There are several different types of humidifiers, but the one I see most often is the bypass humidifier. This type is attached to the side of the ductwork, usually just above the furnace. Water runs into the top of the humidifier, usually through a small copper tube, then runs down and soaks the filter pad. This water feed tube has a valve to shut the water off, so if your humidifier isn’t working be sure the valve is open.
As air from the HVAC ducts moves through the wet filter pad it picks up the moisture and sends it into the house. The excess water that comes off the bottom of the filter pad is drained away, often to a floor drain.
You’ll need to replace the filter pad on a regular basis – probably at least once a year. It can get caked with minerals from the water making it hard for water to soak into it. Then it isn’t wet enough to give off moisture to the air flowing through it. You can buy filter pads at pretty much any hardware store or home center, or of course on-line. You’ll need to get one that’s specific to the size and brand of humidifier you have.
Now we get to the “problem” with a bypass humidifier, and why it’s called a “bypass” type. Air needs to move through this type of humidifier for it to pick up moisture. But when you slap a humidifier up on the side of your ductwork air doesn’t want to move through it. So a bypass duct is installed from the supply side of the ductwork to the return side, and this bypass duct goes through the humidifier. So this causes air to move through the humidifier and pick up moisture.
Here’s a typical example of a bypass humidifier.
The duct on the right side is bringing cool air down (blue arrow) where it eventually makes a left turn and goes into the furnace blower compartment. Then the furnace heats up the air and blows it up (red arrow) through ducts and past the humidifier. But the bypass duct lets a little bit of air (green arrow) flow through the humidifier and back to the return duct. This air picks up the moisture from the humidifier as it flows through.
Here’s a picture (different furnace) with the humidifier cover removed so that you can see the filter pad.
You can also see that there’s a damper built into the humidifier (red arrow above). In the winter, when you’re using the humidifier, you want to be sure and have this damper open so that air can flow through the bypass duct and through the humidifier.
In the summer, when you’re not using the humidifier, you can close the damper (red arrow below points to the lever to open and close the damper) to prevent air from moving through, thus helping your air conditioner work just a little bit better.
What should the humidity level be set to in your house? Really, whatever is comfortable for you is best. Most informed sources recommend somewhere between about 30-60% relative humidity, while others say 30-50%, and still others say 40-60%. If the relative humidity is too low then you’ll probably not be comfortable with it so dry in the house. And if it’s too humid then you run the risk of mold and mildew growth and condensation on the windows. The humidistat to control the humidifier isn’t going to give anything close to an accurate reading of the relative humidity in the house, so if you want to know for sure you’ll need to get a hygrometer to measure the humidity. Or of course, just set it to what’s comfortable for you.