Nortec NHPC, NHTC manual 10-10, Load Calculation Summary Sheet, Page 14, Outdoor relative humidity

Table 1. Outdoor/Indoor Relative Humidity Conversion Chart

Outdoor relative humidity

100%

95%

90%

85%

80%

75%

70%

65%

60%

55%

50%

45%

40%

35%

30%

25%

20%

15%

10%

5%

0%

2	4	5	6	7	9	12	17	19	23	29	36	43	52
2	3	4	6	7	9	12	16	17	22	28	34	41	50
2	3	4	5	6	8	11	15	16	21	26	31	39	48
2	3	4	5	6	8	11	14	15	20	24	29	37	45
2	3	4	5	6	7	10	13	15	19	23	27	35	42
2	3	4	4	5	7	10	12	14	18	22	26	33	39
1	2	3	4	5	6	9	11	13	17	20	24	31	36
1	2	3	4	4	5	8	10	12	15	19	23	29	34
1	2	3	3	4	5	7	9	11	14	17	21	26	31
1	1	3	3	4	4	7	8	10	13	16	19	24	29
1	1	2	3	3	4	6	8	9	12	14	18	22	26
1	1	2	3	3	4	6	7	8	11	13	16	20	24
1	1	2	2	3	4	5	7	7	10	12	14	18	21
1	1	2	2	2	4	5	6	6	9	10	12	15	18
0	1	2	2	2	3	4	5	6	7	9	11	13	15
0	1	1	1	2	3	3	4	5	5	7	9	11	13
0	1	1	1	2	2	3	3	4	5	5	7	9	10
0	0	1	1	1	1	2	3	3	4	4	5	6	8
0	0	0	1	1	1	2	2	2	3	3	3	4	6
0	0	0	0	0	0	1	1	1	2	2	2	3	3
0	0	0	0	0	0	0	0	0	0	0	0	0	0
-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.

Table 2. Load Calculation Summary Sheet

Determine the moisture required in the space (Table 3)
	Grains from Table 3 at space temp. ______ x Indoor RH		= A ______ gr/ft3
Determine the moisture level of incoming air (Table 4)
	Grains from Table 4 at space temp. ______ x Outdoor RH		= B ______ gr/ft3
Therefore: moisture to be added: (moisture) M - A - B ______ gr/ft3				→M ______ gr/ft3
Determine the volume of air to be humidified. Choose the largest
value.
1.	Natural ventilation: Volume x number of air changes.		______ ft3/hr
2.	Exhaust air: CFM x 60 min/hr		______ ft3/hr
3.	Make-up air: CFM x 60 min/hr		______ ft3/hr	→C ______ ft3/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.

10-10

Page 14 2008-10-01