How to Calculate Temporary Heating Requirements on Construction Sites

Winter doesn’t halt construction. Whether you’re pouring concrete during January or installing drywall in an otherwise exposed building, you will need to heat your work area. In many cases, it’s also a legal requirement.  But too many contractors guess at their temporary heating needs instead of calculating them. Guessing can cost you money, delay your project, and put workers at risk. Here’s a step-by-step guide to determining your exact construction heating needs:

Simple Steps to Calculate Construction Heating Needs 

Why Temporary Heating is Important 

Cold weather affects concrete curing, paint drying, glue not sticking, you name it. Materials just don’t react like they should when temperatures fall too low. That’s why most contractors need heaters on job sites during winter months. Concrete won’t cure well below 50°F. Paint, adhesives and even mortar bond strength diminishes in cold weather. Low temperatures also reduce worker productivity and can cause injury. Temporary heating keeps your people and materials protected. But using the wrong size heater can be just as bad as not using one at all. An undersized heater will consume fuel all day without reaching the desired temperature. An oversized heater wastes money and can cause excess moisture inside an enclosure. Take the time to do the calculation correctly before you purchase or rent heaters. It will save you time and money on every cold-weather project.

Step 1: Determine your square feet or volume of space

The first piece of information you need is how many square feet or cubic feet of space you want to heat. Remember to account for ceiling height because we are heating volume, not just area. Square feet x Height = volume in cubic feet Example: The work area is 60 feet long by 40 feet wide by 10 feet tall. 60’ x 40’ x 10’ = 24,000 cubic feet If you have more than one room or space to heat, calculate each one and add them together for total cubic feet.

Step 2: Determine Your Temperature Rise (DeltaT) 

Delta T or DeltaT is the amount of heat your heater needs to produce. It is the difference between outside air temperature and desired temperature indoors. DeltaT = Desired temperature – Outside temperature  Typical job site temperatures are 50-60°F. This protects materials and keeps workers comfortable.  DeltaT = 55 desired temperature – 15 outside temperature = 40 degrees Remember temperature will always fluctuate. Use the lowest expected temperature for the area your project will be located during the time frame you need heat. Don’t use an average temperature. Calculate for the worst day, not an everyday scenario.

Step 3: Determine Your Building Factor 

Not all structures are created equal. A tent or open building frame covered in plastic sheeting will leak heated air quite differently than a finished wall and roof. Use one of these building factors based on your structure: 0.135 for a sealed, insulated building  0.145 for a non-insulated building or open building 0.160 for tents, temporary buildings or similar applications This multiplier adjusts for airflow through the structure. Cold air enters through openings and leaks. A framed tent with plastic sheeting allows much more cold air infiltration than a fully constructed wall and roof.

Step 4: Calculate BTU Requirements 

Now it’s time to crunch the numbers. Multiply the figures together using this formula:  Total BTUs per hour = Volume x DeltaT x Building Factor From our example above:  24,000 cubic feet x 40 degree DeltaT x 0.145 = 139,200 BTUs per hour This is how many BTUs per hour your heater(s) needs to produce to heat the space. When purchasing or renting heaters, look for units that meet or exceed this number. It’s always better to have too much capacity than not enough on a cold night.

Step 5: Add Safety Margin 

The above formula will give you a good starting point. But construction job sites are notorious for losing heat in ways the basic formula can’t account for. Add an additional 10 – 20% to your total for: Doors, entranceways swinging open frequently  Wind blowing across open areas or raised structures Concrete Curing (Release some heat but absorbs moisture from air) Ground Contact – cold ground absorbs heat as it rises Example above: 139,200 x 1.15 = 160,000 BTUs per hour to use as a goal

Step 6: Select Heater Type 

There are many make and models of heaters out there. Knowing how many BTUs per hour you need narrows down your choices. Now you need to choose the right type of heater. Direct-Fired Heaters send combustion products right into space. These are the most common type of heater you will see on a construction site. Direct-fired heaters require ventilation but are very efficient. Indirect-Fired heaters burn fuel outside of the heated space. They send clean, dry air into the work area. These are best for indoor spaces or locations where workers are present 100% of the time. They cost more to rent but do help prevent exhaust from entering the air your workers breathe. Electric heaters are better for spot heating or small spaces. There are no emissions to worry about, but you typically get fewer BTUs per dollar than a fuel-fired heater.

Step 7: Consider Fuel Sources and Runtime 

If your heater is sized at 160,000 BTUs and runs for 12 hours, it will consume a calculable amount of propane or diesel. Most heater manufacturers provide fuel consumption information so you can estimate how much fuel you need each day/week. Propane: One gallon = 91,500 BTUs of usable energy  Diesel: One gallon = 130,000 BTUs of usable energy  Don’t forget to adjust for heater efficiency. This is typically 80% to 90% for most construction heaters.

Final Words 

Calculating how much heat you need on a construction site is not difficult. Measure your space, calculate temperature rise, select the proper building factor and choose heaters with matching or greater capacity. Follow these steps and you’ll protect your workers, materials and schedule all winter long.