If you’ve taken a good look at your furnace after your basement remodel, you’ve probably noticed an insulated tube running ceiling to floor. That tube is a combustion air duct, and it’s essential to your heating/cooling systems.
Air Movement
In order to understand what this duct does, we first need to understand how air moves in a house.
Keep in mind the following two ideas:
1. Air always flows from high pressure areas to low pressure areas.
2. When air temperatures get higher, so does air pressure.
In every house, exhaust fans, water heaters, furnaces and clothing dryers pump air outside every day. This makes the air pressure inside the house lower than the air pressure outside.
The other thing that lowers air pressure inside is called the “stack effect.”. It’s where that first idea comes in. Since heat rises, your upper floors become warmer than the bottom levels, causing air pressure upstairs to be slightly higher than the air pressure outside. Since both of these things cause a difference in pressure, air either forces its way in or out, depending on where the pressure is lower. Think of it this way: when Minnesota summer hits, the AC goes on. As air cools inside, the air pressure also lowers. Since the pressure outside is higher than the pressure inside, outside air gets inside through tiny leaks.
Combustion Air Duct
The combustion air duct solves the problem. This duct leads to an air intake vent in the house’s wall, which is basically a hole. It works by giving outer air a way into the house to replace what was pumped out by appliances and fans. This is important to you as the homeowner because it discourages air leakage through gaps in windows that need to be replaced. Once the air pressure inside and outside is balanced, your home stays sealed longer, and monthly utility bills are lower.
If you want to test how well yours works, run your exhaust fans for five minutes, and then hold your hand over the end of the duct in your basement. If it’s working properly, there will be air coming out of it.
We’ll cover what can go wrong with the vent and how to fix it in our next post.
