It's like this. Warm air heating systems work by drawing the air that's already in a room through what we call a "return" heating duct. This is usually in a central location in your home, and down near the floor. The air goes into this duct and returns to the furnace where it gets heated and sent back upstairs to the rooms through smaller, "supply" ducts.
A fan inside the furnace is responsible for moving the air. The fan looks like one of those wheels that hamsters like to run on. The whole works is very much like your hair dryer. Instead of having an electric coil, though, the furnace has either an oil or gas flame. Naturally, the flame is contained inside a metal box. We call that box a heat exchanger because that's exactly what it does! The flame is on one side, and the air is on the other.
As the air returns from your rooms it passes through a filter that's inside your furnace. The filter's job is to help clean the air that you breathe. You should change your filters at the beginning of every heating season. It also pays to check them during the heating season to make sure they're not clogged.
But let's get back to that hair dryer. Imagine what would happen if you put the palm of your hand over the hair dryer's air inlet while it was running. You'd be keeping the air from entering the dryer so not too much would be happening on the business end of the dryer, would it? It would take you an awfully long time to dry your hair because if nothing can get it, nothing can get out.
I have a friend who moved his couch once. He put it right over the return air duct. Then he called me and told us there was hardly any warm air coming out of his supply air ducts! I told him about the hair dryer.
Here, let's take this a step further. Have you ever noticed how the screen on your hair dryer gets clogged as the months go by? That fan is sucking in air like crazy and anything that's in the air eventually winds up getting caught in that screen - lint, dust, hair from the family pets, you name it. As the screen gets clogged, the hair dryer poops out, right?
So you take a hairbrush and give the screen a good going-over. You see all that grunge fall off the screen and into the sink. When you turn the hair dryer on again, it works so much better because the air can move.
Pretty simple, eh? Now think of the filter in your furnace as the screen in your hair dryer. It's the same basic principle. And that's why you have to keep those filters clean. Give that air a chance to move. You’ll be more comfortable, and you’ll save money on fuel.