| Parameter | No Recovery | With Selected Recovery | Unit |
|---|
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
Book a Demo →This DOAS sizing calculator estimates a dedicated outdoor air system from occupancy and design conditions. Enter the floor area, space type (ASHRAE 62.1 Table 6-1 rates and default density), Ez, summer and winter outdoor conditions, site elevation, supply temperature and leaving dew point, return air conditions, and the energy recovery device; the tool returns the outdoor air CFM, design occupants, total and sensible cooling coil capacity, heating coil MBH, and the cooling tons energy recovery saves.
A DOAS conditions ventilation air separately from space sensible cooling, which improves humidity control and pairs well with fan coils, VRF, chilled beams, or radiant systems. The summary table contrasts the system with and without energy recovery so the value of an ERV is clear.
| Quantity | Equation |
|---|---|
| Outdoor air (62.1) | Vbz = Rp · Pz + Ra · Az; Pz = D · Area ÷ density; Vot = Vbz ÷ Ez |
| Outdoor humidity | W from DB/WB at site pressure (ASHRAE Fundamentals Ch. 1, Eq. 33; Hyland–Wexler saturation) |
| Recovery-conditioned air | T = T‑OA − εs(T‑OA − T‑RA); W = W‑OA − εL(W‑OA − W‑RA) |
| Total coil load | BTU/hr = 4.5 × CFM × (h‑entering − h‑leaving) |
| Sensible coil load | BTU/hr = 1.08 × CFM × (T‑entering − T‑supply) |
| Heating coil | BTU/hr = 1.08 × CFM × (T‑supply − T‑entering) |
Outdoor air follows the ASHRAE 62.1-2019 zone calculation (Table 6-1 rates; Vbz = Rp·Pz + Ra·Az, Voz = Vbz ÷ Ez). For a 100% outdoor air system the intake is the sum of the zone Voz values; the occupant diversity D belongs to the multiple-zone recirculating equation, so leave it at 100% unless you are deliberately applying 62.1’s short-term averaging. Table 6-1 rates are based on standard air (0.075 lb/ft³); the table permits, but does not require, adjusting for actual density, and holding the mass flow is the conservative reading. So the OA here is standard CFM and the coil loads use the standard-air factors 4.5 (= 60 × 0.075) and 1.08 (= 60 × 0.075 × 0.24) without altitude scaling; site elevation changes the psychrometric state (humidity ratio from wet bulb and dew point at local pressure) and the actual CFM the fan moves. The coil dehumidifies to the dew point you enter (never adding moisture). A wet coil leaves air at about 90% RH, roughly the dew point + 3 °F, so when the supply dry-bulb is warmer than that — a neutral-air DOAS — the coil load is taken to dew point + 3 °F and the summer reheat back up to the supply dry-bulb is reported separately. Moist-air enthalpy is h = 0.240·T + W·(1061 + 0.444·T). Energy recovery assumes balanced supply and exhaust flow at the entered return conditions; the ERV options apply one effectiveness to both temperature and humidity (εs = εL), the HRV transfers sensible heat only, and in summer recovery is bypassed when it would add load (outdoor enthalpy below return for an ERV, outdoor dry-bulb below return for an HRV). Use the manufacturer’s AHRI 1060 sensible and latent ratings for a selection. If the wheel can be stopped or bypassed for frost control, size the heating coil for the no-recovery value shown in the summary.