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How to Size a Construction Heater: BTU Formula, Heater Types and Fuel
An undersized construction heater runs flat out and never catches up. An oversized one burns fuel you didn't need to buy and can push a space well past a comfortable temperature. Use this guide to estimate the BTU output you need, then choose the heater type and fuel that suit the space. Shop the full range in our portable construction heaters collection.
Step 1: Estimate the BTUs you need
The quick sizing formula used across the heater industry is:
BTU/hr needed = cubic feet × temperature rise (°F) × insulation factor
- Cubic feet = length × width × ceiling height, in feet.
- Temperature rise = the temperature you want inside minus the coldest outside temperature you expect to work in.
- Insulation factor: about 0.17 for an uninsulated building, 0.133 for average insulation and 0.10 for a well-insulated, tight space.
This is an estimate, not a full heat-loss calculation. Open doors, tarped walls, wind and high ceilings all raise the load, so when the answer falls between two heater sizes, choose the larger one.
To convert an electric heater's rating, use 1 kW = 3,412 BTU/hr. A 15 kW electric heater puts out about 51,000 BTU/hr.
Worked examples
| Space | Cubic feet | Rise | Factor | Estimate |
|---|---|---|---|---|
| 2-car garage, 24 × 24 ft, 10 ft ceiling | 5,760 | 40°F (20°F → 60°F) | 0.133 | about 30,600 BTU/hr |
| Workshop, 30 × 40 ft, 12 ft ceiling | 14,400 | 60°F (5°F → 65°F) | 0.133 | about 115,000 BTU/hr |
| Framed building under construction, 40 × 60 ft, 14 ft ceiling | 33,600 | 50°F (15°F → 65°F) | 0.17 | about 285,600 BTU/hr |
For the garage, a 35,000–45,000 BTU heater leaves headroom. The workshop calls for a heater of about 125,000 BTU/hr. The building under construction needs about 285,000 BTU/hr, from one large indirect-fired unit or from two smaller heaters placed at opposite ends.
Step 2: Choose the heater type
Forced-air (direct-fired)
A forced-air heater, often called a salamander or torpedo heater, blows air across an open flame. That makes it efficient and quick to heat a large area. The combustion gases, including carbon monoxide, stay in the space being heated, so use it outdoors, in open structures or where the manufacturer's ventilation requirements can be met. It is the usual choice for concrete curing, rough framing and open job sites.
Indirect-fired
An indirect-fired heater burns fuel in a sealed combustion chamber and moves heat through a heat exchanger. The exhaust is vented outside the work area, and the heater delivers clean, dry air through ducting. It is the right choice for enclosed or occupied spaces, drywall and finish work, and long-term heating. It costs more and weighs more than a forced-air heater of the same output.
Radiant and convection
Radiant (infrared) heaters warm the people and surfaces in front of them, not the air, which works well at a workstation or in a drafty bay where warm air would blow away. Convection heaters spread warmth evenly in every direction for general area heat.
Electric
Electric heaters produce no combustion gases, so they are the cleanest option indoors. Their limit is the power supply: larger units need 240V or three-phase service. Match the voltage, phase and amperage on the product page to the power available on site.
Step 3: Choose the fuel
| Fuel | Good for | Keep in mind |
|---|---|---|
| Propane (LP) | Portable, clean-burning heat on most job sites | Cylinder size and outdoor temperature limit how much the tank can supply |
| Natural gas | Shops and buildings with a gas line | Needs a gas connection, so it is less portable |
| Kerosene / diesel | High output with fuel on board, remote sites | Has a fuel odor; many units need electric power for the fan |
| Electric | Enclosed spaces where combustion is not wanted | Depends on available voltage, phase and amperage |
Heaters by BTU range
| Output | Typical space | Models |
|---|---|---|
| Up to 50,000 BTU/hr | Garages, small rooms, workstations | HeatStar HS35FA 35K propane forced-air · DEWALT DXH45LP 45K propane radiant · HeatStar HS50K 50K kerosene/diesel · L.B. White Volt 9 electric (30,700 BTU/hr) |
| 50,000–100,000 BTU/hr | Shops, small framed structures | DEWALT DXH90FAV 50K/90K propane · HeatStar HS80KT 80K kerosene/diesel · HeatStar HSP70ID 70K indirect-fired |
| 100,000–200,000 BTU/hr | Workshops, additions, mid-size job sites | DEWALT DXH125FAV 125K propane · HeatStar HS170QFAVT 170K propane · HeatStar HSP200ID 180K indirect-fired |
| 200,000–400,000 BTU/hr | Large buildings under construction, warehouses | DEWALT DXH215HD 215K kerosene/diesel · HeatStar HSP300ID 285K indirect-fired · HeatStar HS400FAVT 400K propane |
| 400,000 BTU/hr and up | Commercial construction, concrete curing | HeatStar HSP500ID-A 400K/500K indirect-fired · HeatStar HS6000DF 600K direct-fired |
Accessories that change the result
A thermostat cycles the heater to hold a set temperature instead of running it nonstop, which saves fuel. Ducting lets an indirect-fired heater sit outside and deliver heat exactly where it is needed. See the HeatStar digital thermostat, the 16 in. x 25 ft duct and the 12 in. x 25 ft duct. Check that each accessory lists your heater model.
Safety basics
- Fuel-burning heaters produce carbon monoxide. Never use a direct-fired heater in an enclosed space unless the manufacturer's ventilation instructions are followed, and use a CO monitor.
- Keep the clearances to combustibles listed in the owner's manual, and keep fuel cylinders and cans away from the heater outlet.
- Look for an automatic shutoff if the flame goes out, a tip-over switch and a high-limit switch. These are standard on most job-site heaters.
- Follow local codes and your site's rules for fuel storage and heater use.
Need help sizing?
Send us the dimensions of the space, the insulation level, the coldest outside temperature you expect and the fuel or power available. We will recommend a heater and confirm it is in stock before you order. Contact Urban Supply.