When a Building Needs More Fresh Air
DOAS projects usually start with complaints: meeting rooms that feel stuffy by 2pm, CO2 readings above target, humidity that swings with the weather, or a tenant whose lease requires better indoor air quality. Sometimes the trigger is a fit-out that adds occupants beyond what the original ventilation was designed for.
A dedicated outdoor air system (DOAS) brings in fresh outdoor air, filters it, conditions it and delivers it to occupied spaces, separately from the rooftop units that handle heating and cooling.
DOAS vs Make-Up Air Unit?
| DOAS | Make-up air unit | |
|---|---|---|
| Purpose | Ventilation for people | Replace air exhausted by hoods or garages |
| Conditioning | Heating, cooling, dehumidification | Usually heating only |
| Energy recovery | Common | Rare |
| Typical buildings | Offices, clinics, schools | Restaurants, commissaries, garages |
Kitchens and garages need a make-up air unit. Offices, clinics and schools with IAQ needs usually need a DOAS.
Does a DOAS Replace My RTU?
No. Your RTUs keep doing heating and cooling. The DOAS takes over ventilation, which often helps the RTUs: they no longer have to cool or heat large volumes of raw outdoor air through their economizers. Many buildings add a DOAS rather than oversizing RTU replacements.
What Does Energy Recovery Save?
An energy recovery wheel or plate core (ERV) transfers heat, and with wheels moisture, between exhaust air and incoming fresh air. In a Toronto January, that means incoming air arrives much warmer than outdoors; in a humid July, it arrives drier and cooler. The DOAS and RTUs both work less.
Sizing and Controls
We size outdoor air to ASHRAE 62.1 for your occupancy, then add CO2 demand control so ventilation drops when rooms empty. Filtration up to MERV 13 suits clinics and offices with IAQ requirements.
Cost and Incentives
DOAS projects are quoted per site because airflow, recovery type and controls vary. You get a written quote within 24 hours of the site assessment. Energy recovery and high-efficiency equipment can qualify for incentives; see Save on Energy retrofits. We install DOAS units for office parks in Markham, clinics in North York and schools across the GTA, often alongside a new RTU installation.
