Flow through versus Bi-pass

By-Pass Flow Through Humidifier

The most popular type of humidifiers are the Flow-through type. Called "flow-through" because a portion of the air from the supply side of the furnace is detoured to force it across a wetted humidifier pad. The humidified air is then routed back to the return side of the furnace, blended with air from the cold air returns, heated by the furnace and delivered to the conditioned space.

The most remarkable feature of all flow-through models is the patented PerfectFlo^TM water distribution tray. The tray allows for speedy and complete humidifier pad water saturation and efficient humidification.

The unique tray also ensures fast delivery of water to the humidifier pad assuring that the pad is wet at the start of the blower cycle and wets the entire humidifier pad evenly providing optimal evaporation.

Pros: Bypass flow-through humidifiers are low-cost and low maintenance. Depending on the hardness of the water supply, the only necessary maintenance is to replace the humidifier pad once a year. The drain tube should also be checked yearly and cleaned if there is mineral build-up inside.

Bypass flow-through models cost less than other models - and are easier for contractors to install because there is no heating element and no fan or drum motor.

Cons: First, because there is no separate fan or motor, bypass flow-through models deliver rated humidification only when the furnace is operating. In some applications, there may not be enough furnace run-time to deliver the needed humidification. Also, there is a higher rate of water use. Relatively little of the water used in a bypass flow-through humidifier is actually converted to humidity. Most of the water is used to flush the humidifier pad to minimize the clogging effect of mineral build-up.

For example, Honeywell tests show that the bypass flow-through humidifiers from other manufactures use as much as six gallons of water per hour (gph) to convert .75 gallons to humidity, an efficient ratio of 1-8. In an area with high water/sewer cost, this could translate to up to a $5 - $10 monthly increase in the homeowner's water/sewer bill.

If water conservation is a concern, the efficient Honeywell HE260 bypass humidifier can help. It uses just 3.5 gph to convert .75 gallons to humidity, an efficiency ratio of about 1 to 5. While a competitor's six gph model could use 72 gallons of water for each 12 hours of operation, the Honeywell model would use only 42 gallons under the same conditions. The Honeywell model is 40 percent more water efficient.

Powered Flow Through Humidifier

Pros: Like bypass flow-through, powered flow-through are hard-water tolerant and minimal maintenance - just change the pad.

The fan inside a powered unit blows heated supply-side air across the humidifier pad so its water efficiency is higher. Powered models can typically produce about one gallon more humidity per day using the same size humidifier pad as a bypass model.

The additional electricity needed to run the fan is minimal - about equal to a 25 watt light bulb - and could be more than offset by the energy-savings possible with lower thermostat setting when the air is kept at an optimal humidity level.
In addition, powered flow-through humidifiers do not require a bypass duct, so they can be installed in tighter spaces. In fact, there is a specific market niche for powered models - geographic areas like the Southeast and Southwest where homes are built on slabs and where HVAC systems are installed in closets, with minimal room for the mechanicals.

Cons: Because it includes a separate fan, a powered flow-through humidifier costs more to purchase and operate than an un-powered model.