Theoretical Chimney Draft
Natural draft from the hot-vs-cold density difference: D_t = 0.52 x B x H x (1/T_o - 1/T_m) with temperatures absolute (Rankine), in inches of water column, plus a net-available estimate (factor 0.5-0.8). This is the no-flow draft; the barometric pressure must be altitude-corrected (thinner air cuts the draft) and T_m is the mean flue temp. A design aid, not a venting sign-off.
Formula and source
T_o_R = ambient_temp_f + 460; T_m_R = mean_flue_temp_f + 460; D_t = 0.52 x baro_psia x stack_height_ft x (1/T_o_R - 1/T_m_R) [in wc]; D_net = net_factor x D_t.
ASHRAE Handbook HVAC Systems (chimney/vent) / NFPA 211 theoretical chimney draft, by name.
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