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Central Humidifiers for Forced Air Heating
Systems
If you are looking for a way to add moisture
to your house without than having to routinely
refill and clean a portable humidifier, you
should consider installing a central humidifying
system. This article discusses systems for
homes that have forced air heating systems.
The basic concept of a central humidifier
is simple. A small water line that is connected
to the household plumbing carries water to
a humidifying unit located somewhere in the
ductwork of the forced air furnace. Inside
the humidifying unit, the water evaporates
into the passing air and thereby increases
the air's humidity level. Where central humidifiers
differ, however, is in how the evaporation
is accomplished, what happens to any excess
water, and what controls are available to
regulate the humidification:
- The most common method of converting incoming
water to airborne moisture is to allow the
air to flow through some form of pad, screen,
or grillwork that is kept constantly damp
by water to trickling through it. The greater
the surface area and the greater the flow
of passing air, the greater the amount of
evaporation. Some designs use a gravity grill
in which water is distributed at the top
and allowed to drip down the grill as air
passes through it. Others use a pad wrapped
around a drum that is kept wet by rotating
through a small pool of water. To boost the
quantity of air that passes over the moist
surfaces, some humidifiers include their
own fans instead of relying upon the air
movement created by the blower in the furnace
system. The most elaborate (and most effective
but most expensive) means to achieve evaporation
is to heat the incoming water to the point
that it turns into steam that mixes with
the air flow, much like a tea kettle in your
furnace ductwork.
- Ideally, all the water flowing into the humidifying
unit would evaporate, but in reality most
systems have to contend with leftover water.
The usual solution is to provide a drain
tube, but this means that the humidifier
must have access to a drain pipe. Some designs
retain and recycle the incoming water until
it all evaporates, but the trade-off for
this approach is the reduction in fresh water
can be used to carry away any mineral deposits
and potential mold build up. Thus, while
drainless designs conserve water and do not
need drains, they require more frequent cleaning
and maintenance.
- The basic control for humidifiers is a humidistat,
which turns the humidifier on or off according
to what it senses as the level of air humidity--just
as a common household thermostat controls
temperature. However, if the humidifier relies
on the furnace blower to force air through
it, the unit will only work when the furnace
is running. Since a forced air furnace cycles
on and off
according to temperature, there can be times
when the humidifier should be on but the
furnace is off. Humidifiers that contain
their own fans can independently (and thus
better) regulate the humidity.
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