What is the difference between an upflow and downflow furnace?
The answer is about more than just the obvious – which way the air flows out of the furnace. The important issue is that your climate determines which is best for your home.
Here is the basic answer explained in this article:
- Upflow furnaces blow heated or cooled air upward and are installed in the lowest level of your home. An upflow furnace is the best choice in cold climates where it is used for heating more than for air conditioning.
- Downflow furnaces blow heated or cooled air downward and are installed in attics. They are the best choice for warm climates where air conditioning is used more than heating.
What is the difference between an upflow and a downflow furnace? In mechanical terms, the answer is simple and discussed below.
The question is a little more complex when discussing the question “Is an upflow or downflow furnace better?” for your home.
Basic Furnace Design and Operation
It’s worth noting that your home has two sets of ducts. Supply ducts carry heated or cooled air, called treated air, to the rooms of your home. Return ducts carry “spent” air back to the furnace. A few homes do not have return ducts. The furnace pulls air in from living areas to heat or cool.
Here is how a furnace heats:
- The furnace burner burns natural gas (NG), propane (LP) or home heating oil to create heat.
- The heat is transferred into the ducts (ductwork) by passing through a heat exchanger surrounding the chamber that prevents combustion gases including carbon monoxide from mixing with air in your home.
- The transferred heat is circulated by a blower, also called a circulating fan. When heating, the blower pulls cool air into the furnace through the return ducts. It heats it and pushes the warmed air through the supply ducts.
This pulling and pushing of air is why these heat systems are called forced air heating.
How is a Furnace Used for Air Conditioning?
The same basic process happens with air conditioning.
Here’s how a furnace cools:
- The blower pulls warm, humid air into the furnace.
- A radiator-like coil and refrigerant collect heat from the air. The refrigerant circulating through copper lines carries it outside where it is released through the outdoor coil. The cooled air is pushed through ducts into your home. But there’s a second benefit.
- As a result of this process, the indoor coil becomes very cold. The blower pulls in warm, humid air. As it passes over the cold coil, moisture in the air condenses on the coil and is drained out of the house. Dehumidifying the air is an important part of air conditioning.
What is an Upflow Furnace?
An upflow furnace is built with the combustion chamber on top and the blower fan on the bottom. The furnace pulls in cool air from return ducts. The air is pushed over the very hot heat exchanger to warm it. It pushes the air through the supply ducts and grates in each room.
The advantage of this design is that cool air naturally sinks, so pulling it into an upflow furnace located in a basement or crawlspace takes less energy. Conversely, hot air rises, so pushing it out of an upflow furnace is efficient too.
Upflow Furnace Installation Location
An upflow furnace is installed on the lowest level of the home. This is often a basement or crawlspace. When homes are built on a slab, an upflow furnace is installed on the first floor or in the garage.
What is a Downflow Furnace?
A downflow furnace is designed opposite to an upflow furnace. The blower is at the top of the furnace. The combustion chamber is below.
When heating a home, the blower pulls in cool air and pushes it down, over the combustion chamber and heat exchanger, and into your home. Compared to an upflow furnace, pushing heat down takes more energy because heat naturally rises.
When your HVAC system is in air conditioning mode, the downflow performance of the furnace is efficient. Cool air naturally sinks, so pulling in warm air that naturally rises and pushing down cool air takes less energy.
Downflow Furnace Installation Location
A downflow furnace is installed in an attic or the highest level of a home.
What is a Horizontal Flow Furnace?
If you’ve been reading along, then you likely understand how a horizontal-flow furnace works. Untreated air is pulled into the unit, treated (warmed or cooled, depending on whether the HVAC system is in Heat mode or AC mode) and forced back into the living spaces of your home.
There are few horizontal-only furnaces. Instead, most are upflow/horizontal or downflow/horizontal furnaces. A simple change of the air outflow portal switches any furnace to a horizontal model. The change is often made during installation.
Furnace Cost: Upflow vs. Downflow
Furnace cost estimates vary only slightly for the two types of furnaces. However, it probably won’t make a difference to you. Based on your climate and the construction of your home, you will either be given upflow furnace cost estimates or estimates for a downflow furnace.
Gas furnace cost information is available in our Gas Furnace Buying Guide that includes features and cost factors to consider, the cost of all furnace brands including Carrier, Lennox and Trane and miscellaneous cost factors most sites overlook. You’ll find the most accurate installed gas furnaces costs on PickHVAC.
Furnace Efficiency: Upflow vs. Downflow
Upflow furnaces are more efficient for heating. Downflow furnaces are more efficient for air conditioning.
When an Upflow Furnace is Better
From the discussion above, it is clear that an upflow furnace is preferred for cool and cold climates. We recommend upflow furnaces for Zones 5, 6 and 7.
The key questions are: Do you heat your home more than you use air conditioning? If so, an upflow/horizontal flow furnace is the right choice because it works with warm air’s natural tendency to rise and cool air’s tendency to fall. An upflow furnace will reduce your heating costs and make your home more comfortable.
The upflow furnace should be installed in the lowest part of your home, when possible in the basement/crawlspace (upflow position) or on the first floor in a home built on a slab.
When a Downflow Furnace is Better
If you cool more than you heat, a downflow furnace is better. A downflow furnace is usually better in Zones 1, 2 and 3. In Zone 4, rely on your furnace contractor to make a recommendation about furnace type.
This requires room in the attic or upper floor for furnace installation. If you plan to install the unit on the roof, then a downflow heating/cooling package unit is recommended because they are designed for outdoor installation.
If your home has a low attic, a furnace installed for horizontal flow is the next-best choice.
In short, a downflow furnace is the energy-efficiency choice in climates where the cooling season is long, warm and/or humid.