Heating Info From Warmair.net
Bypass Humidifiers
The most common furnace humidifier is the bypass humidifier. It makes use
of the pressure differential between the return and supply plenums
on a furnace to move air through it with minimal electrical consumption.
Constructed of metal or plastic, it consists of a cabinet, reservoir,
rotating drum with motor, and float. Preferably mounted on the
supply plenum, warm air is forced into the cabinet and flows over
the rotating drum. The drum is a sponge or foam fabric stretched
over a plastic frame. A small motor slowly spins the drum through
a tray of water fed from the domestic water supply and regulated
by a float valve. As the wet drum is exposed to the flow of warm
air, water evaporates and humidifies the air. Ducted into the
return air plenum, it is mixed with supply air and ducted to the
house.
The same humidifier design can be altered slightly and become a power humidifier.
With a larger cabinet and no duct to the return plenum, a fan
moves air across the wet drum or wet screens and blows it back
into the supply plenum.
Heat pump and electric furnace air is not warm enough to evaporate water as easily
as a fuel burning furnace, so a steam humidifier is needed. Fabricated
of metal, a float valve keeps the required amount of water in the
reservoir. Mounted in the return or supply ducts or plenums, line
voltage current is passed through a heating element immersed in
the reservoir to make steam. The control wiring will include a switch
to turn the blower on during operation for increased humidification
even when the thermostat is not calling for heat.
Airstream humidifiers mount in the bottom of a supply duct. Shaped like a half of a watermelon to
form a reservoir, a float valve maintains the proper water level. Discs made
of screen rotate with the help of a motor into the water then around into the warm
air stream to allow the water to evaporate.
Two types of humidifier spray water directly into the warm air duct while the air is flowing. One uses
the ultrasonic principal to atomize water droplets for evaporation. Tabletop
versions of this design are known as cool mist humidifiers. The other spray
type uses air and water pressure mixed in a nozzle to atomize
the water similar to an aerosol spray. These designs are effective,
but any minerals dissolved in the water will drop out as solids
and collect in the ductwork or be blown out the supply registers
as a fine powder.
The earliest furnace humidifiers were ceramic wicks stacked in a tray of water that was suspended
over the heat exchanger in the supply plenum. Very few if any of these still
exist, going the way of the old furnace that has been torn out and replaced
over the years.
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