How to Replace the Evaporator Pad on a Furnace Humidifier

A home humidifier attached to the return duct with a bypass connection to the supply hot air duct.
Marvin Samuel Tolentino Pineda / Getty Images
Project Overview
  • Total Time: 30 mins - 1 hr
  • Skill Level: Beginner
  • Estimated Cost: $10 to $30

Although not all forced-air furnaces are equipped with them, a central humidifier mounted on the furnace can be a very important piece of equipment in certain climates. The central humidifier works to maintain correct humidity levels in the home, which can be especially important in perennially dry climates or in climates with intensely cold, dry winter air. Proper humidity levels are important for protecting wood furniture and flooring, and it also helps to reduce skin problems and breathing-related issues.

Types of Humidifiers

Whole-house, central humidifiers come in several types, including:

  • Reservoir (drum) humidifiers: These use a fairly simple design, operating on the basic physics of evaporation. Moving air in the system's forced-air ductwork is diverted through a chamber containing the humidifier unit. This design works by means of an absorbent pad attached to a rotating drum that picks up water from a reservoir pan. The moving air simply absorbs evaporating moisture and distributes it through the house to raise the humidity level. The water in these humidifiers can grow stagnant if the unit is not running constantly, so problems with mold and bacteria sometimes occur with this type of humidifier.
  • Flow-through drip-style humidifiers: This type also works through the process of evaporation, but here the water source flows or drips constantly across an absorbent pad rather than sitting in a reservoir. These humidifiers are less prone to mold and bacterial problems since the water never sits stagnant; however, they are slightly more expensive than the reservoir-type humidifier.
  • Steam humidifiers: This style puts active steam into the air by superheating it into water vapor. This type is a good choice in very dry climates where the other humidifier designs struggle to sufficiently increase humidity levels. But these units are quite expensive, adding $1,600 or more to the cost of a forced-air system.

In modern furnaces, the most common type of central humidifier is the flow-through design, which offers a good compromise between affordability and lower health risks. A flow-through or drip-style humidifier uses a media element to transfer water vapor into the supply air of the furnace. This absorbent material is known as the evaporator pad, evaporator panel, drip pad, or water wick. Whatever it is called, the evaporator pad is a specially-coated screen that has water constantly dripping across it in small amounts while the furnace operates.

Evaporator pads should be replaced annually and inspected monthly. The steps may vary a little depending on your manufacturer and model but will generally follow the process outlined here, based on a GeneralAire Model 1099.

Humidifier pads

What You'll Need

Equipment / Tools

  • Adjustable wrench
  • Rubber gloves
  • Sponge


  • New evaporator pad
  • Commerical lime dissolver or household vinegar
  • Spray-type mold and mildew cleaner


Replacing a Humidifier Evaporator Pad

  1. Turn Off the Humidifier

    There is generally no reason to shut off the furnace to complete this project. Just set the humidistat to 0 percent humidity, which turns the humidifier off. Then proceed to remove and inspect the evaporator pad.

  2. Remove the Top Retaining Side Clips

    The first step is to remove the top cover, which conceals the evaporator pad. For some units, this will require removing the side retaining clips holding the top cover of the cabinet assembly to the bottom cabinet housing. With other units, you will need to remove nuts holding the water distributor trough cover to the top cabinet cover (but you do not have to remove these nuts if you are not going to separate the trough cover from the top cover).

    Be careful not to lose these small parts; make sure to put them in a safe place.

    Removing the top fasteners
  3. Disconnect the Water Distributor Tube

    Use an adjustable wrench to loosen the nut securing the water distributor tube (the water inlet feed tube) to the solenoid/water inlet valve assembly, then swing the tube out of the way of the cover.

    Water distributor
  4. Remove the Top Cover

    With the retaining clips removed and the water distributor tube out of the way, lift and separate the top cover from the bottom cabinet housing. Then, lift the evaporator pad out of the bottom cabinet housing.

    Top of humidifier
  5. Inspect and Clean the Pad (Optional)

    If the pad appears to be in relatively good shape, it may require only a cleaning rather than a full replacement. In that case, you can use a commercial cleaner such as Lime-Away or CLR. Follow the manufacturer's instructions for dilution strength when cleaning the pad. Soak the pad until the deposits dissolve. Rinse the pad with clean water and repeat if necessary.

    Soaking the pad in a 1:3 solution of household white vinegar and water can also remove calcification from the pad.

    cleaning evaporator pad
  6. Prepare the New Evaporator Pad

    If the old evaporator pad is clearly clogged and calcified, it will need to be replaced. This will be clear if you visually compare it to a new pad. If a replacement is warranted, remove the new evaporator pad from its wrapping and set it aside. Don't install until you clean the interior of the humidifier cabinet.

    New evaporator pad
  7. Clean the Humidifier Cabinet

    Clean the interior of the cabinet housing with a commercial mold and mildew spray, then clean the sides, bottom, and underside of the top cover.

    interior of cabinet housing
  8. Insert the New Evaporator Pad

    Insert the new humidifier evaporator pad into the lower cabinet assembly. It will slide down the guide tracks on the side of the housing to ensure a proper position.

    inserting evaporator pad
  9. Replace the Humidifier Cover

    Carefully place the top cover in place over the evaporator pad. Just as the side cabinet had guide rails, the top cover will have its own guides to properly position the top of the pad. The top will be properly fitted when the sides of the top are flush with the cabinet and the top is setting squarely.

    Insert the retaining clips to hold the top and bottom cabinet assembly together, then reconnect the water distributor tube to the top cover.

    replacing humidifier cover
  10. Tighten and Check the Connections

    With the water distributor tube in place, slightly tighten the nut holding the tube to the solenoid. Turn the humidistat to 35 percent (to turn it back on) or whatever your desired humidity level is. Check for leaks at the water distributor tube connection and gently tighten if any leaking is found. Finally, check for proper drainage in the condensate drain.

    Tightening and checking connections
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