Fact Checked

What is a Downflow Furnace?

Harriette Halepis
Harriette Halepis

A downflow furnace is a type of furnace that allows cold air to enter the top of a furnace unit, where it is warmed, and exit the bottom of the furnace. Once warmed, the air then enters a home through air vents. Contrastingly, an upflow furnace includes a fan that blows cold air over a heated surface. This air is them warmed and sent throughout the house via furnace ducts. While upflow furnaces are generally placed within the basement area of a home, a downflow furnace is usually installed within the main portion of a house.

The main parts that makeup a downflow furnace include a combustion chamber, heat exchanger, and thermostat. When natural gas flows through the combustion chamber, this gas is allowed to mix with air that is inside of the chamber. Once the air has become adequately mixed, a pilot light ignites the gas-filled air, which creates warmth within the heat exchanger. Finally, the air is pushed through a filter, in order to remove any impurities, and it is sent throughout a home with the help of a ventilation system.

A thermostat is one of the main parts of a dowflow furnace.
A thermostat is one of the main parts of a dowflow furnace.

A downflow furnace's thermostat controls the air temperature inside of the unit. Most units are equipped with thermostats that have temperature sensors. When a sensor detects that the air inside of a home is too cold, the unit automatically raises the temperature of the air inside of the furnace. As soon as a home reaches the right temperature, the furnace shuts off. This process is then repeated as the temperature inside of a home changes throughout any given day.

While a downflow furnace can adequately warm any home, these furnaces can also be dangerous if installed incorrectly. When the air inside of a downflow furnace is ignited, carbon monoxide is produced. Carbon monoxide is toxic to both animals and people, which is why all downflow furnaces must include vents that push carbon monoxide outside of a home.

As a response to safety concerns, ventless downflow furnaces have recently been invented. Manufacturers of ventless furnaces claim that these units do not produce harmful amounts of carbon monoxide. In order to ensure that this dangerous substance remains harmless, all ventless furnaces include automatic shut-off sensors. If carbon monoxide levels become too high, these sensors will cause the furnace to close down. Regardless of the type of downflow furnace chosen, it is crucial that all furnaces be tested on a regular basis. This way, any furnace defects or problems can be eliminated right away.

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Discussion Comments


@elizabeth23- My uncle used to work with heating and cooling and he told me something similar once.

What I really learned from him, though, is that even the most high efficiency furnace is only as good as a home's insulation. Every building loses heat through the roof, but good insulation around the roof, windows and doors can help minimize that heat loss, which can make more of a difference sometimes than even getting a new furnace.


@FernValley- I don't know where you heard that, but actually, the opposite is usually true. When you compare downflow furnaces to upflow furnaces, another common type of furnace, the more efficient one is clearly upflow.

Heat rises naturally, so upflow is a more natural pattern. Of course, that means that upflow furnaces work best if they can be put in a basement, and no everyone has one to put it in.


I heard that these types of furnaces can be very energy efficient. I've never had one myself, though. Does anyone know if they really are more efficient than other types of furnaces?

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    • A thermostat is one of the main parts of a dowflow furnace.
      By: Mariusz Blach
      A thermostat is one of the main parts of a dowflow furnace.