We get a lot of these upflow vs. downflow furnace questions. It’s about time we address what is the difference between upflow and downflow furnace. Let’s get into it by first pointing out the main difference (all the consequences, efficiencies, etc. flow from this 1 difference):
- Upflow furnace produces a down-up direction of airflow.
- Downflow furnace produces an up-down direction of airflow.
Basically, the premise is that the upflow furnace will push the air upwards while the downflow furnace will push the air upwards. That “flow” is upflow and downflow is in regards to the direction of airflow.
Here is a quick summary of the differences between upflow and downflow furnaces (we’ll explain why these differences happen further on):
| Differences: | Upflow Furnace | Downflow Furnace |
| Climate: | Best for colder climates (Canada, Illinois, Minnesota, New York, etc.) | Best for warmer climates (California, Texas, Florida, etc.) |
| Furnace Location: | In the basement | In the attic |
| Cooling/Heating Prioritization: | Best for heating | Best for cooling |
| Filter Slot Location: | Lower side of the furnace | Upper side of the furnace |
| Efficiency: | Depends on the furnace itself | Depends on the furnace itself |
| Fuel: | Gas, electricity, oil, diesel | Gas, electricity, oil, diesel |
| Cost: | On par with downflow furnaces | On par with upflow furnaces |
Note: If you are wondering if you have an upflow or downflow furnace, you just need to check 2 things (location, filter slot location). If the furnace is in the basement and it has the filter slot on the lower side of the furnace, you have an upflow furnace. If the furnace is in the attic and it has the filter slow to the upper side of the furnace, you have a downflow furnace.
Now, let’s look at why these differences matter on a one-by-one basis:
Cold/Warm Climate For Upflow vs. Downflow Furnaces
This is the key difference. You will almost never see someone in Texas installing a downflow furnace. Similarly, you will almost never see someone in Illinois installing an upflow furnace. Why is that?
Simple. Most HVAC systems that include a furnace can both provide heating as well as cooling.
In hot Texas, the cooling is more important. In cold Illinois, the heating is more important.
Compared to downflow furnace (located in the attic), the upflow furnace (located in the basement) is better for heating. That’s because cold air has a higher density than hot air. The result of this – as we all know – is that the hot air rises (to the attic), and cold air goes down (to the basement).
Here is what happens when we use a furnace for heating:
- Upflow furnace will take that cold air, heating it up, and push it upwards into our home. That’s why upflow furnaces are best for colder climates; they will provide more optimum heating.
- Downflow furnace will take a bit warmer air, heating it up, and push it downwards into our home. The point of a furnace is not to heat up that warmer air; it’s to heat up the colder air. That’s why downflow furnaces are not ideal for colder climates; they do not provide optimum heating.
Now, when we apply our HVAC system for cooling, here is what’s happening:
- Downflow furnace will take hot air, cool it down, and push the cool air downwards into our house. That’s why downflow furnace is best for hot climates; it provides more optimum cooling.
- Upflow furnace will take less hot air, cool it down, and push the cool air upwards into our house. Again, the point of cooling is to cool hot air (not already cool or just warm air). That’s why upflow furnace is not ideal for cooling.
Because of these characteristics, we install these different types of furnaces in different locations in the house. You will also see why the furnace air filter positioning is specific for both types:
Location Of Upflow Vs Downflow Furnaces + Filter Positions
Because upflow furnace pushes the air upwards, it’s only natural that we position them in the lower part of the building. Primarily, you install an upflow furnace in these locations:
- In the basement. Most upflow furnaces are located in the basement. It then pushes air up to the 1st, 2nd, 3rd floor, right up to the attic.
- On the first floor. This is usually reserved for houses that don’t have a basement. The lowest part of the house is thus the 1st floor.
Downflow furnace location is exactly reverse. You position them in the upper part of the building. Here are 2 of the most common places where you install a downflow furnace:
- In the attic. This is the most common location for downflow furnaces. The furnace then pushes the air in the downward direction on the upper floor, lower floors, and to the basement (if needed).
Attics are the go-to location choice for downflow furnace installs. - On the upper floor. If you don’t have an attic (modern architecture flat roof houses, for example), you would put the downflow furnace on the upper flow. The air is pushed downward to lower floors and, if needed, to the basement.
Now, the position of the air filter slot in the furnaces is also important. It’s basically the main thing you can use to immediately distinguish between an upflow or downflow furnace. Here is where the air filters are positioned:
- Upflow furnace: Air filter slot is located in the lower part of the furnace. This is between the supply vent air input and the furnace’s heating element. The direction of the furnace filter in upflow furnace should point upwards (upward arrow direction).
- Downflow furnace: Air filter slot is located in the upper part of the furnace. Similarly, you put the furnace filter between the supply vent air input and the heating element. Contrary to the upflow furnace, the downflow furnace is supplied with air from the top of the furnace (upflow furnace is supplied from the bottom).
- Horizontal furnace: Just to add the 3rd type of furnace. Horizontal furnace gets its air supply from the left, and the direction of supply airflow is to the right. That’s why you always point the arrow on the filter to the right as well.
You can read more about how to correctly position the furnace filter here. You can also see what happens if you put it backwards (bad idea) here.
These are the key differences. Not a lot – all based on the direction of airflow – but the differences are quite vital to understand when you are buying a new furnace. Let’s shortly look at similarities between upfront and downflow furnaces as well:
Similarities Between These 2 Furnace Types
Basically, all other characteristics are the same. This includes:
- Basic inner furnace mechanism.
- Types of fuels. You have gas upflow and downflow furnaces, electric ones, oil ones, and so on.
- Cost. The upflow furnace usually costs the same as the downflow furnace.
- AFUE values. These are energy efficiency ratios.
- Types of air filters used.
All in all, the upflow and downflow furnaces are different, but they are also very similar. We have got some questions about whether it makes sense to convert upflow furnace to downflow furnace, and visa versa. In a very large majority of cases, this doesn’t make sense.
We hope that you now understand the differences between these 2 furnace types. Based on this, you can make a better educated decision when buying a new furnace for your home.