How to Properly Size a Unit Heater

In situations where central heating systems are inaccessible or impractical, unit heaters are a compact, powerful substitute. They don’t require any ductwork and some units have modular and portable capabilities. Spaces like garages, workshops, and other commercial and industrial areas benefit from the presence of unit heaters.

However, for a unit heater to work adequately, it needs to be the proper size to emit enough heat. Sizing a unit heater can be difficult if you don’t know what measurements and specs to pay attention to. Just buying a garage heater for a garage, or a commercial heater for a warehouse, won’t always produce the best results.

The largest factor to consider when buying unit heaters is BTUs. BTU, or British Thermal Unit, is a measurement that indicates the heating capacity of different types of heaters. Specifically, it measures the amount of energy required to heat one pound of water by one degree Fahrenheit.

Even though unit heaters heat air, not water, BTUs are still the metric of choice for measuring their heating capacity. BTUs also measure air conditioning capabilities, but instead indicate how much air a unit can remove from an area.

When looking at the number, you can understand that the larger the number, the larger of a space the heater will cover. Standard BTUs for unit heaters range anywhere from 10,000 on the lower end to over 100,000. Household unit heaters typically need lower BTUs, while commercial and industrial heaters typically require much higher BTUs.

Choosing the right BTU is important because it can affect more than just the physical temperature of your space. If you get a heater with too many BTUs, it will run for shorter periods and waste energy while doing so. This will cause excess wear on your unit over time and needlessly drive up your heating expenses.

Unit heaters with insufficient BTUs are equally problematic. They will run for longer than they need to and likely not ever heat an entire space adequately. This can create cold pockets and humidity imbalances. An underperforming unit heater will cost more to operate and inflate your energy bill unnecessarily.

Unit heater sizing calculators exist to allow you to quickly get the number you need. But calculating it yourself is just as simple, only requiring a few measurements and a bit of multiplication. All you need to gather are a few metrics and details about your space.

First, you need to measure the square footage of your space. Do this by measuring the length and width of the area, then multiplying those numbers. Then, you can measure the length from floor to ceiling to get the ceiling height. Multiply that number by the square foot measurement to get the cubic footage of your space.

Once you have the cubic footage, you can then multiply it by what HVAC technicians call the “insulation factor.” This factors in how insulated your space is to determine if the heater will need to work more or less than usual to heat the area.

The different insulation factors are:

4 = Excellent Insulation

5 = Average Insulation

7 = Poor Insulation

Multiplying the cubic footage by the insulation factor will tell you how many BTUs your space needs. Whether you choose one or multiple unit heaters, just ensure that the total BTU measurement exceeds the minimum calculation. To simplify, the formula for a BTU calculator is:

Cubic Footage x Insulation Factor = British Thermal Unit (BTU) Measurement

For example, let's say the space you're trying to heat has a square footage of 500, a ceiling height of 10, and average insulation. The minimum BTU a unit heater would need to have to properly cover the space would be 25,000. Another rule of thumb you can use to calculate BTU that is even simpler is 20 BTU per 1 square foot. Use this in cases where getting measurements or determining insulation may be more difficult.

After you’ve determined how many BTUs you need, you can then decide on the type of unit heater that you want. Gas and electric heaters are the most common and have the most variety in BTU measurements to choose from. Browse HomElectrical's selection of unit heaters and sort by BTU rating to find the exact unit heater you need.