A common complaint HVAC professionals across the country have to deal with during the summer is the large temperature differences in two-story homes. The second floor is typically burning up, while the first floor is extremely cold. It is common for temperature differences of 10°F or more to be found in homes with this problem.

These temperature-related issues are often corrected simply by renovating the duct system to cure duct leakage and air-balancing issues, which created pressure-related problems in the building. There are instances, though, when you can have the perfectly designed and installed HVAC system and these temperature complaints still exist.

My Second Floor Smells Like My Attic

One statement frequently heard with this issue is that, for some reason, the second floor seems to have a faint smell similar to the attic during the summer. Customers with this issue often mentioned the air on their second floor smelled hot, as well. Ever hear either of those before? The reason these comments are often made is that the second floor really did smell like the hot attic.

It usually isn’t your customer’s imagination when statements like this are made, as heated attic air is actually making its way into the house. How could this happen when everything is right with your system? It could be unintentional holes in the building and natural pressure differences working against you. When this occurs, the HVAC system might not be the area you need to focus your attention on.

Reverse Stack Effect

“Stack effect” is used to describe the action occurring when air moves in and out of the building through leakage. The stack effect can be a major mover of airflow in a building. During the winter months, it causes air to be brought in at the lower levels of a building and exhausted at the higher levels of the building. During the summer, the stack effect reverses itself and causes air to be brought in at the higher levels of a building and exhausted at the lower levels of the building.

Stack effect is temperature-driven. The greater the temperature difference between the indoors and outdoors, the greater the stack effect is. Remember, cold air is heavier than hot air, and it easily moves down an open staircase or landing from the second floor to the first floor. With many attic temperatures reaching the 120°-140° range across the country, this can easily yield a 45°-70° temperature difference between the hot attic and the cooler inside of a home.

The size of the holes in the building and their locations also influence stack effect. Remember, to have airflow you must have an opening and a pressure difference across that opening. If there are no holes in a building, theoretically, there isn’t going to be any airflow. Most buildings aren’t that airtight, though, and have quite a few leaks in them that you have to contend with.

The largest areas of concern for the openings that contribute to this problem are typically in the ceiling and the floor areas. Just think of all the holes in a ceiling that can leak air into a building. Attic access panels, can lights, and unsealed boot penetrations are just a few that come to mind.

These holes give air a pathway to move in and out of. Holes in the ceiling allow hot attic air into the home, while holes on the first floor allow cooler air to be moved to the outside.

Control the Air First

In order to control air, you must first contain it. The hot second floor and cold first floor problem may be reduced tremendously just by controlling the amount of uncontrolled air entering the building through leakage. As an HVAC professional, your goal is to condition the environment of your customers. To do this properly, you have to first understand the environment you are attempting to condition.

An HVAC system is essentially a closed-loop system. The building is the connection point between the supply ducts and return ducts, making it part of the duct system. Since the building is one side of the duct system, controlling the load of the building really becomes another opportunity for you.

By being able to identify which side of the duct system (traditional or building) is the bigger problem, you can develop a game plan for how to approach each unique situation. Instead of simply focusing on the HVAC system, look at the building side of the duct system, as well. You may find untapped opportunities for needed air-sealing and insulation improvements your customers want and need.

Publication date: 8/4/2014

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