Makeup air — also called replacement air — is outside air brought into a building to replace air removed by exhaust, from welding stations, paint booths, kitchen hoods, dust collection, or process ventilation. That volume has to come from somewhere. If you don't bring it in deliberately, the building pulls it through whatever openings exist: door gaps, dock seals, and flue vents.
The result is negative pressure, and most facility managers recognize the symptoms before they know the cause. Exterior doors get hard to open. Cold drafts pour in at floor level. Combustion appliances backdraft — a safety issue, not a comfort one. Exhaust fans stop moving their rated airflow because the building can't feed them. Heating bills climb because unconditioned air is entering at uncontrolled points.
A properly sized makeup air system replaces that volume in a controlled way, at a controlled temperature, in a controlled location.
Direct-fired vs. indirect-fired makeup air
A direct-fired unit fires a burner directly in the airstream. Thermal efficiency is very high as nearly all of the combustion heat enters the space.
An indirect-fired unit fires into a heat exchanger and vents combustion products outside, keeping the supply air separate. Indirect gets specified where combustion products in the space are unacceptable. Direct-fired gets specified where efficiency matters and dilution is adequate.
Where a building has significant ceiling stratification, there's a third option that burns no fuel at all — see Energy Jet unfired makeup air.
Radiant Energy designs and supplies makeup air for both commercial kitchen and industrial applications. We solve air quality and negative pressure problems.
Custom features include mix boxes and v-bank filtration to HEPA efficiencies. Units can be equipped with steam, water, or DX coils where cooling is required.
A dedicated outdoor air system conditions ventilation air separately from the equipment handling space heating and cooling. Rather than asking rooftop units to temper outside air alongside their recirculating load, a DOAS treats the outdoor air stream on its own — filtering, heating, cooling, and dehumidifying it before it reaches the space.
The reason to separate them is dehumidification. Equipment sized for sensible cooling load often can't remove the latent load outside air brings with it, especially at part load in a Midwest summer. That's how buildings end up cold and humid at the same time. Handling ventilation air on a dedicated path lets each system be sized for what it's actually doing, and usually allows the primary equipment to be smaller.
DOAS is commonly specified where ventilation rates are high relative to floor area, where humidity control is a process or product requirement, or where a building needs to meet current ventilation standards without replacing the entire HVAC system.
Replacement coils
We supply replacement coils for commercial and industrial air handling equipment: chilled water, hot water, steam, steam distributing, DX evaporator, condenser, and booster coils. Standard construction is copper tube with aluminum fin. For high pressure applications above 200 psig, high temperatures, or corrosive atmospheres, coils are built in stainless steel or 90/10 cupro-nickel.
Most replacement coil work starts as an emergency. A coil fails, the unit is down, and the original manufacturer either no longer builds that model or quotes a lead time the building cannot absorb. Coils are built to match existing dimensions, connection locations, row and fin configuration, and capacity regardless of who made the original. A discontinued air handler does not mean replacing the whole unit.
Where access is tight and a full-length coil will not physically get to the opening, divided coils ship in sections and bolt together in the field with a gasketed joint.
Getting a replacement coil right depends entirely on the field dimensions, and a quarter inch in the wrong place means it will not fit the casing. We will come out and measure the coil ourselves. We take the dimensions, casing depth, row and fin configuration, connection sizes and hand, and material, so the coil arrives correct the first time and drops into the opening without field modification. Send us the location and we will schedule the site visit.
Frequently asked questions
How do I size makeup air for my exhaust?
Makeup air volume is driven by total exhaust CFM, usually targeted slightly below total exhaust so the building stays marginally negative relative to outside. That controls contaminant migration without pulling air in through the building envelope. Actual sizing depends on the exhaust equipment, process requirements, and how the building is zoned. We produce ventilation load calculations and equipment selections for the specific project.
What is the difference between direct-fired and indirect-fired makeup air?
A direct-fired unit fires a burner in the airstream. Thermal efficiency is very high because nearly all combustion heat enters the space, but products of combustion enter with it, so the application requires adequate dilution. An indirect-fired unit fires into a heat exchanger and vents combustion products outside, keeping supply air separate. Indirect is specified where combustion products in the space are unacceptable.
What is a DOAS and when do I need one?
A dedicated outdoor air system conditions ventilation air on its own path rather than asking the primary HVAC equipment to handle it alongside the recirculating load. It is specified where ventilation rates are high relative to floor area, where humidity control matters to the process or product, or where a building needs to meet current ventilation standards without a full system replacement.
Do I need carbon monoxide or NO2 detection?
Facilities running combustion equipment or vehicle traffic indoors, such as warehouses with propane forklifts, vehicle maintenance bays, and loading docks, commonly require CO and NO2 detection tied to ventilation control. Requirements depend on the occupancy and applicable code. Your mechanical contractor or engineer of record makes that determination, and we supply and help specify the detection and control equipment.
Can I get a replacement coil if the original manufacturer no longer makes it?
Yes. Replacement coils are built to match existing dimensions, connection locations, row and fin configuration, and capacity. The original manufacturer does not need to still offer the part, or still exist.
How do you make sure a replacement coil fits?
We come out and measure it. Field dimensions are what determine whether a coil drops into the opening or needs modification on site, so we take the measurements ourselves rather than working from what is on a nameplate.
What is makeup air?
Makeup air, also called replacement air, is outside air brought into a building to replace air removed by exhaust. Without it, the building goes negative and pulls replacement air through door gaps, dock seals, and flues.
How do I know if my building has a negative pressure problem?
Common signs: exterior doors that are hard to pull open or that slam, cold drafts at floor level near entries, combustion equipment that backdrafts or trips, exhaust fans that do not move the air they should, and heating costs that seem high for the space.
Can makeup air be added without increasing heating costs?
In buildings with significant ceiling stratification, an unfired system can temper incoming air using heat already at the roof deck rather than burning fuel. See our Energy Jet unfired makeup air systems.
We carry replacement parts to get your system back up and running as soon as possible.
We've compiled answers to the most commonly asked questions. If you need help, start here.
Send us a project and we'll produce the equipment layout, specifications, and energy comparison you need. No design fee for mechanical contractors.
