Duct Heat Gain/Loss Through Unconditioned Space
Duct heat gain or loss through unconditioned space, and the supply-air temperature change.
Example
You enter
- Duct insulation R (h-ft2-F/Btu) 4
- Duct surface area (ft²) 100
- Ambient minus in-duct temp (F, signed) 65
- Airflow (cfm) 1000
You get
- Heat gain (+) / loss (-) 1625 Btu/h
- Air temperature change 1.50 F
Details, formula, and sources
Conductive duct heat gain/loss Q = U A dT (U = 1/R) and the resulting supply-air temperature change dT_air = Q/(1.08 CFM). 1,000 CFM of 55 F air in R-4 duct, 100 ft^2, a 120 F attic warms 1.5 F; R-8 halves it. Steady-state conduction, no leakage. A design aid.
U = 1/R_duct; Q = U A dT (Btu/h); dT_air = Q / (1.08 x CFM).
The conductive duct heat-gain/loss relation from the ASHRAE Handbook - Fundamentals duct-design method, by name.
The Q = U A dT conduction relation and the 1.08 sensible-air constant are standard ASHRAE / duct-design values. The ductwork design and ambient conditions govern.
Estimate. AHJ and licensed professional govern.
Field names used by the API: R_duct, A_ft2, dT_F, cfm, Q_btuh, dT_air
- Conduction Q = U A dT, U = 1/R_duct, mean dT across the runASHRAE Fundamentals / duct design
- Air-temp change dT_air = Q / (1.08 x CFM), the sensible-air constantASHRAE
- No leakage/latent conduction only; no air leakage, radiant gain, or latent transferscope of this tile