5
SIZING
What is meant by a humidity load requirement?
The humidity load requirement is the daily amount of vapor needed to maintain 35% RH in specific home sizes and is often listed in minimum
gallons per day (GPD) needed.
Square feet of space
Structure tightness 500 1000 1500 2000 2500 3000
Humidity needed (GPD per AHRI)
Tight 0.1 2.2 4.4 6.5 8.6 10.7
Average 1.3 4.5 7.8 11.1 14.3 17.6
Loose 2.6 7.2 11.8 16.4 21 25.6
Chart 2: Humidity requirements in GPD based on home size and construction type (from AHRI Guideline F). Loads assume people
living within home.
How is structure type defined?
AHRI defines structure type as follows:
'Tight' construction = Well insulated with vapor retarders, tight storm doors, windows with weather stripping, dampered fireplace and
using 1/2 air change per hour of air infiltration.
'Average' construction = Insulated with vapor retarders, loose storm doors and windows, dampered fireplace with 1 air change per hour
of air infiltration.
'Loose' construction = Generally built before 1930 with little or no insulation, no storm doors, no insulated windows, no weather stripping,
no vapor retarders, undampered fireplace, and with 1-1/2 air changes per hour of air infiltration.
Taking humidity load requirements into consideration, contractors can better pinpoint the ideal TrueSTEAM model based on the run time to
meet 35% RH in the home. This is important since run times affect the homeowner operating cost, the system fan noise and actual indoor
humidity achievable.
Home size
Humidity
needed**
TrueSTEAM
HM506
(6.5 GPD)
HM509
(9 GPD)
HM512
(12 GPD)
Square feet GPD per AHRI Average hours per day
500 0.1 0.2 0.15 0.12
1000 2.3 8.5 6.1 6.0
1500 4.4 16.2 11.7 8.8
2000 6.5 24.0 17.3 13.0
2500 8.6 24* 22.9 17.2
3000 10.7 24* 24* 21.4
* Gray indicates model is undersized for the home size, and cannot achieve 35% RH under these conditions.
** Based on people living in a home with tight construction and 8-foot ceilings.
Chart 3: TrueSTEAM average run times to meet 35% RH.
Using run time averages per TrueSTEAM model and house size, you can determine the humidifier that is best suited for meeting
35% RH in the home. For example, you can see that for smaller homes, there is little difference in the run times required between
units. Thus, choosing a smaller size TrueSTEAM which draws fewer amps to achieve 35%RH in about the same time as the larger
model is more economical. As the home gets larger, the run time increases for the smaller size TrueSTEAM, which then makes it
logical to upsize to the larger model. Taking into consideration the hourly run time and amp draw per model, Chart 4 shows the total
daily operating costs, based on national averages (source: Department of Energy web site, Nov 2008).
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