How to Use a Makeup Air Heater to Prevent Residential Backdrafting

It’s no secret that U.S. homeowners are becoming increasingly attracted to the idea of installing high-powered, commercial-quality kitchen equipment in their homes. The widespread availability and increasing affordability of these devices makes them a common sight in American kitchens.

At the same time, residential building contractors are designing homes that are more airtight than ever before. As a result, house depressurization and its associated risks are on the rise, making reliable makeup air solutions a necessity for many households.

Introducing House Depressurization and Makeup Air

House depressurization occurs when the air pressure inside the house is lower than that of the atmosphere outside. This creates a system of negative pressure. According to the EPA, this can cause exhaust gases, such as those produced by combustion-based kitchen appliances, to backdraft into the home. This accumulation is dangerous for residents and can promote the growth of mold, placing the integrity of the structure itself in peril.

In order to create an indoor pressure system that pushes exhaust gases out, home contractors typically install either engineered openings or mechanical ventilators. However, this approach does not provide adequate air infiltration for larger kitchen ranges.

According to the Pennsylvania Housing Research Center, exhaust hood systems capable of exhausting in excess of 400 cubic feet-per-minute (CFM) require the installation of a complementary makeup air system. The system must be automatically controlled to start and operate simultaneously with the exhaust system, in accordance with International Residential Code M1503.4.

Since the unit needs to both push air inside the home and heat the air to a temperature that does not negatively impact the home, an appropriately powered unit is necessary.

Calculating Power Requirements

Although simply choosing a makeup air unit with the same CFM-rating as the kitchen exhaust seems like a logical idea, actually choosing the right unit is a little more complex than that. This is mostly due to the difference in temperature the unit has to compensate for. In order to determine how powerful a makeup ventilation system is needed for a particular kitchen appliance, we need a few figures:

• The exhaust rate of the device
• The desired temperature rise
• The heating capacity of the makeup air heater
• The electrical power available(voltage/phase)

High-powered commercial cooking ranges generally include information about their exhaust rates, given in CFM. If that information is not immediately available from the appliance manufacturer, it may be necessary to arrive at an estimate based on the type and power consumption of the unit. The American Society of Heating, Refrigerating and Air Conditioning Engineers sells a handbook that contains the information needed to make this estimate.

Arrive at the desired temperature rise by subtracting the coldest likely design temperature from the home’s desired room temperature. This number will be multiplied by the CFM of the appliance and then divided by the number of BTUs-per kilowatt the makeup air system uses, arriving at the desired kilowatt performance of the heater.

HVAC manufacturers like Thermolec typically include BTU-per-kilowatt figures in their product specification sheets. So if a homeowner needs to displace 400 CFM even when the outside temperature is 20°F and the ambient room temperature is 70°F, using a Thermolec Makeup Air Model FER-8-6-240, the equation would look like this:

(70-20) x 400 ÷ 3.412 = 5.86 Kw

How do we know that this is the appropriate figure for that particular model? Going back to the specification sheet, we can see that the FER-8-6-240/1 has an 8-inch collar size and delivers a maximum of 6 Kw—meaning that it’s the correct model for this example even though it has a reported CFM of 300.

This is typical of the Upper Midwest, where state regulations explicitly require units be powerful enough to compensate for large differences in temperature. As a result, people who live in colder climates will need to adjust accordingly by calculating the actual CFM using the above equation.