A whole house humidifier connects to your furnace and water line
A whole house humidifier installs directly onto your furnace's supply duct or plenum—the metal box where heated air leaves the furnace. Water runs to it from a nearby cold water line, and a small drain line carries away excess water. The unit sits between your furnace and the ductwork, so humidified air travels through your entire home instead of just one room.
Installation takes a few hours if you have basic tools and are comfortable working near your furnace. You will need to cut into ductwork, run water and drain lines, and connect the humidifier to your furnace's humidistat (the humidity sensor). If your furnace is in a tight space, or if you are unsure about cutting metal ducts or soldering copper lines, a heating technician should do this work.
Key Takeaways
- A whole house humidifier mounts on your furnace's supply plenum and requires connections to cold water, a drain line, and your furnace's power and humidistat wiring.
- You will need to cut an opening in the ductwork, install a bypass duct (on bypass models), and run water and drain lines from nearby plumbing.
- The humidifier connects to a humidistat that measures indoor humidity and turns the unit on and off to maintain your target level.
- Furnace location, ductwork access, and plumbing proximity all affect whether you can do this yourself or should hire a technician.
Types of whole house humidifiers and where they mount
The two main types are bypass humidifiers and fan-powered humidifiers. A bypass model uses the furnace blower to pull air through a water-soaked pad, so it only adds humidity when the furnace is running. A fan-powered model has its own small motor and can add humidity even when the furnace is off, but it uses more electricity.
Both types mount on the supply plenum—the metal chamber where air exits the furnace before entering the ducts. Some furnaces have a plenum on top; others have one on the side. You will cut an opening in the plenum, slide the humidifier into place, and seal it with sheet metal screws and duct sealant. A bypass model also needs a return duct opening so air can cycle back to the furnace after passing through the pad.
Tools and materials you will need
Gather a drill with metal bits, a reciprocating saw or tin snips (for cutting the ductwork), a screwdriver, adjustable wrench, a level, duct sealant or mastic, sheet metal screws, and a flashlight. You will also need flexible water supply line (usually 1/2-inch diameter), a shutoff valve for the water line, a drain line (typically 3/4-inch PVC or vinyl tubing), and a humidistat if your furnace does not already have one.
Before you start, turn off the furnace at the thermostat and the breaker. Locate the cold water line nearest to your furnace—often in a basement or utility room—and the nearest floor drain or sump pit for the drain line. If neither is close, you may need to run lines a longer distance or hire a plumber.
Step-by-step installation on the furnace
First, mark the opening on the supply plenum using the humidifier's template (usually included in the box). The opening is typically 6 to 8 inches wide. Use a drill to start a hole, then cut the opening with a reciprocating saw or tin snips. Smooth any sharp edges with a file.
Slide the humidifier into the opening and find it with sheet metal screws—usually four screws, one at each corner. explore duct sealant around the seams where the humidifier meets the plenum to prevent air leaks. If you are installing a bypass model, cut a second opening in the return duct (the duct that pulls air back to the furnace) and install the bypass duct connection there, sealing it the same way.
Check that the humidifier is level using a level tool. An unlevel unit may not drain properly or may allow water to pool inside. Once the humidifier is find and level, you can move on to the water and drain connections.
Running water and drain lines
Locate the cold water line and turn off the main water shutoff valve. Cut the cold water line with a tubing cutter (a small hand tool that costs a few dollars), then install a tee fitting and a shutoff valve. The shutoff valve lets you stop water to the humidifier without shutting off the whole house. Attach the flexible water supply line to the shutoff valve and run it to the humidifier's water inlet, securing it with hose clamps.
The drain line is simpler: attach a vinyl or PVC tube to the humidifier's drain outlet and run it to a floor drain, sump pit, or laundry sink. The drain line does not need to be pressurized, so it can sag slightly, but avoid creating a trap where water could pool. If you do not have a drain nearby, you may need to run the line a longer distance or install a small pump to lift water upward.
Connecting the humidistat and power
The humidistat is a wall-mounted sensor that measures indoor humidity and signals the humidifier to turn on when humidity drops below your target level. If your furnace does not have a humidistat already, you will need to install one on an interior wall away from kitchens and bathrooms (where humidity is naturally high). Run a thin wire from the humidistat to the humidifier's control box, usually a distance of 20 to 50 feet depending on your home's layout.
The humidifier's control box plugs into a standard outlet near the furnace. Some units also require a wire connection to the furnace blower so the humidifier knows when the furnace is running. Check your humidifier's manual for the exact wiring diagram—this step varies by model. If you are uncomfortable with electrical connections, a technician can handle this part.
Testing and adjusting humidity levels
Once everything is connected, turn the furnace back on at the breaker and thermostat. Set the humidistat to 35 to 50 percent relative humidity—a comfortable range for most homes. Turn on the furnace and let it run for a heating cycle. You should see water flowing into the humidifier's pad or reservoir.
Check for leaks around the water line connections and the drain line. A small drip at a connection can usually be tightened with a wrench; a leak inside the ductwork means the humidifier is not sealed properly and needs more duct sealant. After a few days, use an inexpensive humidity meter (available at hardware stores) to check whether the humidifier is raising humidity to your target level. If humidity is too low, the pad may need cleaning or the water line may be blocked; if it is too high, lower the humidistat setting.
When to call a heating technician instead
Call a professional if your furnace is in a cramped space where you cannot safely cut ductwork or reach connections. Also call if your home has a boiler instead of a forced-air furnace—boiler systems require a different type of humidifier. If your water line is soldered copper and you are not comfortable cutting and soldering, a plumber can do that part. And if your furnace has a complex control board or if you are unsure about any electrical connection, let a technician handle the humidistat wiring.
A heating technician typically charges $300 to $600 for installation labor, depending on your region and how accessible your furnace is. This is worth the cost if it means avoiding damage to your ductwork or furnace.
Frequently Asked Questions
Can I install a humidifier on a furnace in an attic?
Yes, but it is more difficult because you have to run water and drain lines up to the attic, and the drain line must slope downward to work properly. You may need a condensate pump to push water downward if the drain cannot flow by gravity. An attic installation also makes maintenance harder because you have to climb up to clean the pad.
What if my furnace does not have a humidistat?
You will need to install one. A humidistat is a small wall-mounted device that costs $50 to $150 and runs a thin wire to the humidifier's control box. It measures the humidity in your home and tells the humidifier when to turn on. Without one, the humidifier will run constantly and may over-humidify your home.
How often do I need to clean the humidifier pad?
Most pads need cleaning every one to three months during heating season, depending on your water hardness. Hard water leaves mineral deposits on the pad, which reduces water flow and humidity output. You can rinse the pad under running water or replace it entirely—replacement pads cost $20 to $50.
Can I install a humidifier on a heat pump?
Yes, if your heat pump has a furnace backup (called a dual-fuel system). The humidifier installs on the furnace part just as it would on a gas furnace. If you have only a heat pump with no furnace, you will need a portable or ductless humidifier instead.
What happens if the drain line freezes?
If your drain line runs through an unheated space like a basement wall or crawlspace, it can freeze in winter and block water from draining. Prevent this by insulating the drain line with foam pipe wrap or by running it through a heated space. If it does freeze, you can thaw it with a heat lamp or by running warm water through it.