Aprilaire 700M Reference Manual page 5

The Story of Humidity
Humidification – the process of adding moisture to the air – is one of the most important aspects of total indoor comfort,
yet it's one of the least understood.
One reason for this is that humidity is an intangible. It can't be seen, touched, or smelled. It has no color, no sound. However,
its presence, when properly controlled, offers many proven benefits. Humidification during the heating season not only
provides comfort, it helps protect the home and its furnishings from the harmful effects of air that's too dry. In addition,
physicians often recommend humidity to guard against dry skin, hair, and scratchy throats. It also reduces undesirable
winter-time static electricity.
Relative humidity (RH) indicates the amount of water vapor, in percent, actually in the air compared to the maximum
amount that the air could hold under the same conditions. The warmer the air, the more moisture it can hold. Air in a
home heated to 70°F can hold about eight grains of moisture per cubic foot. That is 100% RH. If there are only two grains
per cubic foot in the home, this is ¼ of the air's capacity to hold moisture. Therefore, the RH is also ¼ or 25%. The air
could hold four times as much water.
But, the important thing is what happens to air when it's heated. The outdoor-indoor RH conversion chart below
illustrates this.
Locate outdoor RH on the left of
the chart and outdoor temperature
at the bottom. Indoor RH is where
the vertical and
columns meet. The calculation
assumes that outdoor air is
brought into the home and heated
to 70°F.
Example:
(See shaded figures)
Outdoor RH is 70%; outdoor
temperature is +10°F; when
this air is brought into the home
and heated to 70°F , the indoor
RH drops to 6%.
This substantial reduction of RH from outdoor to indoor air is taking place in every unhumidified or under-humidified
home where the air in the home is heated.
To solve it, we add moisture artificially, so there's more water available for that thirsty air to exercise its ability to hold
more water. We humidify because there are benefits that are as important as heating to overall indoor comfort and
health during the heating season.
100% 2
95% 2
90% 2
horizontal
85% 2
80% 2
75% 2
70% 1
65% 1
60% 1
55% 1
50% 1
45% 1
40% 1
35% 1
30% 0
25% 0
20% 0
15% 0
10% 0
5% 0
0% 0
-20
OUTDOOR-INDOOR RELATIVE HUMIDITY CONVERSION CHART
(figures in chart are percentages)
4 5 6 7 9
3 4 6 7 9
3 4 5 6 8
3 4 5 6 8
3 4 5 6 7
3 4 4 5 7
2 3 4 5 6
2 3 4 4 5
2 3 3 4 5
1 3 3 4 4
1 2 3 3 4
1 2 3 3 4
1 2 2 3 4
1 2 2 2 4
1 2 2 2 3
1 1 1 2 3
1 1 1 2 2
0 1 1 1 1
0 0 1 1 1
0 0 0 0 0
0 0 0 0 0
-10 -5 0 +5 +10
OUTDOOR TEMPERATURE (˚F)
12 17 19 23 29
12 16 17 22 28
11 15 16 21 26
11 14 15 20 24
10 13 15 19 23
10 12 14 18 22
9 11 13 17 20
8 10 12 15 19
7 9 11 14 17
7 8 10 13 16
6 8 9 12 14
6 7 8 11 13
5 7 7 10 12
5 6 6 9 10
4 5 6 7 9
3 4 5 5 7
3 3 4 5 5
2 3 3 4 4
2 2 2 3 3
1 1 1 2 2
0 0 0 0 0
+15 +20 +25 +30 +35
36 43 52
34 41 50
31 39 48
29 37 45
27 35 42
26 33 39
24 31 36
23 29 34
21 26 31
19 24 29
18 22 26
16 20 24
14 18 21
12 15 18
11 13 15
9 11 13
7 9 10
5 6 8
3 4 6
2 3 3
0 0 0
+40 +45 +50