Masonry Magazine October 1981 Page. 17
The steady-state heat losses for the vented wall may be assumed to equal those of an unvented wall and thus the total resistance of the various wall thicknesses will be the same. The average temperature of the interior surface of the glass may be determined by using Equation 8 and the maximum average daily temperature from Table 1 of Technical Notes 43.
To = T. + [(L/R) + (Ro/Ro)] × (T-T (8)
where: To Interior surface temperature of the glazing material, in °F.
T, Average maximum exterior temperature, in °F.
For an 8-in. thick vented thermal storage wall, in Washington, D.C., in January when T, = 41.2°F:
To = 41.2 + [[(0.17/4.15) (72-41.2) +(1.45/4.15)] X × (72-41.2
To 53.2°F, or approximately 53°F
Similarly, for the other wall thicknesses:
12-in. wall, 52.1°F, or approximately 52°F
16-in. wall, 51.1°F, or approximately 51°F
24-in. wall, 49.6°F, or approximately 50 F
The average amount of convective thermal energy from the system during operating hours may be roughly predicted by Equation 9 with the unadjusted maximum exterior surface temperature.
4 = 0.30 x [[(Tomar + T)/2] - T 1.25 (9)3
where: q Average convective thermal energy, in Btu/sq ft/hr.
The energy provided by the convective loop for the 8-in. thick thermal storage wall is approximated using Equation 9 to be:
q = 0.30 x [(127+53/21-72]
q11.12 Btu/sq ft/hr
Similarly:
12-in. wall, q. 10.74 Btu/sq ft/hr
16-in. wall, q. 10.36 Btu/sq ft/hr
24-in. wall, q 9.98 Btu/sq ft/hr
The average amount of total thermal energy, q, supplied by the vented thermal storage wall may be approximated by adding the product of the average energy supplied by the convective loop times the quotient of the number of operating hours divided by 24 hr per day to the average amount of radiant energy available. Thus, for the various wall thicknesses, considering an operating time of 7 hr per day in January, in Washington, D.C.:
8-in. wall, q [11.12 × (7/24)] +12.07-15.31 Btu/sq ft/hr
12-in. wall, q [10.74 × (7/24)] +10.52 13.65 Btu/sq ft/hr
16-in. wall, q [10.36× (7/24)] +8.98 12.00 Btu/sq ft/hr
24-in. wall, q= [9.98 × (7/24)] +8.47 11.38 Btu/sq ft/hr
Without the use of night insulation, the vented thermal storage wall will probably supply less thermal energy to the interior than the unvented system. The selection of vented over unvented systems will be dependent upon whether or not night insulation is economical and when the maximum amount of heat is required.
The time lag for the available radiant energy is the same for equivalent thicknesses of vented and unvented thermal storage walls. The calculated time lag for the thermal storage wall may be determined by using Equation 10:
1 = (w/2) x 1/(π×8/n) (10)2
where: t Time lag, in hours.
The values for time lag and a comparison of various thicknesses of vented and unvented thermal storage walls is provided in Table 1. From Table 1, the unvented wall thickness selected by consideration of the amount of radiant energy available, the surface temperatures and time lag would probably be a 12 or 16-in. thick wall. The vented wall would probably be about 8 to 12-in. thick.
Additional Mass
The amount of brick masonry required in the building, as interior flooring, interior walls, or a fireplace/ chimney assembly, in addition to the thermal storage wall may be determined by using Equation 11.
VAVR-VTW (11)