Masonry Magazine February 1981 Page. 34
0.130 x
10.77 Btu/day/°F/ft² of floor area
8.00 Btu/day/F/ft² of floor area
- 0.175 times the floor area, or:
0.175 x 1500 ft² of floor area = 263 ft
The new system selected, 200 ft² will provide less heat than required to maintain interior temperatures of 68° to 70° F on clear winter days. If R-8 night insulation is added to the system, the adjusted value 0.175 is reduced by 15 per cent to account for the system performance with night insulation and is found to be:
0.85 0.175-0.149
The collector, south-facing glazing, area is determined to be:
0.149 x 1500 ft of floor area = 224 ft²
Thus, this system 200 ft of south-facing double glazing with R-8 night insulation will supply most of the heat required to maintain interior temperatures of 68° to 70°F on clear winter days.
It is important to note that these values are conservative as compared to the actual performance of the direct gain system and overheating or larger temperature fluctations may result because the average maximum daily temperatures from Table 1 of Technical Notes 43 are used, and not specifically average clear day or sunny day exterior temperatures. Average clear day winter exterior temperature at or near the site should be used with this sizing procedure, but these values are often not readily available and the maximum average monthly winter exterior temperatures from Table 1 of Technical Notes 43 may be used as conservative substitutes.
Clerestories
When using clerestories to admit direct sunlight into a space, place the clerestory approximately 1 to 1.5 times the height of the brick storage wall in front of the wall. If an 8-ft high thermal storage wall is to receive solar radiation transmitted through a clerestory, the clerestory should be positioned 8 ft to 12 ft in front of the thermal storage wall. The ceiling of the clerestory should be light in color to reflect and diffuse sunlight down onto the storage wall.
Glazings
When the south-facing glazing is glass, it should be kept as near vertical as possible. Many glass manufacturers do not recommend that glass be tilted more than 15° from vertical, because slopes of more than 15° from vertical may result in structural failure of the glazing assembly. This may be overcome by using glazings other than glass, but the manufacturer's recommendation for the use of the alternate materials in passive solar systems should be followed. Some plastics, for instance, may discolor or deteriorate because they are affected by ultraviolet light degradation which would result in decreased performance of the passive solar system.
TABLE 1
Area of South-Facing Glazing Required
for Direct Gain Systems as a Ratio
to the Floor Area"
| Average Winter Clear Day Exterior Temperature | 36° | 40° | 44° | 48° |
|---|---|---|---|---|
| 20° F | 0.24 | 0.25 | 0.29 | 0.36 |
| 25 F | 0.22 | 0.23 | 0.25 | 0.32 |
| 30 F | 0.19 | 0.20 | 0.22 | 0.24 |
| 35° F | 0.16 | 0.17 | 0.19 | 0.21 |
| 40 F | 0.13 | 0.14 | 0.16 | 0.17 |
| 45° F | 0.10 | 0.11 | 0.12 | 0.13 |
"Reprinted with permission from Edward Mazria. Edward Mazria and Associates, Inc., Albuquerque, New Mexico 87196.
Thermal Storage Wall System
The general aspects of thermal storage wall systems are discussed in Technical Notes 43. The sizing of thermal storage wall systems is similar to that of the direct gain systems, except that temperature fluctuations may be controlled by the volume of the brick masonry storage wall. If the brick thermal storage wall is too thin, the space may overheat during the day because heat is conducted through too quickly. If the storage wall is too thick it may become ineffective because insufficient energy will be conducted through it. Brick masonry thermal storage walls are usually 10 to 18 in. thick. The desired thickness is determined by the allowable interior air temperature fluctuation of the space. As the wall thickness increases, the interior air temperature fluctuations decrease. Brick thermal storage wall systems constructed with double glazing have shown the following interior temperature fluctuations, when properly sized:
| Brick Thermal Storage Wall Thickness | Approximate Interior Air Temperature Fluctuation |
|---|---|
| 8 in. | 24° F |
| 12 in. | 11° F |
| 16 in. | 7 F |
| 20 in. or more | Negligible |
These temperature fluctuations may be reduced by using additional mass located in the space. This mass may be interior brick masonry floors and walls.
Table 2 gives the ratio of thermal storage wall area to floor area of the heated space for sizing the system. Values given in Table 2 are for 36° to 48° North Latitude and average maximum clear day exterior temperatures of 20° to 45° F. The sizing ratio applies to a structure having 8 Btu/day/F/ft² of floor area and must be adjusted for total structure heat loss greater or less than the 8 Btu/day/°F/ft² of floor area by multiplying the value obtained from Table 2 by the ratio of the