Tempered Make-Up Air Units
Every cubic foot of air an exhaust system pulls out of a building has to be replaced, and a tempered make-up air unit controls the temperature of that replacement air before it enters the space. Without tempering, outdoor air floods in through doors, louvers, and envelope gaps at whatever temperature the weather delivers.
Negative pressure builds. Doors fight back. Heating and cooling systems burn fuel trying to compensate.
Tempered air falls between untempered and fully conditioned. It is heated or cooled enough to prevent freeze damage, condensation, and comfort problems, but it is not held to a precise setpoint or humidity level.
RWA Ventilation Solutions specs tempered MUA across every heat source and cooling method available from US manufacturers, matching the configuration to the exhaust load, climate, and fuel supply at the site. Every unit ships with the manufacturer’s warranty behind it.
Have questions about Tempered MUA Units? Need a price quote?
Key Takeaways
Tempered vs. Untempered vs. Conditioned Air
Tempered air is outdoor air heated or cooled to a moderate range, typically 55-68°F, before entering the building.
It prevents freeze damage, condensation, and cold drafts without the precise setpoint and humidity control that define conditioned air.
Untempered air enters at whatever the outdoor temperature happens to be, with no thermal treatment at all. The distinction matters because each category drives a different equipment selection, different energy load, and different code obligation.
| Attribute | Tempered | Untempered | Conditioned |
|---|---|---|---|
| Discharge temperature | 55-68°F | Outdoor ambient | 68-75°F (heating) / ~55°F (cooling) |
| Humidity control | No | No | Yes |
| Typical equipment | Gas-fired or hydronic make-up air unit | Gravity intake or unheated fan unit | Full AHU with DX or chilled water coil |
| Code purpose | Freeze protection, comfort, pressure balance | Ventilation only (mild climates) | Full IAQ and thermal comfort (ASHRAE 55) |
Facilities that exhaust large volumes but operate in mild climates may only need untempered replacement air.
Tempered make-up air units divide into three families based on whether the application needs heating, cooling, or both. Climate zone, exhaust volume, and process load determine which family fits.
Heated Only
The most common configuration. Outdoor air is warmed before delivery to offset winter intake temperatures.
- Direct gas-fired (most widely specified)
- Indirect gas-fired (when combustion byproducts cannot enter the airstream)
- Electric (where gas is unavailable or prohibited)
- Hot water (tied to existing hydronic systems)
- Steam (industrial plants with steam infrastructure)
- Recirculating (blends return air with outdoor air to raise supply temperature)
Cooled Only
Treats outdoor air before delivery when warm-season temperature or humidity would overload the space.
- DX cooling (self-contained refrigerant circuit)
- Chilled water (connected to a central chiller plant)
- Evaporative cooling (effective in dry climates)
- Ammonia cooling (industrial cold storage and process applications)
- Recirculating cooling (uses return air to reduce outdoor air load)
Heated and Cooled
Combines both treatments in a single unit for year-round tempering.
- Rooftop packaged systems (heating and cooling in one cabinet, curb-mounted)
- DOAS (dedicated outdoor air systems that decouple ventilation from space conditioning)
- Direct gas-fired with DX cooling (gas heat plus refrigerant-based cooling in one unit)
Code and Energy Cost Factors
Mechanical codes set specific triggers for when replacement air must be tempered. The energy cost of ignoring those triggers scales with every cubic foot of untempered air the building pulls in.
The energy penalty is measurable. For every 1,000 CFM of untempered outdoor air entering a heated building, annual gas consumption rises by roughly 2,100 therms in a 6,500 HDD climate.
At current rates, that is $2,000 to $2,600 per year. A facility exhausting 5,000 CFM faces $10,000 to $13,000 annually, before equipment strain or comfort complaints.
Warmer climates trade heating penalties for cooling loads when summer intake arrives at 95°F or above.
Specifying the Right Tempered Make-Up Air Unit
The spec starts with four inputs: exhaust airflow being replaced, climate data for the site, available fuel, and process load requirements. Those inputs determine the heat source, the cooling method, and the unit configuration. Getting one wrong means rework, change orders, or a system that never performs.
RWA carries tempered MUA from multiple US manufacturers across every configuration in the taxonomy. No single-brand bias. The recommendation matches the application, not a product catalog.
Every unit ships with the manufacturer’s full warranty.
Call 847-262-3457 ext. 23 to start the spec process, or submit a request through the contact form with your exhaust volume and application details.
FAQ
What does tempered air mean in HVAC?
Tempered air is outdoor air heated or cooled to a moderate range before entering a building. The target is typically 55-68°F. The goal is freeze protection, pressure balance, and occupant comfort without the precise setpoint and humidity control that define fully conditioned air. Most mechanical codes reference a 10°F maximum differential between supply and space temperature.
What is the difference between tempered and conditioned air?
Tempered air receives thermal treatment only. It is heated or cooled enough to prevent problems but is not filtered to IAQ standards or held to a humidity setpoint. Conditioned air is filtered, dehumidified or humidified, and maintained at a precise temperature per ASHRAE Standard 55. Different equipment, different cost, different code obligation.
What temperature range qualifies as tempered air?
Industry practice and IMC Section 505 converge on 55-68°F. Below 55°F risks heat exchanger damage in downstream equipment. Above 68-70°F crosses into conditioned air territory. The practical rule is supply air within 10°F of room temperature, warm enough to prevent cold drafts and condensation but not brought to full comfort setpoint.
When does mechanical code require tempered make-up air?
IMC Section 505 triggers tempering when the makeup air temperature differential exceeds 10°F from space temperature or when the added load exceeds HVAC system capacity. ASHRAE 62.1 requires outdoor air intake matching exhaust volume. In any climate where outdoor air falls below design thresholds, tempering is implied.
Scott Williamson
Scott Williamson is an engineer specializing in industrial ventilation, including make-up air units.
His expertise has been recognized by leading business publications like HubSpot and Tech Bullion. Scott helps business owners find high-performance solutions that balance energy savings with daily operational needs.