Short #36 – Stack Effect (Podcast)

In this short podcast episode, we talk through stack effect. We explain what it is and what sorts of comfort issues it can cause in a home.

Most of us understand that hot air rises even though heat itself doesn’t rise. The stack effect is precisely a version of that piece of common knowledge; hotter air is less dense than cooler air, so it floats above the cooler air. In hotter air, the molecules move a lot faster than they do in cooler air, so they can start to separate from each other, which reduces the overall air density.

For the most part, we don’t work pressurize air in HVAC work (not refrigerant), but we do change the temperature. The temperature changes cause the difference in air densities to emerge.

If we’re dealing with a furnace system in a two-story house or a home with high ceilings, we see that stack effect in action. When that hotter air rises and cooler air sinks, the hotter air makes way for a vacuum that draws colder air into the building. While that hot air rises, the colder air comes in under doors and through low cracks. Although the air that’s coming out of the appliance is warm, it can’t do much to heat the space before rising.

The reverse stack effect can also happen. When you have poorly sealed can lights or cracks in the ceiling, the colder, denser air will sink and create negative pressure near the highest point of the room. When we have that negative pressure, hot air can get pulled in from the attic or other undesirable locations. In Florida, we have to worry quite a bit about the reverse stack effect, whereas the stack effect is more of a concern for colder climates.

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