Make-up air units: direct-fired and indirect-fired

A make-up air unit (MUA) replaces air that is exhausted from a building. Kitchen hoods, paint booths, industrial process exhaust, parking garage fans and large general exhaust systems all remove air that must come from somewhere. Without make-up air, the building goes negative: doors are hard to open, exhaust hoods underperform, combustion appliances can backdraft, and cold outdoor air leaks in through every crack.

In Canada, the outdoor air an MUA brings in must usually be heated for much of the year, sometimes from -30°C (-22°F) or colder. Most gas-fired MUAs are either direct-fired or indirect-fired, and the difference matters for efficiency, safety, cost and where the unit is allowed.

Direct-fired make-up air units

In a direct-fired unit, a gas burner sits directly in the airstream. Outdoor air passes over the flame, and all the products of combustion, mainly carbon dioxide and water vapour, with small amounts of carbon monoxide and nitrogen oxides, enter the supply air.

Advantages

• Very high heating efficiency. Nearly all the fuel's heat goes into the air, because there is no heat exchanger or flue losing heat.
• High temperature rise and wide modulation, allowing control from mild to severe cold weather.
• Simple construction with no heat exchanger to crack or corrode.
• Lower cost for a given capacity.

Limitations

• Combustion products enter the space, so units are designed and certified to limit concentrations. Product standards for direct gas-fired heaters set limits on CO, CO2 and NO2 added to the airstream.
• Traditional direct-fired make-up air heaters are designed for 100 percent outdoor air and must not recirculate building air. Recirculating direct-fired heaters exist under a separate product standard, with additional requirements for ventilation and controls, and are mainly used in industrial and warehouse spaces.
• Moisture added by combustion can contribute to condensation in cold buildings.
• Not appropriate for many occupancies, such as sleeping areas, and use may be restricted by the authority having jurisdiction.

Direct-fired units are certified to the applicable ANSI Z83 / CSA standard for their type and installed under CSA B149.1.

Indirect-fired make-up air units

In an indirect-fired unit, combustion happens inside a heat exchanger. The flue gases are vented outdoors, and the supply air is heated by passing over the outside of the heat exchanger. No combustion products enter the airstream.

Advantages

• Clean supply air suitable for most occupancies.
• Recirculation allowed, so the same unit can serve as a heating and ventilation unit with mixed return and outdoor air.
• Can be combined with cooling in packaged units.

Limitations

• Lower efficiency than direct-fired, because of flue losses. Typical non-condensing units operate around 80 percent thermal efficiency; condensing units reach the 90s but need condensate management.
• Heat exchanger stress from large temperature rises and cold outdoor air entering the unit can cause condensation and corrosion if not designed and controlled correctly.
• Higher cost and weight per unit of capacity.
• A flue and vent termination are required.

Comparison

Electric and hydronic make-up air units are also common, especially where gas is unavailable or for electrification. Heat pump make-up air units are increasingly available but face challenges heating 100 percent outdoor air at very low temperatures.

Sizing a make-up air unit

Airflow

Start with the exhaust. List every exhaust system the MUA must offset and its airflow, including diversity if some exhaust systems do not run at the same time. Then decide how much of that airflow the MUA should supply. In most commercial applications, the target is a slightly negative or neutral building relative to outdoors, with the space containing contaminants (such as a kitchen) slightly negative relative to adjacent spaces. Our guide to building pressurization covers the balance.

Heating capacity

Heating capacity is based on the airflow and the temperature rise from the outdoor design temperature to the supply temperature. Use the winter design temperature from the National Building Code climatic data or the applicable provincial code.

A simple sensible heat estimate in SI units is: heat (kW) = 1.2 x airflow (m3/s) x temperature rise (K), adjusted for air density at altitude. In Calgary and other high-elevation locations, air density is noticeably lower, which affects both fan performance and heating capacity.

As an example, heating 2.0 m3/s (about 4,240 cfm) from -30°C to 18°C requires roughly 1.2 x 2.0 x 48, or about 115 kW (about 393,000 Btu/h) of delivered heat at sea level.

Supply temperature

Supply temperature is often lower than room temperature in kitchens and industrial spaces where internal gains are high, which saves energy. In spaces without large internal gains, supply temperature must be close to room temperature to avoid discomfort.

Controls

1. Interlock with exhaust. The MUA should run whenever the exhaust it serves is running. In kitchens, this is usually interlocked with the hood fans.
2. Modulating burner control. Burners should modulate to hold discharge temperature as outdoor temperature changes. Simple on-off control causes cold blasts and overheating.
3. Variable airflow where exhaust varies. Demand-controlled kitchen ventilation and variable industrial exhaust need matching variable make-up air.
4. Safety controls. Flame safeguard, high-limit, airflow proving switches and, for direct-fired units, controls that ensure the unit only operates with adequate outdoor air.
5. Building pressure monitoring in critical applications.

Commercial kitchens

Kitchen exhaust hoods are governed by NFPA 96 and local code requirements, and make-up air is a key part of making them work. Too little make-up air causes hood spillage and smoke escaping into the dining room. Make-up air supplied too close to the hood, or at high velocity, can disrupt the capture plume. Our guide to commercial kitchen ventilation and NFPA 96 covers hood design and air distribution.

Whether a direct-fired unit is accepted for a kitchen depends on the authority having jurisdiction, local codes and the occupancy. Indirect-fired units are the more common choice where the kitchen shares air with dining or other occupied areas.

Testing and balancing

An MUA should be tested and balanced together with the exhaust systems it serves. That includes measuring exhaust and supply airflow, verifying building and space pressure relationships, confirming burner modulation and discharge temperature, and checking safety interlocks. A testing and balancing report should show the measured airflows against design.

Frequently asked questions

Can a direct-fired unit be used in an office?

Generally not. Direct-fired make-up air units are most common in industrial, warehouse and some commercial kitchen applications. Codes and authorities restrict their use in many occupancies, and indirect-fired or other heating is normally required for offices.

Why is my kitchen cold in winter?

Common causes include make-up air units that are off, set to low discharge temperature, or not modulating; untempered make-up air delivered near the hood; or exhaust running when the kitchen is not cooking. An airflow and temperature check usually identifies the cause.

Do make-up air units need energy recovery?

Heat recovery is difficult with greasy kitchen exhaust and some industrial exhaust because of fouling. In cleaner exhaust streams, energy recovery can reduce make-up air heating significantly.

Key takeaways

• Make-up air units replace exhausted air and must heat it in Canadian winters.
• Direct-fired units are highly efficient but release combustion products into the airstream and are restricted in where they can be used.
• Indirect-fired units keep combustion products out of the air and allow recirculation, at lower efficiency and higher cost.
• Size from the exhaust airflow, the winter design temperature and the required supply temperature, adjusted for altitude.
• Interlock MUAs with exhaust, use modulating controls, and verify airflow and pressure with air balancing.