Humidifiers Introduction


The purpose of humidifiers is to add moister to the air, adding comfort and energy efficiency during the winter heating season.  As heating systems warm the air, the relative humidity of the air decreases.   Heated air feels dryer than cool air when the amount of water vapor in the air remains constant.   A heating system does not necessarily remove moisture; it only makes the air feel dryer.  A humidifier adds water vapor to the air to retain a comfortable level of relative humidity.

The lower the relative humidity in the air, the more moisture evaporates off skin; cooling your body as the water evaporates.  This phenomenon is the reverse effect of hot humid summer days when your body heats up because it cannot evaporate skin moisture to cool your body.   In winter, a home with inside air temperature of 72F (22.2 C) and relative humidity of 35% will feel much colder than a home with air temperature of 68F (20 C) and relative humidity of 65%.   A humidifier can allow a user to lower the heating thermostat (to save energy) while maintaining comfort levels for occupants. Humidifiers can also mitigate the symptoms of colds, sinus infections, dry skin, and chapped lips caused by excessively dry air.  There are two basic types of humidifiers, from a water use perspective: self-contained room humidifiers; and central humidifiers, usually attached to central forced-air heating systems. 

Room Humidifiers 

The small self-contained units are often called room humidifiers and are not attached to a water supply pipe; requiring the reservoir to be refilled by hand every day or so.  Room humidifiers tend to be water efficient; only using one or two gallons of water per day.   Except for the water used in weekly cleaning and rinsing the reservoir, all water is efficiently converted to vapor.  Room humidifiers are seldom a cause for concern regarding water efficiency because the water use is usually less than two gallons per day, though energy use can vary greatly.  Atomizer models pump water through misting nozzle designed to create a fine spray of water that evaporates before it descends to the floor, using only the small amount of energy to operate the pump.  Wick-pad models use a small fan to push air through a porous saturated pad that constantly absorbs water from a small reservoir, allowing the air to evaporate the water as it passes through the pad.  Steam humidifiers use heating elements to slowly boil the water to steam;  while these units provide the fastest way to humidify, the heating elements often consume more than ten times the energy of the other designs using only pumps or fans.    

Central Humidifiers 

The central humidifier (sometimes called “whole house humidifier” in homes) has great potential to waste water,  These units are usually attached the furnace plenum of a central forced air heating system.  A water supply pipe is attached to the humidifier to provide a constant water supply to the humidifier’s reservoir; automatically refilling the reservoir.  A typical residential unit uses 2 to 12 gallons per day (7.6 L to 45.6 L) when efficiently operating.   Most of these humidifiers include a drain line near the top of the reservoir, leading to a sewer system drain.  The primary purpose of this drain is to allow excess water to be removed in case the refill valve fails to close properly.  If the humidifier reservoir were allowed to overflow, severe damage could occur to the HVAC equipment and the building structure.   When humidifiers are properly designed, operated, and maintained, they provide a very beneficial use of water.  Unfortunately, some humidifiers waste thousands of gallons of water every year. 


 Whenever an appliance or fixture includes a water overflow drain, the potential for hidden leaks exists.  The water quietly flows into the home sewer drain without any visual alerts to the casual observer.  Just like leaking toilet tanks, the water waste can occur for months or years before the leak is discovered.   If the float and or refill valve is faulty, the leak can add up to more than 200 gallons per day (756.9 L), or 6,000 gallons per month (22.7 m3) at a typical home.   Commercial units in large building can waste proportionally greater amounts of water.

Leak detection can be as simple as turning off the humidifier (keep the water supply valve “on”) and see if any water is draining through the drain line at the bottom of the humidifier.  If leaks are discovered, simply shut off the water supply to the humidifier until the unit can be serviced, repaired or replaced as necessary.

Operational Water Losses

Some humidifiers use atomizing spray heads or ultrasonic technology to emit miniscule water droplets in the air, where the water droplets quickly evaporate into water vapor.   Other humidifiers use water saturated pads, where the warm dry air passes through, evaporating the water off the pad similar to that of evaporative coolers (see   The continual evaporation of water leaves an increasing concentration of minerals on the evaporation pads and in the reservoir.  If not removed, the minerals begin to form scale, impairing the humidifier performance.   These evaporative humidifiers sometimes include one of two methods to purge minerals: a) continuous bleed; or b) periodic “blow-down”.       

Continuous bleed is very wasteful of water in any type of water using appliance, central humidifiers included.  In continuous bleed humidifiers, a constant stream of water is allowed to bleed from the humidifier’s reservoir and into to the sewer drain.  Likewise, a continuous stream of potable water going into the reservoir is required to keep the reservoir full.  Humidifiers with continuous bleed systems can waste thousands of gallons of water every month.  At minimum, the humidifier and its water supply should be shut off during months not used.   

Blow-down devices periodically pump water out of the reservoir to remove the high concentration of minerals.   The pump is usually activated by a timer.  The timer frequency is usually based on full load conditions (coldest winter day when furnace operates constantly).  Water is wasted when the timer is not periodically reset to accommodate seasonal equipment usage changes (milder fall and spring days).   The hunidifier and its water supply should be shut off when not needed (summer).  Though not as wasteful as a continuous bleed system, a blow-down system can still waste hundreds of gallons per month when not operated properly.