Sizing Matters
A unit heater must match the real heat load of the space. Oversizing can short cycle the burner. Undersizing can leave workers cold and materials slow to cure. A good estimate starts with the room volume, exposed surfaces, insulation level, and design temperature difference. These items describe how fast heat leaves the building envelope.
Heat Loss Basics
Walls, roofs, floors, windows, and doors lose heat by conduction. Each surface uses a U value. A higher U value means faster heat loss. Air leakage is different. It depends on volume and air changes per hour. Drafty loading bays need more capacity than sealed offices. This calculator combines both paths and then adds the selected safety margin.
Construction Use
Construction spaces often change daily. Doors open. Temporary partitions move. Concrete may still be cold. Crews may need quick warm up before work starts. Use a higher air change rate for open work. Use a larger safety margin for early project stages. Use better insulation settings only when the envelope is finished and sealed.
Output Capacity
The main result is required output capacity in BTU per hour. The tool also converts that value to kilowatts. If you enter heater efficiency, it estimates the input capacity that the fuel system must supply. This helps compare gas, propane, and electric options. It also helps divide capacity across several smaller heaters.
Airflow and Placement
Unit heaters need enough airflow to move warm air through the zone. The airflow estimate uses the chosen discharge temperature rise. A lower rise means more airflow. A higher rise means less airflow but hotter discharge air. Avoid blowing directly on occupants, wet finishes, or sensitive materials. Place heaters so air reaches corners and high loss doors.
Cost Planning
Fuel use is an estimate, not a utility guarantee. Weather, door openings, thermostat setback, and equipment condition matter. Still, the hourly and seasonal cost fields help compare scenarios. Try poor, average, and good insulation cases. Then review the spread before buying equipment. Always confirm final sizing with local codes, manufacturer data, and a qualified HVAC professional. Keep combustible clearances open. Check venting, electrical service, and gas pressure before startup onsite too.