Plenums are an important part of any forced-air heating and cooling system. They are technically part of the ductwork system, but they are directly connected to the furnace and impact how it works. Here is a guide to help you understand what furnace plenums are and the role they play in your heating system.
What Is a Furnace Plenum?
All furnaces have two plenums, which are sometimes referred to as distribution boxes. The return plenum is located just before the furnace and is where cold air flows into the unit. The supply plenum is located next to the heat exchanger where hot air flows out of the furnace. The return plenum is typically located next to the bottom of the furnace and is separated from the unit by the furnace filter. The supply plenum is most commonly at the top of the furnace, but it may also branch off from the side or bottom of the unit.
How Do Furnace Plenums Work?
A furnace plenum is a metal box that supplies cold air to the furnace or hot air to the supply ducts. When your heating system is running, the blower draws air in through the return vents. The air flows through the return air ductwork and into the return plenum. From there, it passes through the furnace filter and into the furnace. Gas is burned inside the combustion chamber to create heat. This causes the heat exchanger to become extremely hot.
Cold air is drawn into the system and forced over the heat exchanger. This raises the temperature of the air, and the hot air then passes into the supply plenum. From there, the blower fan then circulates the hot air throughout the main trunk line and all of the branches of the duct system and then out of the supply vents.
The supply and return plenums are always larger than the ductwork itself, and this is important for ensuring that the furnace works properly. As cold air is drawn in through the return registers, it builds up inside the return plenum. This ensures that there is always sufficient cold air coming into the unit. Hot air also builds up inside the supply plenum to ensure that there is enough air pressure to circulate it throughout the entire duct system.
If the supply plenum isn’t large enough, there won’t be sufficient cold air flowing into the furnace and it won’t create much heat. If the return plenum is too small, it can cause too much heat to build up inside the system and potentially damage the furnace or its heat exchanger. If either of the plenums is too large, the blower fan will have to work much harder to circulate air throughout the system due to the increased air volume.
As with the rest of your ductwork, the plenums can also become damaged and develop air leaks. Any leaks in either of the plenums or the rest of the ductwork will limit the effectiveness of your heating system. This means that your furnace will take much longer to heat your home and your energy bills will be higher. Air leaks also force the blower fan to work overtime, and this can lead to greater repair needs and shorten its lifespan.
Understanding How a Furnace Limit Switch Works
Inside of the supply plenum is a safety mechanism known as a limit switch. Whenever the furnace is heating, the limit switch is constantly measuring the temperature of the air inside the supply plenum. If the temperature ever rises too high, the limit switch will activate to shut the furnace off. This is done to prevent any potential electrical issues, damage to your furnace or safety hazards. If you find that your furnace often shuts off before it has heated your home to the temperature your thermostat is set to, it usually means that the furnace is overheating for some reason and triggering the limit switch.
When the temperature inside the supply plenum is too high, it indicates that the heat exchanger is getting too hot. This can be a dangerous issue as the high heat can cause the heat exchanger to crack. If the heat exchanger is cracked, carbon monoxide and other dangerous fumes can escape the combustion chamber and enter into the supply plenum and ductwork. This is something that needs to be taken care of immediately due to the huge risk of potentially deadly carbon monoxide poisoning.
What Causes Hot Air to Build Up Inside the Supply Plenum?
The most common reason that the temperature inside the supply plenum gets too high is that the airflow is restricted due to a dirty furnace filter. You always need to replace or wash your filter once every one to three months. If not, it will become clogged and drastically limit how much air can flow through it. The less air that flows into your furnace, the less air that will flow out. As a result, the hot air will build up inside the heat exchanger and supply plenum and can cause the unit to overheat and shut down.
Furnaces can also overheat and trigger the limit switch due to issues with either the blower fan or the ductwork and/or vents. If the blower fan begins to wear out or isn’t working properly, it won’t be able to circulate air throughout the ductwork as quickly. This will lead to hot air building up inside the supply plenum and eventually raising the temperature to the point where the limit switch gets triggered.
Closing too many of the supply vents in your home can also cause excess heat to build up inside the plenum. The same problem can occur if too many of the vents are clogged or there is furniture blocking the vents. If you don’t have enough vents open or too many are clogged or obstructed, all of the hot air in the ductwork won’t have anywhere to do. This prevents the system from being able to dissipate all of the hot air quickly enough to prevent it from building up inside the plenum.
The easiest way to prevent this problem is to make sure that you always leave at least 75% of your vents open. You should also make sure to keep furniture at least a foot away from all vents to ensure air can circulate out of them properly. It is also a good idea to inspect all of your vents monthly to make sure that they aren’t clogged with dust, hair and lint. You can easily remove the vents to clean the grates, and you will also want to use a vacuum to suck out any debris from inside the vent itself.
If you’re having any issues with your furnace overheating and shutting down, the HVAC experts at Dream Team Home Services can inspect the plenums, ductwork and your entire heating system to pinpoint the specific cause of the issue. We install, maintain and repair all types of heating systems, and we also work on air conditioning systems and indoor air quality equipment for customers in Media and the surrounding areas. We also have a team of licensed plumbers and electricians that can handle any of your plumbing and electrical repair or installation needs. For more information or to schedule a service call, contact us today.
