The air coming out of your furnace, measured right when it exists the supply vent, should be 40°F to 70°F above the return air temperature.
Most furnaces have a data tag that displays this information. The tag typically displays the temperature of supply air, often around 105°F, and the temperature difference, often between 40°F and 70°F.
Of course, the exact figures vary depending on outdoor temperatures and the thermostat setting, among other factors. Read on to learn more about supply and return air, the ideal temperature difference when the furnace is running, and when you should get worried.
How the Furnace Heats Your Home
Furnaces help raise indoor temperatures during the cold season. It’s a reasonably straightforward process. When you turn on the furnace, it burns fuel (typically natural gas or propane).
The heat exchanger then transfers the heat to air coming from your home. Then the now-warm air comes out at the other end to keep you and your family (and guests) toasty and comfortable. Once warm air enters the home, it circulates through the designated space, such as the entire living room or even the whole house.
Then as it loses its heat content through diffusion and convection, it becomes denser, settles towards the floor, and ultimately re-enters the furnace through the return air plenum for re-heating. The process goes on and on to keep the room in the thermostat setting. It can be a more complicated process depending on the type of furnace.
But hopefully, you get the point – cold air enters the furnace at one end, and warm air exits at the other in a continuous process that goes on until you turn off the furnace.
What is Supply and Return Air?
The cold (or room temperature) air entering the furnace from your home is known as return air. It’s called so because the air is “returning” to the furnace for re-heating. A few things characterize return air;
- It’s cold: Return air is typically at room temperature. This can be extremely low, depending on the time of the year.
- It may be unclean: Return air harbors dirt, dust, pet dander, and so forth. Pretty much everything lingering in your indoor air, including allergens and germs, is found in the return air.
Meanwhile, supply air is the air entering your home from the furnace. It is called so because the furnace is “supplying” your home. Supply air is characterized by the following;
- It’s warm: Supply air is generally tens of degrees (F) warmer than return air. Generally, it’s 40°F to 70°F warmer than return air.
- It’s clean: The furnace has a special filter stationed just after the return vents that traps 99.9% or more of the airborne particles in the return air. Thus, the air coming out of the supply ducts is significantly cleaner and healthier.
What’s Temperature Differential and What’s the Ideal Differential?
The temperature differential is the difference between the return air temperature (measured at the return air vent) and supply air temperature (measured at the supply air vent).
Also known as the “temperature rise,” it’s a figure that tells you how much the thermostat has raised your indoor temperature. If you’re interested in knowing your temperature differential or rise, there are two ways to find out.
First, you can check the data tag on your furnace. Most furnaces have the tag. Look for terms such as “differential,” “rise,” or temperature rise/differential.”
Typically, you’ll find values such as 50°F to 80°F or 40°F to 75°F displayed against the differential term. The first term is the lowest temperature in your home at any time, indicating the return air temperature at the return vent.
Meanwhile, the second figure is the highest temperature in the room within the same period, indicating the supply air temperature at the supply vent.
The difference between the two values gives you the temperature differential. So, in the first example above, the temperature rises from 50°F to 80°F, amounting to a 30°F temperature difference.
Meanwhile, in the second case, the differential is 75°F-40°F = 35°F. Most furnace data tags will also indicate the rise. Alternatively, you can manually measure the supply vent and return vent temperatures using a thermometer and subtract the two values to find the temperature differential.
What’s the Ideal Temperature Differential?
Any rise between 30°F and 60°F is considered a good temperature differential. However, you should aim for at least a 40°F rise.
What if the Temperature Differential isn’t Hitting the Target?
If your furnace’s temperature differential is below 30°F, something is not right. It could be something blocking airflow within the furnace or a heating malfunction that’s compromising heat output. Whichever the case, you need to fix the issue right away.
Why Is My Furnace Blowing Lukewarm Air?
Assuming that the appliance is up and running, the following are common reasons your furnace may be blowing lukewarm air, thus failing to hit the ideal temperature differential;
- Blocked vents/ducts: You will get little hot air or may get none at all if your vents or ducts are partially or fully blocked.
- Leaking ductwork: Alternatively, the ductwork could be leaky. If the warm air leaks away, you’d be left in the cold.
- Dirty air filters: As we mentioned earlier, return air passes through filters to get to the heat exchanger. If the filter is blocked, very little air will pass through to the supply plenum.
- A poor burner flame: A small or sooty burner flame also results in poor heating. Feeble and sooty burner flames are a sign of poor combustion, often due to inadequate air circulation.
- Damaged heat exchanger: You should also check your heat exchanger. If it’s damaged, it may lose its capacity to transfer heat from the burning flame to supply air.
- The furnace is aging: Finally, an aging furnace loses the “strength” to pull in cold air and push out warm air at the desired pace. Thus, the home may be underheated in the end.
How to Fix a Furnace that’s Blowing Lukewarm Air
If your furnace isn’t hitting the desired temperature differential, the following are several steps you can take to rectify the problem.
- Regular maintenance: Regularly clean the furnace, especially the vents, ducts, and air filters, to ensure unrestricted airflow. Maintenance also includes fixing/replacing dents on the ductwork and leaks throughout the furnace and replacing the filter as appropriate.
- Ensure proper air circulation: Your furnace needs unimpeded airflow to operate optimally. So, ensure sufficient clearance all around the unit and the ducts. Sometimes you may need to open the windows for a short period to enhance airflow.
- Replace the appliance: If the poor performance emanates from old age, you may have to replace the furnace. This makes even greater sense when you consider that repairs may not keep the furnace alive for long after a certain age, usually 15 years.
Which is Suitable for Furnace Temperature?
According to the Department of Energy, the best temperature setting for winter is 68°F, though any setting from 68°F to 72°F is acceptable.
However, if you’re conscious about your energy spending and carbon footprint, you may want to lower the setting considerably to save on power. According to the DOE, homeowners can save 5% of their annual energy budget by adjusting the thermostat down by 2-3°F.
It means you can save as much as 8% of your energy bill by lowering your thermostat by 7-8°F for eight hours a day. Another saving opportunity is during the night.
Granted, sleeping in the cold isn’t ideal. However, your body generates tons of heat when sleeping, meaning you don’t need as much heat when you retire to bed.
So, you can keep your thermostat at or below 68°F for the night.
How to Maintain the Perfect Furnace Temperature in Your Home this Winter
- Insulate your home: You can keep your rooms warmer while using up to 30% less heating energy if you insulate your home.
- Use a programmable thermostat: According to the Environmental Protection Agency, a programmable thermostat can save you 10-30% in energy costs yearly.
- Schedule professional maintenance: A professional tune-up before the onset of the heating season keeps your furnace running more efficiently and can save you from expensive mid-season repairs.
There you have it. If you’ve been wondering how hot the air from your furnace should be, the ideal range is 40°F to 70°F above return air. A higher value doesn’t necessarily mean better performance. However, a temperature difference below 30°F is considered underperformance and must be diagnosed right away.