The air coming out of the vent when heating should be 85 to 90 degrees for most homes, or 15 to 20 degrees warmer than the air in the room.
The air is hotter when it leaves the furnace – and the maximum allowable temperature rise through a furnace is usually 30 to 70 degrees based on the manufacturer’s specifications. The air cools slightly by the time it reaches the vents.
What Temperature Should Air Be Coming Out of the Vent when Heating?
There is no specific number for this temperature because it depends partly on the temperature of the air being pulled out of the room and into the furnace.
But generally, as noted, the temperature of the air coming out of a vent when heating should normally be between 15 to 30 degrees warmer than the air in the room where your thermostat is – the air in the cold air return ductwork.
It should always be somewhere in this temperature range, and we’ll explain why later.
So, if the air in your room is 65F – if that is the thermostat setting, in other words, air out of the vent should be somewhere between 80F and 95F, perhaps a little warmer if the vent is one of the first vents in the warm air supply ductwork.
How Does that Work?
Your heater – whether it is a furnace or a heat pump – works with a very important device called a heat exchanger. When your thermostat calls for heat, your heater causes the heat exchanger to become very hot. Air from your rooms will be drawn into the ductwork of your heating system and blown across the heat exchanger. Since the heat exchanger has a lot of surface area, the air that has passed over it will now be warmer. How much warmer? This depends on what the “temperature rise” of your furnace is.
Temperature rise will vary with different makes and models of furnaces, but it will probably be rated at between 30 to 70 degrees F, though most is in the 40-50F range. If you are interested, you can find this number on the rating plate inside one of the covers on your furnace, or in the specifications page in your User’s Manual.
How to Measure Change in Air Temperature
HVAC technicians usually measure this temperature by placing a temperature probe close to the heater on the “return side”, where the air is coming from your rooms to be heated, and another probe on the “supply side”, where the heated air is going into the ductwork leading to your vents. The difference between these two temperatures is called Delta T, and should be within the range specified for your furnace.
For example, if the return air coming from your rooms is 65 degrees, and your furnace is supposed to have a temperature rise of 30 to 70 degrees, that means the supply air should measure between 95 and 135 degrees about three feet from your heater.
But that doesn’t mean the air coming out of your vents will be that warm because this temperature will drop as it travels through the ductwork. And if your system has been designed correctly, the air coming out of the vents will be in that 15 to 30 degree range warmer than the room temperature that we mentioned earlier.
Return to the Example
So, in our example of 65 degree return air, you should have warm air coming out of your vents at a temperature of 80 to 95 degrees F. As this warmer air gradually heats the overall room temperature, the return air will then become warmer, meaning that the supply air will also be warmer. This will continue until the thermostat is satisfied and the heater turns off.
In the case of a high-efficiency furnace, it may not turn off completely, but shift (modulate) down to a much lower level of heating to maintain your desired temperature without that “warm-cool-warm” effect that no one likes.
Does it Really Matter?
Yes, if you want your heating system to be functioning at its best and keeping you warm and comfortable. If it is outside the recommended range of 15 to 30 degrees, it could be caused by a number of different issues that we have listed below.
How Can I Check It?
All you really need to measure the difference between the return air and air coming through your vents is a thermometer, either an infrared thermometer or other digital thermometer that you can hold in your hand.
Turn up the thermostat to get the furnace running, and let it run for at least 2 to 3 minutes. Now check the room air temperature where the air is being drawn into the system in a return vent. You can recognize a return vent because they are usually larger than supply vents, do not have louvers that can be closed, and they will pull on a piece of paper or cloth held near them when the system is running.
Write down this temperature.
Now measure the temperature of the air coming from a supply vent. A supply vent will usually have louvers inside that you can see and can be adjusted by a small lever on the front of it. Naturally, this air should be warmer than the air that you measured by the return vent, and will be blowing outward when the system is running.
Write down this temperature.
Simply subtract the lower number (return air) from the higher number (supply air), and this will give you the Delta T of your heating system. If everything is adjusted and working correctly, it should be 15 to 30 degrees.
This chart may help you figure what the supply air should be with some common room temperatures.
Room Temperature and Temperature of Air Through Vents When Heating
| Room Temperature | Optimal Supply Vent Air Temperature |
| 65 F | 80-95 F |
| 67 F | 82-97 F |
| 70 F | 85-100 F |
| 72 F | 87-102 F |
| 74 F | 89-104 F |
If your temperature difference (Delta T) is above or below the 15 to 30 degree range, here are some possible issues that may be causing it. Some issues you can probably resolve yourself, and some for which you may have to call in a professional.
Issues that You Can Resolve
Dirty Air Filter – If the air filter in your heating system is dirty, it will hinder the airflow through the furnace and the Delta T will increase because sufficient heat is not being removed from the heat exchanger. Normally, if it gets too hot, safety devices built into the furnace will shut down the system to prevent damage, but simply changing the air filter can resolve this issue.
Return Air Vents Blocked – Furniture that is pushed against a return vent on the wall or a carpet or rug covering a return vent in the floor will also restrict the airflow of your system. This can cause poor circulation, little air coming from vents, and even overheating of the system.
Issues Requiring a Competent HVAC Technician
Return and/or Supply Ducts too Small – It is not impossible that the duct system was not properly engineered when the furnace was installed. If that were the case, this problem would probably result in a high Delta T, possibly even to the extent that the high limit switch would trip and shut down the furnace until it cooled down.
Burners Not Adjusted Properly – This could cause either high or low Delta T, depending on whether the burners are over or under firing. Fan Speed Too High – This situation will result in too much air passing over the heat exchanger and the Delta T to be lower than recommended. This will probably make your house feel cool or drafty much of the time.
