How an Aprilaire humidifier adds moisture to your home
An Aprilaire humidifier pulls dry air from your home, passes it through a wet filter, and returns it to your heating system to distribute throughout your house. The water in the filter evaporates into the air as it passes through, raising the humidity level. The humidifier sits inside your furnace ductwork or attached to your furnace itself, so it works with your existing heating system rather than as a standalone unit.
The process is passive — no mist or steam is created. Instead, the humidifier relies on the warm air from your furnace to absorb moisture from the filter. When your furnace runs, it pulls air through the humidifier, and that air picks up humidity before being pushed through your ducts and into your rooms.
Key Takeaways
- Aprilaire humidifiers use a wet filter that moisture passes through, with no visible mist or steam produced.
- The humidifier connects to your furnace and uses your heating system's air circulation to distribute moisture throughout your home.
- A water panel or evaporator pad inside the unit absorbs water from a supply line and releases it into the airstream.
- A humidistat sensor monitors indoor humidity and signals the humidifier to turn on or off based on your target setting.
- Aprilaire models vary in size and installation method, but all work by evaporating water into warm air rather than creating spray or steam.
The water supply and evaporator pad
Water enters the humidifier through a small line connected to your home's water supply. This line feeds water to an evaporator pad — also called a water panel — which is the core of the unit. The pad is made of a porous material that absorbs and holds water, similar to a sponge.
As warm air from your furnace flows across this wet pad, the water evaporates into the airstream. The pad sits in the path of the air your furnace is already moving, so no extra fan or pump is needed to push the moisture into your home. The warmer the air and the drier your home, the faster the water evaporates from the pad.
How the humidistat controls moisture levels
A humidistat is a sensor that measures the humidity in your home and tells the humidifier when to work. You set your target humidity level — typically between 30 and 50 percent — on a dial or digital display on the humidifier or on your thermostat, depending on the model.
When humidity drops below your target, the humidistat signals the humidifier to allow water to flow to the evaporator pad. When humidity reaches your target level, the humidistat cuts off the water supply. This on-and-off cycle keeps your home at the humidity level you choose without overshooting.
Some Aprilaire models include a humidistat built into the unit itself. Others connect to your thermostat, so you control humidity from the same screen where you set temperature. Either way, the sensor prevents your home from becoming too humid, which can lead to mold or condensation on windows.
Installation location and ductwork connection
Aprilaire humidifiers are installed in one of two ways: either mounted directly to the side of your furnace or installed in the return ductwork — the ducts that pull air back into your furnace before it heats and circulates again.
The humidifier sits between the return air and the supply air. Dry air from your home enters the return ductwork, passes through the humidifier's evaporator pad, picks up moisture, and then continues into your furnace where it gets heated. The warm, now-humid air is then pushed through your supply ducts into your rooms.
Installation requires a water line to be run from your water supply to the humidifier and a drain line to be connected so excess water can flow away. A professional installer handles these connections during setup. The humidifier also needs a small electrical connection to power the humidistat and water valve.
Maintenance and filter replacement
The evaporator pad is the part that wears out and needs replacement. Depending on your water quality and how often the humidifier runs, the pad typically lasts one to two seasons. Hard water can cause mineral buildup on the pad, reducing how much moisture it can release, so pads in hard-water areas may need replacement more often.
Aprilaire sells replacement pads for each model. Changing the pad is a straightforward task — you turn off the water supply, remove the old pad, and slide in a new one. Some models have a bypass damper that closes when the pad is removed, preventing air from bypassing the humidifier when it is not in use.
The water supply line should also be checked periodically for leaks or mineral deposits. If water is not flowing to the pad at the expected rate, the supply line may be clogged and may need flushing or replacement.
Different Aprilaire models and their features
Aprilaire makes several humidifier models, ranging from basic units to those with digital controls and higher capacity. The Model 400 is a common entry-level option with a straightforward humidistat dial. The Model 500 and 600 series offer larger capacity and are designed for bigger homes or those with very dry climates.
Some models include a bypass damper that automatically closes when the humidifier is not running, preventing heated air from escaping through the humidifier when humidity is already adequate. Others have a manual damper that you adjust yourself. Digital models allow you to set humidity in smaller increments and may include features like automatic shutoff if the water supply runs dry.
All Aprilaire models work on the same basic principle — evaporating water into warm air — but capacity, control options, and installation method vary. Your furnace size and your home's square footage determine which model is right for you.
Why evaporation is better than mist or steam
Some humidifiers create visible mist or steam, which can leave mineral deposits on furniture and walls if your water is hard. Aprilaire's evaporative approach avoids this problem because the water fully dissolves into the air rather than being sprayed as droplets.
Evaporative humidifiers are also more energy-efficient than steam humidifiers, which heat water to create vapor. Aprilaire units rely on the heat your furnace is already producing, so they add little to your heating costs. The trade-off is that evaporative humidifiers work best when your furnace is running — during heating season — and cannot add moisture when your furnace is off in summer.
Frequently Asked Questions
Can I use an Aprilaire humidifier if I don't have a furnace?
No. Aprilaire humidifiers are designed to work with forced-air heating systems. If you have a boiler, radiant heat, or no central heating, you would need a standalone humidifier instead. A professional HVAC technician can tell you whether your system is compatible.
How much does it cost to run an Aprilaire humidifier?
Operating costs are low because the humidifier uses your furnace's existing fan and heat. The main cost is water consumption and occasional pad replacement. Water use varies by humidity level and how often your furnace runs, but most homeowners see minimal impact on their utility bills.
What humidity level should I set my Aprilaire to?
Most experts recommend keeping indoor humidity between 30 and 50 percent. In winter, 30 to 40 percent is often better to prevent condensation on windows. Your humidistat dial or digital display shows the current humidity and lets you adjust your target. Start at 40 percent and adjust based on comfort and any condensation you see.
What happens if the water supply line freezes?
If your water line runs through an unheated space, it can freeze in very cold weather. This stops water from reaching the pad and the humidifier stops working. Insulating the water line or running it through heated areas of your home prevents freezing. If it does freeze, the line will thaw once temperatures rise.
Do I need to drain my Aprilaire humidifier in summer?
You should turn off the water supply when you stop using your furnace for the season. This prevents water from sitting in the pad and supply line during months when the humidifier is not running. Draining the system is not always necessary, but shutting off the water supply is a good practice.