Despite “BTU” appearing on residential and commercial heating and cooling equipment, few people understand what it means.
BTU stands for British Thermal Unit and represents the amount of energy required to raise 1 pound of water 1° F at sea level. Common heating and cooling equipment are rated in terms of how many BTUs per hour they can add to or remove from the air. The higher the BTU rating, the more heating or cooling capacity the unit has. The heating ventilation and air conditioning (HVAC) industry uses BTU to size heaters and air conditioning units for different applications.
In the construction world, the temporary heating industry also uses BTU to determine the number and size of heaters needed for a project. It is also used to estimate the maximum fuel consumption required for a project. (One gallon of propane produces 91,333 BTU. One cubic foot of natural gas produces 1,032 BTU). Lastly, BTU is used to show the change in heat output (expressed in BTU) on vaporization charts for propane heating systems (click to view vaporization chart).
Even though its usage is common, there are several issues associated with using BTU for temporary heating equipment for construction projects.
- A heater’s BTU rating is, actually, the maximum output of the burner. BTU does not always equate to a heater’s discharge temperature, which is the best measure of the heater’s output.
- Heating systems rarely operate at their maximum output. So fuel consumption estimates, based on BTU, will inflate the amount of fuel needed.
- Considerations are not usually made for airflow or air distribution, which is measured in cubic feet per minute (CFM).
- It’s important to consider the size of space that needs to be heated. Sometimes a multiplier of cubic feet is used in BTU calculations, sometimes square feet is used.
- The enclosure type (permanent or temporary) is, typically, not factored in BTU calculations. Of course, a temporary enclosure will not seal as tightly or be as insulated as a permanent structure.
Also, manufacturers use different BTU calculation methodologies, which makes it challenging to determine true heating requirements and difficult to compare different units.
Fortunately, there is a more accurate method to determine temporary heating requirements for construction projects. It comes down to two Ds: discharge temperature and distribution of air.
These two elements determine how effectively a heating unit can utilize its BTU rating to heat an area. In other words, the heater that generates a higher, constant output temperature and higher CFM will perform better—this may be the heater with a lower BTU-rating!
So, how is this method applied? First, determine the area that needs to be heated, in cubic feet. Next, determine the CFM required for an air change in this space. Target an air change every 100 minutes for permanent enclosures and 50 minutes for temporary enclosures. As an example, a permanent enclosure measuring 700,000 cu. ft. would require 7,000 CFM (700,000 cu. ft. divided by 100 minutes). Then, look for a heater that will provide the required CFM at the highest, constant discharge temperature.
Comparing the resulting Babfar heater in this example with a competitor’s BTU methodology oversized their unit by over 500,000 BTU! Right-sizing a heater to the application heats your site properly, ensures your project runs smoothly and saves on rental equipment costs and fuel costs.
Keep in mind this methodology is a guide. There are other considerations must be made, such as the configuration of the building (open floor or finished hallways and offices), pressurization of the building envelope, ambient temperature, and more. Always consult with an HVAC specialist.
This example illustrates the need to look beyond common BTU estimates to determine the exact heating requirements of your next project. Remember the two Ds: discharge temperature and distribution of air!