What Size Heater Do I Need for My Garage?

Temperatures plummeting across the nation not only means colder weather outside but frigid garages as well. This is especially true in homes that don’t incorporate a central heat and air vent into these areas. Getting the right heater for your garage can be difficult as there are different types of heaters and a seemingly complicated formula to properly calculate a size that works for you.

Terms to Know Beforehand

Before you can calculate the size heater you need for your garage, you first need to know a few things on the language surrounding these machines. Understanding these terms will allow you to more accurately choose the heater that is best for you. The machine's power is often quantified by the amount of wattage. Often written shorthand as watts, this is a unit of power that equals one joule of energy per second. In terms of calculation, a watt is measured by the amperage (the amount of electricity inside of the machine) by the voltage (the amount of pressure needed to move the electricity). Most machines run off of 120 volts but can have varying amperage - some larger machines use 240 volts to run. All in all, wattage can be a bit confusing and what you probably want to look at is the amount of British Thermal Units (BTU). A BTU is the amount of energy required to heat a single pound of water by a single degree - this comes out to roughly 1055 joules. One watt equals just under three and a half BTU. Knowing the amount of BTUs will determine whether or not you have the right machine to increase the temperature of your garage. Knowing how much heat you need is determined by the temperature rise. You want to subtract the outside temperature from the ideal internal temperature. For example, an outside temperature of 30 degrees Fahrenheit would be taken away from the ideal internal temperature of 65 degrees Fahrenheit, equating to 35 degrees of temperature rise. From here you also need to understand what type of insulation you have. The effectiveness of the insulation is determined by the R-value. The higher the R-value the less heat you lose over time. Fiberglass, being the most common insulator in homes, has an R-value typically ranging from 2.9 to 3.8.

Types of Garage Heaters

There are roughly two types of garage heaters you can expect to find - gas and electric. Electricity is the most common and is heated via a metal implement - the electricity passes through this point and heats up the surrounding air. The amount of heat created is equivalent to the amount of current applied to the metal squared, multiplied by the electrical resistance of the said metal. Often times these machines are assisted by some sort of fan to move the heat around. The majority of the heat relies on infrared heating - these rays are absorbed directly into solid objects in your garage and yourself to be more effective. These type of heaters are better with inferior insulation areas. Gas heaters, by their nature, have a much different mechanism for heating up your garage. Propane, or natural gas, is fed through the heater and activated via a sparker. This burning fuel warms the heat exchange - thus activating a fan that blows the hot air through the garage. This type of heater directly warms up the air, making them more effective at high insulation areas. There are also some gas heaters that work with an infrared tube to create infrared rays making them more like an electric heater. Gas heaters are often much more powerful than electric ones and are usually the only way to heat up a larger space, especially in really frigid weather conditions. That being said, they do suffer quite a bit from energy efficiency. While thermodynamics forbids any system from being 100 percent energy efficient, electrical models get much closer to this than gas ones. Upwards of 20 percent of gas used in a gas powered heater can be burned off, requiring ventilation if used in an enclosed space. Finally, electric heaters are usually cheaper when compared to gas models.

Calculating Your Garage Heater Size

There are two ways to calculate the heater you require for your garage - either by wattage or BTU. Watts are easy enough to follow as they can be quantified by 10 watts per square foot of space. For example, in an area that is 20 feet by 24 feet you would need 4800 watts of power (it's just the total square footage, 480, multiplied by 10). Gas heaters are often rated by the amount of BTU they produce and the calculations get slightly trickier. Taking the amount of wattage you need - 4800 in our example - you then need to multiply it by 3.41 (roughly the amount of BTUs a single watt equates to). This comes out to 16,368 BTUs needed to warm your space. For those that want a more accurate measurement and don't mind doing a bit more math, then you need to calculate the amount of air by volume inside your garage. If we use or original example of 20 by 24 garage and multiply it by the height of the ceilings - let's say 8 feet - then you get the number of cubic feet of your air. 480 square feet multiplied by 8 feet is 3,840 cubic feet. For increased accuracy, you then need to multiply the R-value of the insulation into the cubic foot number. R-values on the outside of the home can range anywhere from 13 in warm climates to 27 in cold ones. If you have really good insulation you can simplify this number to 0.5, average insulation to 1, weak to 1.5, and 5 if you have no insulation. Take this number and multiply it by both the cubic feet and the amount of temperature rise that you want. Roughly the formula comes out to this: (Insulation * Cubic Feet of Garage * Temperature Rise) / 1.6 = # of BTU. In an area of 3,840 cubic feet, with a 35 degree temperature rise with average insulation (or 1) would come out to this: (1 * 3840 * 35) / 1.6 = 84,000 BTU. To get this down into watts, merely divide the amount of BTUs by 3.41. At 84,000 BTU this comes out to approximately 24,633 watts.

Get the Right Size Heater for Your Garage

Getting the right size heater for your garage isn’t necessarily a difficult thing as long as you know what to look out for. Knowing the differences between BTUs and watts is usually quantified by the differences between gas and electric. While both machines work well, electric heaters are much more energy efficient. On the other hand, gas heaters can typically do a larger space. When calculating the size that you need, you first should know the amount of square footage, the height of your ceilings, the level of insulation and the desired amount of temperature increase from the outside.

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