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Page 14
2008-10-01
Table 1. Outdoor/Indoor Relative Humidity Conversion Chart
Table 2. Load Calculation Summary Sheet
Outdoor relative humidity
100%2456791217192329364352
95%2346791216172228344150
90%2345681115162126313948
85%2345681114152024293745
80%2345671013151923273542
75%2344571012141822263339
70%123456911131720243136
65%123445810121519232934
60%12334579111417212631
55%11334478101316192429
50%1123346891214182226
45%1123346781113162024
40%1122345771012141821
35%112224566910121518
30%01222345679111315
25%0111233455791113
20%011122334557910
15%00111123344568
10%00011122233346
5%00000011122233
0%00000000000000
-20° -10° -5° 0° +5° +10° +15° +20° +25° +30° +35° +40° +45° +50°
Outdoor temperature
Chart shows what the residual indoor RH would be at 70°F under varying outdoor conditions if a proper humidification
system were not installed in the building. Studies indicate that the recommended RH should be between 40% and 60%
for optimum benefits to the occupants.
Determine the moisture required in the space (Table 3)
Grains from Table 3 at space temp. ______ x Indoor RH
= A ______ gr/ft
3
Determine the moisture level of incoming air (Table 4)
Grains from Table 4 at space temp. ______ x Outdoor RH
= B ______ gr/ft
3
Therefore: moisture to be added: (moisture) M - A - B ______ gr/ft
3
→M ______ gr/ft
3
Determine the volume of air to be humidified. Choose the largest
value.
1. Natural ventilation: Volume x number of air changes.
______ ft
3
/hr
2. Exhaust air: CFM x 60 min/hr
______ ft
3
/hr
3. Make-up air: CFM x 60 min/hr
______ ft
3
/hr →C ______ ft
3
/hr
Therefore: Gross humidification load = L (load) = MxC = ______ lbs/hr →L ______ lbs/hr
7,000
NOTES: 1. 7,000 grains = 1 pound
2. If HVAC system uses economizer cycle, check load using formula 6.