Makeup-Air Unit Tempering Load (Sensible, Latent, Total)
The heater or coil a makeup-air unit needs.
Example
You enter
- Makeup airflow (cfm, = exhaust) 2000
- Outdoor air temperature (°F) 20
- Target supply temperature (°F) 65
- Heater thermal efficiency (0-1) 0.8
You get
- Temperature rise 45.0 F
- Sensible tempering load 97200 Btu/h
- Latent load 0 Btu/h
- Total load 97200 Btu/h
- Required heater input 121500 Btu/h
Details, formula, and sources
When a kitchen hood or process fan exhausts air, IMC 508 requires a roughly equal makeup supply, tempered so it does not dump raw winter air into the space. Sensible load 1.08 x CFM x dT, optional latent 0.68 x CFM x dW (gr/lb), and the gas input at the burner efficiency - a 2,000 cfm MUA lifting 20 F air to 65 F needs 97,200 Btu/h out and a 121,500 Btu/h burner at 80 percent. Sea-level constants, neutral supply, no duct losses. A design aid, not the engineer's stamped design.
dT_F = t_target_F - t_oa_F; Q_s_btuh = 1.08 x cfm x dT_F; Q_l_btuh = 0.68 x cfm x (w_oa_gr - w_target_gr); Q_t_btuh = Q_s_btuh + Q_l_btuh; input_btuh = Q_s_btuh / eta.
The ASHRAE Fundamentals sensible (Q_s = 1.08 x CFM x dT), latent (Q_l = 0.68 x CFM x dW in gr/lb), and total psychrometric load equations and the IMC 508 makeup-air-equals-exhaust requirement, by name; the sea-level constants 1.08, 0.68, and 4.5 are named constants.
The psychrometric load equations and their sea-level constants are public; the IMC makeup-air requirement is readable free at codes.iccsafe.org.
Estimate. AHJ and licensed professional govern.
Field names used by the API: cfm, t_oa_F, t_target_F, eta, dT_F, Q_s_btuh, Q_l_btuh, Q_t_btuh, input_btuh
- Sensible load Q_s = 1.08 x CFM x (T_target - T_oa) at sea-level air densityASHRAE Fundamentals
- Latent load optional Q_l = 0.68 x CFM x dW with the humidity ratios in grains per poundASHRAE Fundamentals
- Makeup requirement makeup air approximately equal to the exhaust; the burner input is the sensible load over the thermal efficiencyIMC 508