Infiltration Heating / Cooling Load (Sensible + Latent)
The load the air the envelope leaks adds to the design day.
Example
You enter
- Infiltration airflow (cfm) 56.5
- Design indoor-outdoor delta-T (°F) 70
You get
- Sensible load 4,271 Btu/h
- Latent load 0 Btu/h
- Total infiltration load 4,271 Btu/h
Checked against published worked examples from: Texas A&M University (I. Choudhury).
Details, formula, and sources
ScopeSensible Qs = 1.08 x cfm x delta-T and latent Ql = 0.68 x cfm x delta-grains, from the natural infiltration cfm a blower-door test produces. In cooling the latent half (the moisture the leaking air drags in) is nearly as large as the sensible, which is why air-sealing pays back on both design days. The 1.08 and 0.68 are sea-level standard-air constants. The infiltration component only, not a stamped Manual J.
Formula or tableq_sensible = 1.08 x cfm x delta_t_f; q_latent = 0.68 x cfm x delta_gr; q_total = q_sensible + q_latent.
Source and editionASHRAE Handbook of Fundamentals air-side sensible (q_s = 1.08 x Q x dT) and latent (q_l = 0.68 x Q x dW_gr) equations, by name; the 1.08 and 0.68 are the sea-level standard-air constants.
Free accessThe air-side sensible and latent equations are public ASHRAE Fundamentals relations; the 1.08 and 0.68 are standard-air constants.
What governsEstimate. AHJ and licensed professional govern.
API field namescfm, delta_t_f, q_sensible, q_latent, q_total
- Sensible q_sensible = 1.08 x cfm x design dry-bulb delta-T (the 1.08 = 60 x rho x cp at sea-level standard air)ASHRAE Fundamentals
- Latent q_latent = 0.68 x cfm x humidity-ratio difference in grains/lb (zero for heating, so the term drops out)ASHRAE Fundamentals
- Component only the infiltration line of the design load, not the whole Manual Jfirst principles