The heating factor for the surfaces in the zone shall be calculated using Equation C-16. (C-15)
where
HLU = Areaopaque × Uow × [HU1 × HDD50 + HU2 × (VS × HDD65)2]
HLUO = Areagrosswall × UO × [HUO1 × HDD50 + HUO2 × HDD65 + HUO3 × EAH × VS × HDD65]
HLXUO = Areagrosswall × {(1/UO) × [HXUO1 × EAH × (VS × HDD50)2 + HXUO2 × EAH × (VS × HDD65)2] + (1/UO2) × [HXUO3 × EAH2 × VS × HDD65]}
HLM = Areaopaque × SHMC × [HM1 + HM2 × G × UO × HDD65 + HM3 × G2 × EAH2 × VS × HDD50 + HM4 × UO × EAH × VS × HDD65 + HM5 × UO × HDD50 + HM6 × EAH × (VS × HDD65)2 + HM7 × EAH2 × VS × HDD65/UO]
HLG = Areagrosswall × {G × [HG1 × HDD65 + HG2 × UO × HDD65 + HG3 × EAH × VS × HDD65 + HG4 × EAH2 × VS × HDD50] × G2 × [HG5 × HDD65 + HG6 × EAH2 × VS × HDD65]}
HLS = Areagrosswall × {EAH × [HS1 × VS × HDD65 + HS2 × (VS × HDD50)2] + EAH2 × [HS3 × VS × HDD50 + HS4 × VS × HDD65]}
HLC = Areagrosswall × [HC1 + HC2 × HDD65 + HC3 × HDD652 + HC4 × VS2 + HC5 × VS × HDD50 + HC6 × VS × HDD65 + HC7 × (VS × HDD50)2]

where
VS = annual average daily incident solar energy on surface
SHMC = sum of the HMC from Equation C-12 for each mass wall in the zone
EAH = effective solar aperture fraction for zone calculated using Equation C-17. (C-17)
ΣSAh = the sum of SAh from Equation C-7 for all vertical fenestration in the zone.
The coefficients used in the above equations depend on the orientation of the surface and shall be found in Table C6.8.3. Terms not defined for Equation C-16 are found under Equation C-14.

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