Theoretical Chimney Draft
Natural draft from the hot-vs-cold density difference.
Example
You enter
- Stack height (ft) 30
- Ambient temperature (°F) 60
- Mean flue-gas temperature (°F) 400
- Barometric pressure (psia, altitude-corrected) 14.7
- Net-available factor (0.5-0.8) 0.6
You get
- Theoretical draft 0.174 in wc
- Net-available draft (estimate) 0.105 in wc
Checked against published worked examples from: ASHRAE Handbook HVAC Systems / NFPA 211.
Details, formula, and sources
ScopeD_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 or tableT_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.
Source and editionASHRAE Handbook HVAC Systems (chimney/vent) / NFPA 211 theoretical chimney draft, by name.
Free accessThe theoretical-draft relation is standard chimney/vent design practice; the venting standard and appliance instructions govern the actual sign-off.
What governsEstimate. AHJ and licensed professional govern.
API field namesstack_height_ft, ambient_temp_f, mean_flue_temp_f, baro_psia, net_factor, draft_theoretical_inwc, draft_net_inwc
- No-flow theoretical D_t is the no-flow draft; net available after flow/fitting losses is about 0.5 to 0.8 of itASHRAE Handbook HVAC Systems
- Altitude and absolute temps barometric pressure must be altitude-corrected; temperatures are absolute (Rankine), T_m the mean flue tempASHRAE / NFPA 211