Especially when it is -15 degrees F. outside?

This is a discussion that almost always comes up with home buyers, either on a detailed viewing of a home or during a home inspection.

Systems that bring in this air are typcially called make-up-air systems or outside combustion air systems.

And they are a building code requirement on newer homes.

And as I write this the make-up-air system in my home is pulling in lots of frigid outside air to my furnace room.

And this is the way it should be.

The steel triangular device here is a vent bringing in outside air. (Yes, that is ~15 inches of snow on the ground.)

This photo is from inside the basement. The big foil covered tube is the other side of the vent outside. The tube is open at the floor. The rim joist insulation is removed to make it a little easier to see.

The reasoning for these systems is simple and three fold:

1. For optimum air quality it is important to not just recirculate the same air in a home. Some fresh air is a must. (An outbreak of Legionnaires' disease in the 70's taught us that.)

2. Furnaces, hot water heaters, fireplaces, stoves and dryers all need air for their combustion. (Unless they are electric.) It is best to bring this air into the home in a controlled fashion.

3. Since air is excaping a home constantly other air must be coming into the home to replace it. It is best to control where that air comes in also.

For example: You turn on a bath exhaust fan in your home. That pulls air out of your home that needs to be replaced. That same amount of air must come into your home somewhere, and if the easiest place for it to come in is near your favorite chair in the living room, you are going to feel that as a draft.

Make-up-air systems generally bring outside air in as far from living space as possible, normally in a basement. In Michigan in the winter an unheated basement will typically be around 50 degrees so the -15 degree air going through the basement picks up heat from the basement walls and floor before it even enters the living space. Once it enters living space it has a chance to get heated up a lot more before it gets to your favorite chair in the living room. The result is a more comfortable home that costs a little less to hear.

And that is why we bring cold winter air into our homes.