Before you rent a space heater, know how warm you want the space to be and how much heat output in BTUs you need to achieve it. To answer the BTUs question, you’ll need to know the square footage of your space, the number of windows and doors and the height of the ceiling. The materials of the floor, walls and ceiling and the degree of insulation matters, too. An outdoor tent, for example, will need more heat to reach a comfortable temperature because it lacks insulation. Also consider the number of people who will occupy the space; more people generate more heat.

How to calculate BTU per hour

A qualified power and HVAC climate specialist can calculate the heat requirements of the space before recommending a portable heater or space heater rental. They will use the insulation factors of the space’s surfaces (walls, ceilings, floors) and the measurements of those surfaces to calculate the total BTU load, then use another formula to calculate the BTU/kW of heat required. If you know your total BTU load, you can figure out how many kW you need in a heater. Multiply the heater’s wattage by 3.412 to get BTU per hour. (A watt is equal to 3.412 BTUs.) A 15kW heater produces 51,180 BTU per hour.

As a very general rule, a well-insulated indoor space requires 10 watts of heating power per square foot, which translates to 34.1 BTU per hour.

When choosing a portable heater rental, the power source is one of the main considerations. United Rentals offers electric heaters, natural gas/propane heaters and diesel heaters, as well as heat pumps. If you choose a fuel-fired heater, you'll need to choose between direct-fired and indirect-fired.

Electric heaters

Electric heaters are the simplest choices for indoor applications because they silently generate clean, dry heat with no exhaust or carbon monoxide. Rent an electric heater to easily warm an enclosed space in a home or office. They can also be used to create warming stations on construction sites and to provide warm, dry air for drying drywall, curing concrete and tackling winter woodworking projects.

Fuel-fired heaters

Natural gas/propane heaters must be used outdoors unless you have a way to extend the flue pipe outside. If the unit is placed outside, the heat can be ducted inside through a window, door or other opening. Residential uses include heating large areas such as a garage or barn. Commercial uses includes heating tents or other outdoor events/worksites that have easy access to ventilation.

Diesel heaters produce drier heat and more precise temperature control than natural gas/propane heaters. They can be used indoors, as long as the exhaust is vented outside, or outdoors for industrial or commercial applications.

Direct-fired or indirect-fired?

Fueled-fired heaters are either direct-fired or indirect-fired. Indirect-fired heaters, in which the flame is contained in a heat exchanger, are safe for use around people, though they are more expensive and less efficient than direct-fired heaters. They are handy for special events and for projects that require drying. Diesel heaters are always indirect-fired. Direct-fired heaters, which are highly efficient and more powerful, are popular as commercial heater rentals. They require plenty of ventilation and are often used on construction sites and in large warehouses and outdoor arenas.

Heat pumps

An alternative to portable heaters, heat pumps can heat or cool large indoor or outdoor spaces, from event tents to jobsites to offices. Available in a variety of sizes, these electric-powered units are fume-free and excellent for worksites where warm, dry air is required. Smaller units plug into a standard outlet. Larger units like this 5-ton heat pump must be hardwired.

If your power outlets are standard 115-volt outlets, you can plug in a small electric heater such as this 1.5kW model. For larger BTU/kW output, you’ll need a natural gas, propane or diesel heater.

A higher-voltage power source, on the other hand, will power the larger electric heaters, like this construction-grade 136k BTU model, which runs on 480v. If you’re not sure what voltage you have, consult your engineer or electrician. Once you verify that you have the voltage you need for the electric space heater rental you want, make sure you have the proper breaker for the amperage.