Sizing - Aprilaire 700M Reference Manual

Sizing

Sizing for humidification is similar to sizing for heating and cooling.
The humidifier capacity required will be determined by various factors such as:
A. The volume of area being humidified
B. The air change rate (infiltration or ventilation)
C. Humidification load determination
A. The volume of area being humidified
The volume can be determined from a floor plan or from measurements taken within the home. It's important to determine
the volume, not just the square feet. Care should be taken to determine ceiling height or any open space such as vaulted
ceilings. If the basement is heated and ventilated, its volume should also be included.
B. The air change rate (infiltration or ventilation)
The amount of infiltration was probably calculated when computing the heating and cooling load. AHRI defines structure
types as follows:
• Tight construction
doors, windows with weather stripping.
• Average construction
doors and windows, dampered fireplace.
• Loose construction
insulation, no storm doors, no insulated windows, no weather stripping, no vapor retarders, undampered
fireplace.
AHRI has also established a standard guideline of determining capacity ratings, which provides a means of comparing
product including those from different manufacturers. Capacity is determined by the output of the device as if it operated
24 hours per day. (AHRI Guideline F)
Structure tightness 1000 ft
Tight
Average
Loose 12.7
C. Humidification load determination
The typical family of four will generate 2.0 gallons per day of moisture from internal sources such as cooking, showering
and breathing. This amount can be subtracted when sizing a humidifier.
– ½ air changes per hour of air infiltration, well insulated with vapor retarders, tight storm
– 1 air change per hour of air infiltration, insulated with vapor retarders, loose storm
– 1 ½ air changes per hour of air infiltration, usually built before 1930 with little or no
Humidifier Sizing Chart (8' ceilings)
2 1500 ft 2 2000 ft
Humidity needed (GPD per AHRI)
4.3 6.4 8.5
8.6 12.8 17
19.1 25.5
2 2500 ft 2 3000 ft 2
10.6 12.7
21.3 25.4
31.8 38.1
4000 ft 2 5000 ft 2
17 21.2
34 42.6
51 63.6