How To Size Electric Duct Heaters? How To Calculate the Watts of Heat?

How to size electric duct heaters?

Sizing a duct heater can be a hard nut to crack, especially if you’re a first-time user. Among the major components of your home’s duct system, the duct heater is one that you cannot afford to go wrong with.

The sizing and compatibility must be pitch perfect to give you just the right amount of heat in the winter. But, let’s not get ahead of ourselves; it’s important that we establish a background by explaining what exactly a duct heater is meant to do.

A duct heater is part of a home’s HVAC system situated in the duct to heat up streams of gases passing through the duct before they move into the rooms.

While there are many types, including tubular, finned tubular, and open coil duct heaters, electric coil heaters are the most popular.

Electric duct heaters create heat by passing an electric current through coils (which form resistance). It is from these coils that air passing through absorbs heat and moves gradually into the rooms in the building.

However, one major challenge with duct heaters is sizing them correctly. How do you know what size fits your duct? Let’s find out.

How Many CFM is a kW?

It all begins with CFM. CFM stands for Cubic Feet per Minute and this refers to how many Cubic Feet the unit can move air through in a minute.

But since electric duct heaters are measured in kW, it’s important to find out how many kilowatts of electricity can provide a certain amount of airflow CFM. How many CFM is a kilowatt?

Once you’re able to figure that out, you should have an idea of what wattage you require. As a measurement, 1 kW equals 20.91 atmospheric Cubic Feet per Minute. Inversely, that’s 20.91 CFM per 1 kW.

What this means is that you can always tell how much energy is used up per Cubic Feet of airflow. So, before you get a duct heater, you should know its wattage, so that you can calculate the CFM of the unit.

How Many Watts of Heat Do You Need Per Square Foot? (Duct Heater kW Calculation)

How To Size Electric Duct Heaters? How To Calculate the Watts of Heat?

There are a number of factors that determine how many watts of heat you’ll need per square foot. This is very important information if you’re interested in getting the right duct heater for your room’s size.

You should know that the wattage of heat you need depends on the efficiency of your room or home. That is, the insulation of your home and how well it’s able to retain heat goes a long way to determine heat wattage per square foot. Let’s explain this further.

If your ductwork and room are properly insulated, you may only require about 10 watts of heat energy per square foot.

A room that’s fairly insulated to an extent will need 12 watts of heat per square foot, while a room that’s poorly insulated or not insulated at all will require in excess of 15 watts of electricity per square foot.

The height of the ceiling is another factor that should be taken into account. For instance, many experts say spaces with an 8-ft high ceiling will be taken care of by 10 watts per square foot.

By going with the first rule of thumb, it would mean that a 160 sq ft well-insulated room or space will need 1,600 watts of electricity to heat up properly. The figure is reached by multiplying 160 by 10 (which is the number of watts needed for a properly insulated room per square foot).

How To Size an Electric Duct Heater?

The size consideration for selecting an electric duct heater ranges from heating capacity to airflow rate and operating temperature. Also, the dimensions of the appliance are another very important consideration.

To arrive at the heating capacity required in your home, you should use the formula.

          SCFM x temperature rise (°F)

kW = ———————————–  x 1.2


There are other sizing considerations that require formulas.

For instance, to find the BTUH required, calculate as follows:

BTUH = Temp rise x CFM x 1.08

So, whether it’s rated by kilowatts or BTU per hour, you should be able to tell which h is right for your home by using the formulas above.

In case you don’t have an idea of the CFM, you should multiply the rated kilowatts by 3,193 and divide the result by the temperature rise.

In more simple terms, your ductwork is a great index for determining what duct heater you need. You need to be sure that the heater fits the duct’s diameter by measuring the sides and comparing it with the internal dimensions of the duct.

Be mindful that ductwork is traditionally round and you must ensure this is considered in your calculation. Also, be sure that the airflow CFM can run beyond the length of the duct.

A typical 6-inch Duct heater should work well in your home, but if you want something for a bigger house or a larger space, you should consider 8″-10″ units.

On average, these home duct heaters have a rated wattage of between 500 watts to 2,000 watts, depending on the unit. You need to be sure of the home’s footage before you pick a heating product.

Final Words:

Sizing your duct heaters can be a really complex process, mainly because these units are installed in the duct and depend on airflow rate and temperature to function.

Still, we’ve provided various options for you to decide what size of duct heater works for your duct and your home. Sizing it wrong can make everything go awry, so you must be cautious.

Remember, the factors you need to take into consideration are CFM, heating capacity (BTU), wattage, and dimensions, among others.

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