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What Is a Furnace’s Supply Plenum and What is its Role in the Heating Process?

Furnace and ductworkThe more you know about how your furnace works, the better you will be at identifying when a problem has occurred and the easier it will be to understand your technician when he or she explains any issues that your system is encountering. One major component of a furnace that is helpful to know about is the supply plenum. Today we’re going to talk about what a supply plenum is, how you can locate it in your system and a few of the most common problems that can go wrong inside of it!

What is a furnace’s supply plenum?

A supply plenum is an enclosed compartment that’s located just after your heat exchanger if you follow the flow of air through your furnace (see below). When your air handler blows air over your system’s heat exchanger, that warm air is directed into the supply plenum. This is the very last place that heated air visits inside of your furnace before it travels through your ducts and into your rooms, and all of your home’s supply ducts stem from the plenum.

How can you locate your furnace’s supply plenum?

Furnaces come in a number of different configurations depending on their sizes, makes and models. As a result, a furnace’s supply plenum might be on top of, below or next to your system’s heat exchanger.

In order to locate where your supply plenum is, first identify where your return ducts meet your air filter. At that point, follow along the ducts past the filter to the burners and heat exchanger. If you continue to follow your ducts in the same direction, you’ll find the supply plenum just after the heat exchanger and connected to the supply ducts that extend out of your system.

What kinds of problems can a furnace’s supply plenum encounter?

  • Your furnace has a safety device known as a limit switch that measures the temperature inside of your furnace. This device is located inside of your furnace’s supply plenum. If your heat exchanger begins to overheat, the temperature inside of the supply plenum will begin to rise. If the temperature inside the plenum gets too high, the limit switch will shut down your furnace in order to prevent safety issues and breakdowns.
  • Air blockage. There are a number of different reasons why air might get backed up inside of your supply ducts. For example, this can happen when you close the supply vents in any of your home’s rooms. When air is blocked in your ducts, it will back up into the supply plenum. This can increase the temperature inside the plenum, causing the limit switch to go off (as above), and it can also decrease the efficiency of your system because your air handler will have to work harder to push air into the plenum.

Issues with the supply plenum itself will almost always need to be taken care of by a professional like Jerry Kelly. That’s because you can’t access the plenum directly unless you take apart certain other components to get to it.

If you have any questions about a furnace supply plenum, or if you’d like a heating system serviced or installed in your home, contact Jerry Kelly, your St. Charles, MO, furnace installation and repair contractor. We provide service all over the St. Louis area, including towns like Ladue, Lake St. Louis and Manchester, MO.