Where to Find and Remove Your Current Filter

The filter in an Aprilaire whole-house humidifier sits inside a cartridge that clips or slides into the unit's water panel housing. To access it, first turn off the humidifier at the power switch or breaker—never work on a running unit. Then locate the water panel door on the front or side of the humidifier; it is usually a rectangular panel held by one or two clips or a small latch.

Open the door by unclipping or unlatching it, then gently pull the cartridge straight toward you. The cartridge is a plastic frame holding the filter pad inside. You will see the filter itself as a tan or white accordion-shaped pad. Do not force the cartridge—if it sticks, check that you have released all clips. Once the cartridge is out, set it aside in a sink or over a bucket, because water will drip from it.

Key Takeaways

  • Turn off power to the humidifier before opening the water panel door to prevent water damage and electrical hazard.
  • The filter cartridge slides or clips out of the water panel housing and should be replaced every heating season or when it appears brown or clogged.
  • Aprilaire filters come in different sizes depending on your model number, so check your manual or the old cartridge before ordering a replacement.
  • After inserting the new filter, run water through the system for 10 to 15 seconds to wet the pad before turning the humidifier back on.

Choosing the Right Replacement Filter

Aprilaire makes several filter sizes, and using the wrong one will not fit or will not seal properly. The most common models for whole-house systems are the Model 35 (4-inch by 8-inch pad), Model 45 (4-inch by 10-inch pad), and Model 550 (larger commercial size). Check your humidifier's model number, which is usually printed on a label on the unit itself or in your owner's manual.

If you cannot find the model number, look at the old cartridge you just removed—the filter size is often printed on the plastic frame. You can also measure the cartridge width and height to match it. Order the replacement cartridge from a heating supply store, online retailer, or directly from Aprilaire. The cartridge includes both the plastic frame and the filter pad, so you replace the entire assembly, not just the pad.

Installing the New Filter Cartridge

Before sliding the new cartridge into the housing, check that the cartridge frame is clean and the rubber gasket (the seal around the edges) is not cracked or dried out. If the gasket looks hard or damaged, the cartridge may not seal properly and water will bypass the filter. A damaged gasket usually means you need a new cartridge, but if the gasket is just dry, a light coat of silicone-based lubricant can help it reseal.

Slide the new cartridge straight into the water panel housing, aligning it with the guides on the sides. Push it in until it seats fully—you should feel it click or lock into place if your model uses clips. Close the water panel door and find any latches or clips. Do not force the cartridge or door; if something does not fit smoothly, stop and check that you have the correct size and that nothing is blocking the opening.

Priming the New Filter Before Use

A new filter pad is dry and will not work properly until it is wet. After installing the cartridge, turn the water supply to the humidifier back on (if you turned it off) and let water flow through the system for 10 to 15 seconds. You may see water drip from the cartridge or hear it running inside the housing—this is normal and means the pad is absorbing water.

After priming, turn the humidifier back on at the power switch or breaker. The unit will begin to evaporate water into your ductwork. If you notice no moisture output after 30 minutes, turn off the unit and check that the cartridge is fully seated and the water supply is on. A cartridge that is not fully wet or not fully inserted will not produce humidity.

When to Replace Your Filter

Most Aprilaire filters last one full heating season (roughly October through April in cold climates) before they need replacement. However, the timing depends on your water hardness and how often the humidifier runs. If your water is very hard or mineral-heavy, the filter will clog faster and may need changing mid-season.

Signs that your filter needs replacement include brown or tan discoloration on the pad, reduced humidity output even though the humidifier is running, or a musty smell coming from the vents. If the pad is visibly clogged or discolored, do not try to clean and reuse it—a clogged filter reduces airflow and humidity output. Replace it with a new cartridge. If you run your humidifier year-round, check the filter every three months and replace it as needed.

Common Mistakes to Avoid

The most common error is installing the cartridge upside down or backward. The filter pad should face the incoming water flow, and the plastic frame should seat evenly in the housing. If the cartridge does not slide in smoothly or feels crooked, remove it and reorient it. Forcing a misaligned cartridge can crack the frame or damage the housing.

Another mistake is turning the humidifier on before priming the filter. A dry pad will not evaporate water efficiently, and you may think the unit is broken when it is straightforward not wet yet. Always let water flow through for 10 to 15 seconds after installation. Finally, do not assume all Aprilaire cartridges are the same size—using a Model 35 when your unit needs a Model 45 will not fit, and a too-small cartridge will allow water to bypass the filter entirely.

Frequently Asked Questions

Can I clean and reuse my old filter instead of buying a new one?

No. Aprilaire filters are designed to be replaced, not cleaned. Washing a used pad can damage the fibers and reduce its ability to hold and evaporate water. A new cartridge costs between $20 and $50 depending on the model, and replacing it annually is more cost-effective than trying to extend the life of a worn pad.

What happens if I use the wrong size filter?

A filter that is too small will not seal properly, and water will flow around the edges instead of through the pad. This means the water does not evaporate into your home, and you will see little to no humidity output. Always check your model number before ordering a replacement to may support you get the correct size.

Do I need to turn off the water supply before changing the filter?

You do not have to, but it is safer to do so. Turning off the water supply prevents water from flowing into the housing while the cartridge is out, which reduces dripping and mess. If you leave the water on, open the water panel door slowly and be ready for water to drip into a bucket or sink.

How long does it take for the new filter to start working?

After you prime the filter with 10 to 15 seconds of water flow and turn the humidifier back on, it should begin producing humidity within 15 to 30 minutes. If you do not see humidity output after an hour, check that the cartridge is fully seated, the water supply is on, and the power is on at the breaker.

What should I do with the old filter after I remove it?

The old cartridge can be thrown in the trash. It is not recyclable in most programs because it is a composite of plastic and filter material. If you want to dispose of it responsibly, check with your local waste management facility to see if they accept it in regular trash or if there is a special program for HVAC filters in your area.