Direct Fired Industrial Make-Up Air Heaters
Direct fired make-up air heaters put 100% of the burner’s heat into the air they supply. The heaters below move 600 to 92,000 CFM, with burners up to 2,500,000 BTU.
Every model runs on natural gas or propane and installs indoors or outdoors.
Each exhaust fan pulls air out of your building, and something has to replace it.
Without make-up air, doors get hard to open, fans fall short of their rated CFM, and cold air leaks in at every gap. A direct fired heater brings that air back in already warm.
Not sure which direct fired heater fits your building?
Our engineers match the heater to your exhaust load and winter temperature.
This efficiency leads to significant cost savings over other methods in both equipment costs and operating costs.
Six Direct Fired Heater Types and Where Each One Fits
Airflow and air source sort these heaters into six types. Non-recirculating heaters bring in 100% outside air to replace what your exhaust fans remove. Recirculating heaters mix room air back in, so one unit heats the building and ventilates it.
Compact Low CFM Direct Fired Heater
Small exhaust loads, such as a single spray booth or a small shop.
Vertical Industrial Direct Fired Heater
Mid-size exhaust loads where floor or roof space is tight.
Modular Direct Fired Heater
Mid-size loads that need a V-bank filter section or an evaporative cooler intake.
Horizontal Industrial Direct Fired Heater
Large exhaust loads in warehouses, distribution centers, and plants.
Horizontal Industrial Direct Fired Recirculating Heater
Large open buildings that need space heat and ventilation from one unit.
Vertical Industrial Direct Fired Recirculating Heater
The widest airflow range here, for large buildings heated with mixed room and outside air.
Not sure which of the six heater types fits your exhaust fans?
Our engineers match the type, airflow, and burner size to your building.
Is a Direct Fired Heater Right for Your Building
Direct fired heaters release combustion products into the air they supply. ANSI Z83.4 limits them to industrial and commercial buildings with no sleeping quarters. Inside those limits, all of the burner's heat goes into the supply air, with no flue losses.
Good fit for direct fired
- Warehouses and distribution centersDock doors and exhaust fans pull in cold air all winter.
- Manufacturing plantsProcess exhaust removes heated air that has to come back warm.
- Paint boothsBooth exhaust runs at high CFM and needs tempered replacement air.
- Buildings with heavy exhaust loadsAll the burner heat goes into the replacement air, with no flue carrying part of it away.
Choose indirect fired instead
- Any space with sleeping quartersANSI Z83.4 rules out direct fired heaters here.
- Healthcare facilitiesPatient areas need supply air free of combustion products.
- Pharmaceutical productionClean rooms cannot take burner byproducts in the airstream.
- Moisture-sensitive stockBurning gas adds water vapor to the supply air.
Not sure which side your building falls on? See indirect vs direct fired make-up air, or call 847-674-0000.
Combustion products, mostly carbon dioxide and water vapor, enter the space along with the heat. Safety controls hold each unit to the limits in ANSI Z83.4 and Z83.18.
Recirculating models can return up to 85% room air under those standards, so outside air never drops below 15%.
What Size Direct Fired Heater Do You Need
Two numbers size a direct fired heater, your exhaust CFM and your winter temperature rise. Heater airflow matches the air your exhaust fans remove. Temperature rise is the gap between your design outdoor temperature and the supply air temperature.
Burner output follows the standard sensible heat formula, BTU per hour = 1.08 × CFM × temperature rise. A building exhausting 10,000 CFM that heats 0°F outside air to 65°F needs 1.08 × 10,000 × 65, or 702,000 BTU per hour. The 1.08 factor assumes standard air at sea level, so high-altitude jobs need a correction.
Send us your exhaust CFM and winter design temperature. We will match the heater type and burner size, and you can reach us at 847-674-0000.
Adding Summer Cooling to a Direct Fired Unit
A direct fired heater can carry a cooling section for the summer months. The three options are DX cooling, chilled water coils, and evaporative cooling. On this page, the Modular heater takes an evaporative cooler intake module.
Evaporative cooling works best in dry climates, since it cools air by adding moisture. DX and chilled water cool without adding humidity, which suits humid summers.
Direct Fired Make-Up Air Heater Questions
Plain answers on safety, sizing, and recirculation.
Call 847-674-0000 with anything specific to your job.
Yes, when the heater is certified to ANSI Z83.4 or Z83.18 and installed to local code. Those standards set the safety limits for combustion products in the supply air. They also restrict direct fired heaters to industrial and commercial buildings with no sleeping quarters.
Multiply 1.08 × CFM × temperature rise to get BTU per hour. A 10,000 CFM heater warming 0°F outside air to 65°F needs about 702,000 BTU per hour. The 1.08 factor assumes standard air at sea level.
Recirculating direct fired heaters, built to ANSI Z83.18, mix room air with outside air. They can return up to 85% room air, so at least 15% of the supply is always outside air. Non-recirculating heaters, built to ANSI Z83.4, supply 100% outside air.
Authorized Distributor for top USA Manufacturers
We are the official distributors of the USA’s top-rated make-up air unit manufacturers. As official representatives, we’ll be happy to help explain the best make-up air unit for your application. Contact us for a quote or any questions regarding industrial or commercial MUA.
Some manufacturers we work with for make-up air equipment include AbsolutAire and FloAire.
Make-Up Air Units Installed
