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Bypass vs. Fan-Operated: Which Central Humidifier is Best for My Home?


Itchy, dry skin, fly-away hair, scratchy throats, and nosebleeds. What do they all have in common? Dry indoor air. That’s why so many homeowners choose to add a central humidifier. But which type is best for your home?

We asked Keith Hill, resident HVAC expert at Stay Comfy, and here’s what we learned.

Bypass Humidifiers

Bypass humidifiers are the most popular and least expensive of the central humidifiers. A bypass humidifier takes air from the supply side (warm air) and passes it through a duct to the return air of the furnace. It uses the furnace blower motor, so it needs no extra power supply to push the air through the humidifier.

Pros: A bypass humidifier is inexpensive, needs no high voltage power, and can be set up to flush residual minerals away. It will also run quieter than a fan-powered unit, because it’s running off the furnace fan.

Cons: The humidity output from a bypass unit is moderate and the furnace must be operating for the humidifier to work.

Fan-Powered Humidifiers

Fan-powered units use a fan, internal to the humidifier, to take air from the duct the humidifier is installed on and push it through the humidifier and back into the same duct. Since the duct air pressure is the same it requires a fan to push the air. The fan runs on a 120-volt power supply.

Pros: A fan-powered unit produces a higher humidity output than a bypass unit. Since it does not need a bypass duct, it can sometimes be installed in a more confined area where a bypass type would not fit. Like a bypass unit, a fan-powered humidifier can be set up to flush away residual minerals. Since it has its own fan, it can be operated on its own without the furnace running. As a result, a fan-powered humidifier can maintain a more consistent humidity throughout your home.

Cons: A fan-powered humidifier by itself will be more expensive because of the fan and fan motor. It will also cost more to install because of the high voltage power requirement. Because there’s an additional fan running, it may add to the system’s operating noise.

Portable Humidifiers

Portable or room humidifiers are also an option for some homeowners. Humidity seeks its own level, so it can be a good idea to have one or more portable units in your home. This may be especially beneficial if your needs are for relatively light humidification. These units operate based on an internal humidistat, so they’re automatic in that sense. However, since they’re not connected to a water supply, they need to be manually filled with fresh water.

Pros: Portable units require no installation, they’re the least expensive option, and they’re portable, so you can move them from room to room.

Cons: These humidifiers require more maintenance because they need to regularly be refilled and they have no “flushing system” to remove mineral buildup.

A Word About Mineral Buildup

All humidifiers evaporate water, but water contains minerals that do not evaporate. They’re left behind as “lime” (the plumbing and heating industry term for mineral buildup). Both bypass and fan-powered central humidifiers can be set up with a flushing system to continuously purge these minerals down the drain, keeping maintenance to a once a year event. That’s not the case with portable units unless you fill them with distilled water.

So, say goodbye to dry, itchy skin and scratchy throats and invest in a central humidifier. With little maintenance on your part, you'll have perfectly humid air year-round.

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