Lack of makeup air can be the result of which of the following?

• A. Improper insulating techniques
• B. Under-fired appliance
• C. Negative pressure in the building
• D. Loose building construction

Answer: (C)

Makeup air is the outdoor air that must enter a space to replace the air used for combustion and what’s exhausted. When a building is under negative pressure, air is being pulled out faster than it can be replaced, so there isn’t a sufficient supply of fresh outdoor air for combustion appliances. This lack of makeup air can cause improper venting or backdrafting of combustion gases and degrade appliance performance. Negative pressure is typically caused by exhaust devices or tight building envelopes that limit air entry. By contrast, improper insulation tends to affect energy efficiency rather than directly creating a lack of makeup air, under-fired appliances reduce heat but don’t inherently limit air supply, and loose building construction usually allows more air to leak in, providing more makeup air rather than less.