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happens to the temperature of air passing through the wall? It drops to about 63°F at the vapor barrier, down to 17°F at the sheathing, and to 0°F outside. A psychrometric chart shows that 70°F indoor air at 35% relative humidity has a dew point of 41°F. That temperature will occur near the center of the insulation. This is where condensation and trouble would start without a vapor barrier, and with a humidifier that is not controlled.

Properly controlled relative humidity is important. It avoids the damaging effects of air that is too dry, and the damaging effects of relative humidity that is too high.

BENEFIT NO 3—HEALTH Number of cases of Average indoor respiratory diseases Month relative humidity per 1,000 population What do doctors say about humidity and health? Dr. Arthur W. Proetz, an eye, nose, July 59% 17 and throat specialist, writes: August 56% High Good 20 September 51% humidity health 23 October 40% 29 “In the struggle between the nose and the machinery in the basement, November 20% 40 December 12% 58 sometimes the heater wins and January 12% Low Poor 92 sometimes the cooler, but seldom the February 14% humidity health 102 March 18% 89 nose. The nasal mucus contains some April 25% 55 96% water. To begin with, it is more viscous than mucus elsewhere in the May 35% High Good 35 June 57% humidity health 22 body and even slight drying increases the viscosity enough to interfere with the work of the cilia. Demands on the TABLE 9-2. Proper relative humidity can help relieve health nasal glands are great even under usual problems conditions and they cannot cope with extreme dryness indoors in winter.SAMPLE Experience has shown that with approaching winter, the first wave of dry nose patients appears in the office when the relative humidity indoors falls to 25%. It would seem, therefore, that 35% would be regarded as a passing grade but 40% something to shoot at. It boils down to this—a pint of water is a lot of water for a small nose to turn out. In disease or old age, it simply doesn’t deliver and drainage stops and the germs take over.”

Next is a comment by Dr. Joseph Lubart, a common cold expert. He says, in the New York State Journal of Medicine:

“Prevention of the common cold at present is our nearest approach to a cure. The most important prevention measure would appear to be proper

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regulation of the humidity, especially during the heating season with its distressing drying of the indoor air and the creation of an environment favorable to the cold bug.”

In Table 9-2, the right-hand column shows the number of cases per month of respiratory disease per 1,000 population, taken from U.S. Public Health statistics. The left-hand column is typical average indoor relative humidity during the same months. There is an obvious correlation between good health and high relative humidities, and between poor health and low relative humidities. All the facts point toward a positive connection between humidity and health.

WHAT INDOOR RELATIVE HUMIDITY IS CORRECT?

While some humidity conditions may be ideal for health and comfort, they may be less than ideal for other reasons. An indoor relative humidity of 60% may satisfy health Outside Recommended and comfort needs, but it can also damage walls and temperature, °F relative humidity, % furnishings. As mentioned previously, the fogging of +20 and above 35 windows is usually an indication that the relative humidity +10 30 is too high. And the same condensation is taking place 0 25 –10 20 inside walls and other places vulnerable to damage by –20 15 excessive moisture.

TABLE 9-3. Temperature-humidity levels It is, therefore, necessary to set safe limits of indoor relative humidity levels. These levels should give the maximum benefits from correct humidity, without making the structure itself susceptible to damage. It is recommended that the levels in Table 9-3 be followed to ensure these benefits. HOW RELATIVE HUMIDITY ISSAMPLE MEASURED There are two types of instruments normally used to measure relative humidity— hygrometers and psychrometers.

Hygrometers

Hygrometers are the instruments most commonly seen in homes and offices. They are manufactured in a variety of models. Most of them use a device that changes in size as the relative humidity changes. This change actuates a dial which shows the relative humidity.

This type of instrument, when properly calibrated, gives reasonably accurate relative humidity readings. The humidistat supplied with humidifiers is a type

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of hygrometer. For convenience and maximum accuracy, the humidistat should be wall-mounted in the living area. The accuracy of these instruments can be checked by the second type of measuring device—the psychrometer.

Psychrometers

A sling-type psychrometer is an instrument used by humidifier installers and others to determine the exact relative humidity readings needed for calculations and recommendations.

Readings are taken with a sling-type psychrometer as follows:

ª Dip the wick on the wet-bulb thermometer in distilled water, preferably (only one dipping per determination of relative humidity, but never between readings). The progressive evaporation of the moisture in the wick, until it reaches equilibrium with the moisture in the air, is the determining factor of the wet-bulb reading.

ª Whirl the sling psychrometer for 30 seconds. Take the reading quickly on the wet-bulb thermometer first, then on the dry bulb, and jot them down. Whirl the psychrometer again, taking readings at 30-second intervals for five successive readings. Jot down the temperature each time, or Dry-bulb lines until the lowest reading has been obtained and the last reading shows a leveling off or return curve 20 304050 7060 80 90 100 Dry-bulb scale, °F (two or more nearly identical successive readings). FIGURE 9-3. Dry-bulb temperature lines

ª SAMPLE Use a psychrometric chart or table to obtain the 85 relative humidity. 80

THE PSYCHROMETRIC CHART 75

70 °F , le The psychrometric chart simplifies the determining of air a 65 sc b properties and eliminates many tedious calculations. Of ul 60 -b Wet-bulb lines et 55 W special concern are five specific psychrometric terms that 50 45 can be found on the chart: 40 35 25 30 20 ª dry-bulb temperature

ª wet-bulb temperature FIGURE 9-4. Wet-bulb temperature lines

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ª relative humidity

0 0 40 ª absolute humidity 1

ª 30 dew point temperature. % y, it id m hu 20 If you know any two of these air properties, the other e tiv la three can be found on the chart. Re

10 The first term is dry-bulb temperature, the temperature measured by an ordinary thermometer. This temperature scale runs horizontally across the bottom of the chart. The dry bulb temperature lines are the straight vertical FIGURE 9-5. Relative humidity lines lines, as shown in Figure 9-3.

The second air property found on a psychrometric chart is the wet-bulb temperature. This is the temperature 0.024 resulting when water is evaporated off a cloth covering 0.022 0.020 an ordinary thermometer. The wet-bulb scale is 0.018 measured along the curve portion of the psychrometric 0.016 chart, from lower left to upper right. The wet-bulb lines 0.014 run diagonally across the chart, as shown in Figure 9-4. 0.012 0.010 0.008 Specific humidity lines On a complete psychrometric chart, relative humidity 0.006 lines are the only curved lines. The various relative 0.004 humidities are indicated on the lines. Unlike the other 0.002 0 P air properties, there is no coordinate scale for relative humidity values, as you can see in Figure 9-5. FIGURE 9-6. Specific humidity lines The fourth component of a psychrometricSAMPLE chart is specific humidity. This is the actual weight of water 85 vapor in the air. Specific humidity lines run horizontally from a scale that extends vertically along the right side 80 of the chart, as shown in Figure 9-6. 75

70 °F , le Dew point temperature is the fifth air property included a 65 sc in a psychrometric chart. This is the temperature at nt 60 oi Dew point lines p 55 which moisture will condense on a surface. The scale ew D 50 45 for dew point temperature is identical to the scale for 40 35 wet-bulb temperature. However, the dew point lines run 25 30 0 horizontally across the chart, not diagonally (as is the case in the wet-bulb temperature lines). Figure 9-7 shows dew point lines. FIGURE 9-7. Dew point lines

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Putting the five graphs together makes a 35% relative humidity complete psychrometric chart, as shown in Figure 9-8. Now we can use an example to show 56°F wet bulb how a psychrometric chart works. Assume 0.0058 pounds of moisture 42°F dew point that readings have been per pound of dry air taken using a sling psychrometer. The dry- bulb temperature reads 72°F and the wet bulb 72°F dry bulb is 56°F. Two factors are used to determine FIGURE 9-8. Psychrometric chart example relative humidity—dew point and specific humidity.

The 72°F dry-bulb temperature is found on the bottom scale. The wet-bulb temperature scale is on the curved outside line of the chart on the left. The wet- bulb temperature line is 56°F. Extending the two lines, they intersect. From this point, all other information can be determined. The relative humidity, for example, is 35%. The specific humidity is 0.0058 pounds of water per pound of air. The dew point temperature is 42°F.

For further information on the use of the psychrometric chart, see Section 5401 of the RSES Service Application Manual. Section 621 of your SAM discusses the use of the RSES psychrometric calculator. EFFECT OF WATER CHARACTERISTICSSAMPLE Fundamentally, only distilled water, or rain water caught before it reaches the ground, is free from minerals. Water from wells, lakes, and rivers all contain varying amounts of minerals in solution. These minerals are picked up as water moves through or across water-soluble portions of the earth’s surface. In many cases, the level of these minerals is too high to permit the water to be used domestically. Water conditioning equipment is required to remove the objectionable minerals.

It is common knowledge that water evaporated from a tea kettle leaves a residue of lime. But evaporation of water is the only way to create and distribute water vapor into the air in homes. Obviously, mineral residue from evaporation presents a problem.

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Water hardness varies in different localities. Drinking water contains some hardness. It consists primarily of calcium carbonate and/or magnesium carbonate. This hardness is expressed in grains per gallon, as shown in Table 9-4. If a gallon of water of average hardness is evaporated, a residue Class of Hardness, grains % figure of 25 grains remains. If 100 gallons of water are hardness per gallon in U.S. evaporated to provide humidity, 2,500 grains or 5.7 ounces of solids will build up on the Low 3 to 10 30 Average 10 to 25 55 evaporating surface. High 25 to 50 15

HUMIDIFICATION AND HUMIDIFIERS TABLE 9-4. Hardness classifications In the “good old days,” most of the home was closed off for the winter. The family congregated and lived in the sitting room and the kitchen. These were the most comfortable rooms in the house. The old wood stove generated plenty of heat. And the way grandmother used to cook added a great deal of moisture to the air. Pots or kettles usually steamed or bubbled on the stove, clothes dried nearby, and well- watered house plants decked the window sills.

The stews and the soups and the tea kettle acted as humidifiers. The air was moist some of the time, at least in one or two rooms. The only control was how much cooking and washing was done and how many house plants there were. It couldn’t be called high-capacity humidification or controlled humidification. In fact, it couldn’t really be called humidification.

Next came the gravity furnace. It was hand-fired. A register was cut into the floor in the parlor, maybe in the living room. Three or four rooms were used during the winter. The furnace had a small reservoir for water. The furnace heat evaporated it. It was filled everySAMPLE week, if remembered, and it took two quarts of water. Two quarts of water evaporated a week. The only benefit of this type of humidification was the self-satisfaction in thinking that something was being done about keeping the air from becoming too dry.

Then came the forced-air furnace. It was an improvement that initiated a varied assortment of humidifiers, with varied degrees of success.

Types of humidifiers

There are many different humidifiers available today, and they vary widely in price, capacity, and principle of operation. For classification purposes, it is probably simpler and more logical to consider humidifiers as belonging to three general types:

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