What You Need to Know Before Starting
An Aprilaire humidifier connects directly to your home's furnace and ductwork, so installation involves more than unpacking and plugging in. The unit mounts on or near your furnace, ties into the return air duct, and runs a water line to a drain. Most homeowners can handle this in two to four hours if they have basic tools and are comfortable working near their furnace. If your furnace is in a tight space, or if you're unsure about cutting into ductwork, hiring an HVAC technician is a reasonable choice—expect to pay $300 to $600 for professional installation.
Before you begin, check which Aprilaire model you own. The installation steps differ slightly between the Model 400, 500, 600, and 700 series. You'll find the model number on the product box or on a label on the unit itself. read the installation manual from Aprilaire's website or have it on hand—it includes a parts diagram and wiring schematic specific to your model.
Key Takeaways
- Turn off your furnace at the breaker before you start any work, and do not restore power until all connections are complete and the unit is mounted.
- The humidifier mounts on the return air duct, which pulls air back into your furnace; cutting into the wrong duct will prevent the unit from working.
- A drain line must run to a floor drain, laundry sink, or condensate pump, because water will collect inside the unit during operation.
- The power wire connects to a 24-volt transformer on your furnace, and the humidistat (wall control) wires into your thermostat's humidification terminals.
- After installation, run the furnace for 15 minutes with the humidistat set to 50 percent to check for leaks before leaving the system unattended.
Gather Tools and Materials
You will need a power drill, a drywall saw or reciprocating saw, a tape measure, a level, a screwdriver set (Phillips and flathead), an adjustable wrench, and a flashlight. Have a bucket or towels nearby in case water spills during the drain connection.
Check your Aprilaire box for the humidifier unit, mounting brackets, a drain pan (if included), a humidistat, and wire connectors. You will also need to buy a few items separately: flexible ductwork or sheet metal to connect the humidifier to the return duct (usually 6-inch diameter), a drain line kit (typically vinyl tubing and a floor drain adapter), and a 24-volt humidifier transformer if your furnace does not already have one. Most hardware stores carry these items, or you can order them online.
Turn Off Power and Locate the Return Air Duct
Switch off your furnace at the circuit breaker. Do not rely on the thermostat alone—you need the breaker off so no power reaches the unit while you work. Leave the breaker off until you have finished all wiring and mounting.
The return air duct is the large duct that pulls air from your home back into the furnace. It is usually the biggest duct in your furnace area and often has a filter rack or air filter inside it. Trace it from the furnace upward or outward; it may run along the ceiling or through a wall. The supply ducts (which push heated air out) are smaller and branch off in multiple directions. If you are unsure, look at your furnace manual—it will show a diagram of the return and supply sides.
Once you have found the return duct, measure its diameter (usually 16 to 20 inches wide and tall for a rectangular duct, or 6 to 8 inches for a round duct). Mark the spot where you will cut the opening—typically 12 to 18 inches away from the furnace itself, on a straight section of duct. Use a tape measure and pencil to outline the opening size specified in your Aprilaire manual (usually 6 inches round or a 6-by-6-inch square).
Cut the Duct Opening and Mount the Humidifier
Put on safety glasses and a dust mask. Use a drywall saw or reciprocating saw to cut the opening you marked. Work slowly and support the cut-out piece so it does not fall into the duct. Vacuum or wipe away any dust or debris from inside the duct.
Slide the humidifier's inlet collar or adapter into the opening and find it with the self-tapping screws provided. The collar should fit snugly and sit flush against the duct. If your duct is flexible, you may need to insert a support ring first to keep the duct from collapsing around the opening.
Mount the humidifier unit itself on the furnace or on a nearby wall bracket, depending on your model. Use the mounting brackets and hardware included in your kit. The unit should be level and stable—use a level to check. Position it so the drain outlet points downward and the water line can reach a drain without kinking or running uphill.
Connect the Drain Line and Water Supply
Locate the drain outlet on the bottom of the humidifier. Attach the vinyl drain tubing (usually 1/2 inch diameter) to this outlet using the connector provided. Run the drain line to a floor drain, laundry sink, or condensate pump. The line should slope gently downward so water flows by gravity—do not let it sag or loop upward, or water will back up into the unit. If you do not have a floor drain nearby, a condensate pump (a small electric pump that sits in a pan) can push water to a sink or outside.
Some Aprilaire models include a bypass humidistat that controls water flow automatically. If yours does, connect the water line from your furnace humidifier water supply line (or install a saddle valve on the cold water line near the furnace) to the humidifier inlet. The water line should be 1/4 inch copper or vinyl tubing. Do not overtighten fittings—hand-tight plus a quarter turn with a wrench is usually enough.
Wire the Humidifier to Your Furnace and Thermostat
Locate the 24-volt transformer on your furnace. It is a small box, usually mounted on the side of the furnace, with two or more wire terminals. If your furnace does not have a humidifier transformer, you will need to install one—this is a good time to call an HVAC technician if you are not comfortable with electrical work.
Run the humidifier's power wire from the unit to the transformer. Strip about 1/2 inch of insulation from the wire ends and connect them to the transformer terminals marked for humidifier power (usually labeled "HUM" or "W2"). Use wire connectors or terminal screws to find the connections. Do not mix up the wires—check your manual for the correct terminals.
Next, run the humidistat wire from the wall-mounted humidistat (the control you will use to set humidity level) to your thermostat. This wire is usually thin and comes in a two-wire or three-wire bundle. Connect it to the humidification terminals on your thermostat—these are separate from the heating and cooling terminals. If your thermostat does not have humidification terminals, you may need to upgrade to a compatible model. Again, if you are unsure about thermostat wiring, a technician can do this step in minutes.
Test the System Before Regular Use
Once all connections are complete and the unit is mounted, restore power at the breaker. Set your thermostat to heat mode and turn on the furnace. Set the humidistat to 50 percent humidity.
Let the furnace run for 15 minutes and watch for leaks around the drain connection and the duct collar. Check that air is flowing out of the humidifier outlet into the return duct (you should feel a slight breeze if you hold your hand near the outlet). Listen for any unusual noises or rattling.
After 15 minutes, turn off the furnace and inspect the drain pan or floor beneath the unit for water. A small amount of condensation is normal, but a steady drip means the drain line is clogged or not sloped correctly. If you see water pooling, turn off the humidifier and fix the drain before running it again.
Frequently Asked Questions
Do I need a humidifier transformer if my furnace already has one?
Check your furnace manual or look at the transformer label. If it is rated for humidifier use and has available terminals, you can use the existing one. If it is only for the thermostat or if all terminals are in use, you will need a second transformer. Most furnaces built after 2010 have a humidifier-ready transformer, but older units often do not.
What if my furnace is in a tight crawlspace and I cannot reach the return duct?
If the space is too cramped to work safely, hire an HVAC technician. Forcing the installation in a tight space risks damaging the ductwork, the humidifier, or yourself. A professional can often find a better mounting location or run the ductwork differently to fit the space.
Can I connect the humidifier to the supply duct instead of the return duct?
No. The humidifier must connect to the return duct because it needs to pull air from your home, add moisture, and send it back to the furnace. Connecting to the supply duct will cause the unit to malfunction and may damage your furnace. Always double-check your duct identification before cutting.
How often do I need to clean or replace the humidifier filter?
Most Aprilaire models have a water panel or evaporator pad that should be replaced once or twice a year, depending on your water hardness and how often you run the humidifier. Check your manual for the replacement schedule. A clogged panel will reduce moisture output and may cause water to back up into the drain line.
Why is water dripping from my humidifier even though the furnace is off?
Water in the drain pan is normal after the furnace shuts down because moisture condenses as the unit cools. If water is actively dripping or pooling, the drain line may be clogged or kinked. Straighten the line and flush it with warm water. If the problem persists, the water panel may be saturated and need replacement.