Rooftop Make-up Air Unit
A rooftop make-up air unit replaces exhausted air with heated or cooled outside air. Modular units run up to 22,000 CFM at 2 in. W.G. external pressure. Not sure which unit fits your roof? Call 847.674.0000.
Exhaust fans pull air out of the building whether anything replaces it or not. Without make-up air, doors stick, hoods stop capturing smoke, and gas appliances backdraft.
A standard RTU recirculates building air and brings in only a share of outside air. When exhaust outruns that share, the roof needs a unit built for 100% outside air.
Below are the four rooftop types, how they compare to an RTU, and common buyer questions.
Key Takeaways
One of the reasons that RTUs are so popular is because they are self-contained, compact, and easy to install.
Rooftop Make-Up Air Unit Types
Rooftop make-up air units come in four types, set by how each treats outside air. Direct fired units burn gas in the airstream. Indirect fired units heat through a heat exchanger. DX and DOAS units add cooling and humidity control.
The exhaust load and who occupies the space set the choice.
| Type | How it treats the air | Fits |
|---|---|---|
| Direct fired makeup air unit | Gas burns in the airstream, so nearly all the heat reaches the space | Warehouses, plants, high-exhaust process areas |
| Indirect fired make up air unit | A heat exchanger keeps combustion out of the supply air | Kitchens, healthcare, occupied spaces |
| Make-up air unit with DX cooling | A refrigerant coil cools and dries the outside air, with optional gas heat | Spaces that need cooled make-up air |
| DOAS unit | Conditions 100% outside air for ventilation, with humidity control | Schools, offices, labs |
If supply ever runs ahead of exhaust, a pressure relief damper keeps the building from over-pressurizing.
Not sure which rooftop make-up air unit fits your building?
An engineer matches the unit to the words in your spec.
Make-Up Air Unit vs RTU
A make-up air unit and an RTU differ in the air they treat. An RTU, a packaged rooftop unit, mostly recirculates building air. A make-up air unit conditions 100% outside air to replace what exhaust removes.
| RTU | Make-up air unit | |
|---|---|---|
| Air it treats | Mostly return air, some outside air | 100% outside air |
| Main job | Heat and cool the occupied space | Replace air the exhaust removes |
| Sized by | Space heating and cooling load, in tons | Exhaust airflow, in CFM |
| Runs with | The thermostat | The exhaust fans, often interlocked |
An RTU’s heating and cooling are sized for mixed air. Push it to high outside air in winter and the heat exchanger risks condensation and cold supply air.
Buildings with heavy exhaust usually run both. The RTU holds space temperature and the make-up air unit holds building pressure. Compare the full range of make-up air units.
RTUs are known for their energy efficiency, which is essential for reducing operating costs and maintaining a comfortable indoor environment.
Rooftop Make-Up Air Matched to Your Exhaust
RWA carries several manufacturer lines of rooftop make-up air units, so the spec drives the pick.
Our engineers start with the exhaust airflow and the space, then match the heat source, cooling, and controls to it. That includes the interlock with your exhaust fans and the relief path when supply runs ahead.
If an RTU already serves the space, we size the make-up air unit to work alongside it. We spec US-built units backed by the manufacturer’s guarantee.
One call covers the whole rooftop.
Rooftop Make-Up Air Unit Questions
Plain answers on choosing, specifying, and pairing rooftop make-up air units.
Call 847.674.0000 with anything specific to your job.
RTU in HVAC means rooftop unit, a packaged unit that heats and cools a building from the roof. One cabinet holds the fan, cooling coil, compressor, and heat section. Most RTUs recirculate building air, so they are not built to replace large exhaust loads.
A restaurant with an RTU still needs make-up air when the kitchen hood exhausts more than the RTU brings in. International Mechanical Code Section 508 requires make-up air approximately equal to the exhaust air removed. The code also requires mechanical make-up air to run whenever the exhaust runs.
Rooftop make-up air units come from several manufacturer lines, including AbsolutAire, FloAire, Venco, and United Enertech. The lines differ in heat source, cooling options, and airflow range, so the exhaust load decides the line. RWA carries each of them and specs the one that fits.