of water to vinegar, or use a commercial calcium-removing fluid such as Lime Away. Soak the pad until the deposits dissolve. Rinse the pad with clean water and repeat if necessary.

    If the evaporative pad has not had monthly maintenance, it may have significant mineral deposits that flake off as the pad is bumped or twisted. If that is the case, replace the evaporator pad with a new drip pad.

  3. Test the Humidistat

    Check to make sure that the humidistat is calling for humidity when the furnace is running. Using the inlet fill tube located over the humidifier housing, open the water supply to the humidifier and turn the power on to the furnace. When the furnace begins running, the humidistat should call for humidity, the solenoid valve should open, and water should come out of the water inlet tube and drain into the humidifier drain.

    If water does not flow, check the humidistat with a multimeter or voltmeter, checking for 24 volts across the fill valves, solenoid terminals, and wires. If there is no voltage, the humidistat is bad.

    If the humidistat is working properly and there is no water flow from the solenoid valve, then the valve needs to be replaced. This is probably a job for a service technician.


    You can do a “workaround” if you do not have a voltmeter. Turn the humidistat up to 80 percent. If the solenoid valve does not open and allows water to flow, turn the humidistat dial setting down to 20 percent, and then back up to 80 percent. You should hear a “click” sound coming from the solenoid valve. If you hear the clicking sound, the humidistat is working correctly.

  4. Reassemble the Humidifier

    Once all components have been confirmed as working, reassemble the humidifier with a cleaned or new evaporator pad.

Steam-Style Humidifier Maintenance

A steam-style humidifier adds humidity to the air by heating water electrically until it boils to create humidity in the form of steam, even if the furnace isn’t on. The steam is picked up by the system blower then pushed throughout the home vents. These humidifiers can add moisture to air even when the furnace isn't running and work well with heat-pump heating systems.

While annual maintenance tasks are similar for all brands of the humidifier, the exact maintenance routines vary from one manufacturer to another. 

  • Unlikely to breed mildew or mold

  • Controls humidity more precisely

  • Less maintenance

  • Requires a dedicated electrical circuit to boil water

  • The most expensive type of central humidifier

How to Maintain a Steam-Style Humidifier

At least once per year, follow this maintenance procedure to check for scale buildup and eliminate it.

  1. Turn off the Humidifier

    Turn down the humidistat to make sure the system does not enter a heat cycle while you are servicing it. 

    Drain the system of water, following the instructions of your particular humidifier model. With some types, this involves shutting the power off then turning it back on while the humidistat is set to a low setting.

  2. Remove the Overflow Pan

    Wait for the unit to cool, then remove the lid on the humidifier, following the manufacturer's directions. Remove the overflow pan from the unit, loosening the mounting screw if necessary.

  3. Check the Drain Tube

    Remove the main drain tube and check to make sure that it is clean with no deposits clogging it. 

  4. Clean the Parts

    Remove the overflow tank (there may be a small amount of water in it) and clean any lime deposits you find, using a 1:3 solution of water to vinegar or a commercial calcium-removing fluid. Clean the scale from the metal water discharge and water supply tubes.

  5. Reassemble the Humidifier

    Reassemble the tank, taking care to install any O-rings correctly. If the service interval calls for it, replace the water filter, as per the manufacturer's instructions. 

    When finished, turn the power back on and reset the humidistat to the desired level.

Article Sources
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  2. Use and Care of Home Humidifiers. United States Environmental Protection Agency.

  3. Central Humidifiers. International Association of Certified Home Inspectors.

  4. Evidence Brief: Humidifiers in Healthcare. Public Health Ontario.