Gas Vs Electric Vs Hydronic Makeup Air Light Commercial Decision Matrix
Why this matters
Makeup air (MUA) replaces the air that exhaust systems pull out of a light-commercial space, a restaurant hood, a paint booth, a manufacturing exhaust, so the building does not go negative, doors stay usable, and combustion appliances vent properly. The heating source for that tempered makeup air is a real fork: direct or indirect gas, electric resistance, or hydronic coil. Pick wrong and the operator either pays a punishing energy bill, fails a code inspection on indirect-versus-direct fired requirements, or starves a kitchen of the airflow the hood needs to capture grease-laden vapor. Because MUA units run during all occupied hours moving large volumes of outdoor air, the heating-source choice drives both first cost and a large recurring operating cost, and it interacts with available utilities and code. This matrix lays out where each source wins so the selection is defensible to the customer and the inspector.
The options
Direct gas-fired MUA burns gas in the airstream; combustion products enter the space. Very high thermal efficiency (near 92 percent plus, little flue loss) and strong temperature rise from one stage, but limited by code to spaces and applications where products of combustion in the supply air are permitted and where dilution and CO limits are met.
Indirect gas-fired MUA burns gas in a heat exchanger and vents combustion products outside; the airstream stays clean. Lower efficiency than direct (flue and jacket losses), higher first cost for the heat exchanger and venting, but acceptable wherever combustion products cannot enter occupied or process air.
Electric resistance MUA heats with elements in the airstream. Simple, clean air, no venting or gas piping, but 1:1 efficiency and very high operating cost for the large continuous loads MUA represents; often limited by available electrical service.
Hydronic MUA uses a hot-water coil fed by a boiler or other plant. Clean air, no local combustion, modulates well, and leverages an existing boiler plant, but depends on having that plant and adequate water temperature, and adds pumping and coil freeze-protection requirements.
When gas wins
Direct gas-fired wins where a large heating load runs many hours, gas service is present and affordable, and the application permits products of combustion in the supply air (many industrial and some commercial spaces with adequate dilution). Its near-flueless efficiency makes it the lowest operating cost for high-volume, cold-climate makeup air.
Indirect gas-fired wins where the airstream must stay free of combustion products, kitchens serving occupied dining, healthcare, food processing, or any space where code or the owner requires clean supply air, but gas is still the economical fuel and electric service cannot carry resistance heat. It is the common default for restaurant hood makeup air in cold climates.
When electric wins
Electric resistance wins where the heating load is small or the climate mild, where no gas service exists and bringing it in is cost-prohibitive, or where venting is impractical (interior penthouse units, tight rooftops). It also wins where the electrical service has spare capacity and the operator values the simplest possible installation with no combustion, no flue, and no gas-code involvement. In all-electric buildings or jurisdictions discouraging new gas, electric MUA may be the only compliant heating source despite its operating cost.
When hydronic wins
Hydronic wins where a boiler plant already exists with capacity and reasonable water temperature, so the MUA coil is a low-first-cost add that runs on the building's most efficient heat source. It wins in larger light-commercial and mixed-use buildings that already heat hydronically, in retrofits where extending the loop is cheaper than new gas or a service upgrade, and where modulating, even-temperature makeup air is desired. It requires freeze protection (glycol, a preheat coil, or face-and-bypass and pump control) because a large outdoor-air coil can freeze on a cold morning.
Field decision flow
Ask in this order:
Does the application permit products of combustion in the supply air? No (occupied dining, healthcare, food, process clean-air): rule out direct gas-fired; choose indirect gas, electric, or hydronic. Yes: direct gas-fired stays on the table.
Is there an existing boiler plant with spare capacity and adequate water temperature? Yes: hydronic is usually the lowest-operating-cost clean-air option; price the coil and freeze protection. No: continue.
Is gas service present and affordable, and is the heating load and runtime large? Yes: gas (direct if permitted, indirect if clean air required) is usually the lowest operating cost. No: continue.
Is the heating load small or the climate mild, or is electric the only compliant fuel in this jurisdiction? Yes: electric resistance, sized to the available service. Confirm the panel can carry the continuous load.
Confirm makeup-air volume against the exhaust it replaces and the building pressurization target before sizing any heat source; the airflow requirement is fixed by the exhaust, and the heat source only tempers it.
Sizing and code notes
Size MUA airflow to the exhaust it replaces plus the desired slight positive pressurization; under-sizing makeup air starves the hood and pulls the building negative, defeating combustion venting and door operation. Temperature rise is set by the design outdoor temperature and the discharge target (often near room temperature or a tempered setpoint); confirm the chosen source can deliver the rise at design conditions. Indirect-versus-direct fired use, CO limits, and clean-air requirements are governed by the mechanical and fuel-gas codes and the application; verify the local adoption. Hydronic outdoor-air coils need explicit freeze protection in cold climates.
References
- ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality (makeup air and ventilation)
- ASHRAE Standard 90.1, Energy Standard for Buildings (heating-source efficiency and economizer interactions)
- NFPA 96, Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations (kitchen makeup air)
- NFPA 54 (National Fuel Gas Code), direct and indirect gas-fired equipment and venting
- ACCA Manual N, Commercial Load Calculation (makeup-air and ventilation loads)