Makeup-Air Unit Tempering Load (Sensible, Latent, Total)
The heater or coil a makeup-air unit needs: 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.
Formula and source
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.
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