Makeup Air Unit

Overview

A makeup air unit is a dedicated outdoor-air conditioning appliance installed when the building exhaust fans (kitchen hoods, bathroom vents, dryers, dust collectors) remove more air than infiltration naturally replaces. Rather than let the building depressurize and suck unconditioned air through envelope leaks, a makeup unit deliberately supplies filtered, heated outdoor air, "makeup", to rebalance the building and maintain indoor comfort and pressure.

The unit is compact and self-contained: a cabinet that holds a supply fan, heating section, filter stage, motorized intake and relief dampers, and a simple control loop. Sizes range from 0.5 m³/s for a single-family home with a strong range hood, to 20 m³/s (72,000 m³/h) for a commercial kitchen or industrial spray booth. Gas-fired units are common where heating is plentiful; electric units suit mild climates or where gas is unavailable.

Airflow path

Outdoor air is drawn in through a Intake Damper that opens modulating amounts based on building pressure or manual demand. The Pleated Prefilter catches outdoor dust, pollen, and debris, essential in urban or agricultural areas. The air then passes through the Heat Exchanger, where it is warmed by either a gas flame in a sealed-combustion chamber or by electric Heating Element banks. A Outlet Mixing Damper blends cooler bypass air with the heated stream to prevent overshoot, and a Limit Thermostat caps discharge temperature for safety. The Supply Fan Section then pushes the conditioned air into the building supply ductwork at 150–500 Pa.

Heating: gas versus electric

Gas-fired units burn natural gas or propane in a Burner Head, drawing combustion air via a Combustion Blower in a sealed-combustion design. The Gas Shutoff Valve is solenoid-operated and springs closed on loss of Flame Sensor signal, a fail-safe interlock. The Gas Pressure Regulator holds gas pressure stable at the burner. Electric units use banks of 5–10 kW resistance heating elements switched by the Control Board; they are simpler and quieter but consume more electricity.

Cabinet and isolation

The Insulated Cabinet is a double-wall Steel Frame, typically 1.5–3 m long and 0.8–1.2 m tall, with 25–50 mm foam Foam Insulation for thermal and acoustic performance. Access doors at the Access Door points allow filter replacement and burner cleaning. The fan and motor sit on Vibration Isolator elastomer pads to decouple vibration from the building structure; modern units are designed to be quiet enough to run during meal service in a restaurant.

Controls and integration

Simple units run on a single thermostat dial: user sets discharge temperature (say, 35 °C), the controller modulates the heating valve or electric element to hold it, and the intake damper either opens wide or closes in response to a room thermostat. More sophisticated units accept a 4–20 mA signal from a building management system, or integrate via Modbus or BACnet, allowing the building automation system to coordinate makeup supply with exhaust load and building static pressure. The Control Board executes the sequence: ignition lockout during startup, ramp-up of fan speed over 10–30 seconds, modulation of the heating source, and shutdown with purge. A Discharge Temperature Sensor measures actual outlet temperature; a Airflow Sensor can signal low airflow as a clogged filter.

Installation and interconnection

The unit is mounted horizontally in an attic, plenum, or mechanical room, or vertically on a roof pad. The Supply Collar connects to supply ductwork via Flexible Insulated Duct and Duct Vibration Hanger; the Relief Collar ties to a relief or exhaust duct that vents displaced air back to the outdoors, typically through the building roof or a wall louver. A gas-heated unit requires a Gas Shutoff Valve supply line and exhaust vent for combustion products. All electrical connections are field-wired on-site; thermostats and controls are commissioned to match the building envelope and exhaust load.