It depends on the type of heater. Generally, heat pumps produce heated air in the range of 85°F and 92°F. Meanwhile, furnaces produce much hotter air, between 130°F and 140°F. Remember that the ideal indoor temperature is about 72°F.
Additionally, it’s important to note that the point at which you take the measurement matters significantly. The 130-140-degree range applies to measurements taken at the furnace.
Supply at the furnace is up to 70°F hotter than return air. However, if you measure at the register, the supply air will be about 40°F warmer than the return air. This means that you should expect supply air temperatures around 110°F to 120°F.
If the temperature coming out of your vents is lower or higher than the typical range, there could be a problem. But, more importantly, lower or higher supply air temperature can cause several issues, including cold conditions and overheating, respectively. That’s why you need to check your supply air periodically. It will tell you whether your heating appliance is working optimally.
Let’s find out the typical supply and return air temperature values, how to measure the values, and what to do if your furnace is producing too hot or lukewarm air. Remember that this guide applies primarily to furnaces.
Understanding the Key Terms
Before we go any further, it’s important to know the difference between the two terms, i.e., return air and supply air.
- Return air is the air entering the furnace or any other HVAC system, whether it’s an air conditioner or dehumidifier. It’s the stale air flowing from your room into the HVAC appliance.
- Supply air: This is the air flowing from the HVAC appliance to your home. It is conditioned air. For instance, the warm air released into the home raises indoor temperatures during the heating season.
The difference between the return and supply air is Delta T. Delta means difference while T stands for temperature.
Determining Supply Air Temperature
There are two main ways to determine supply air temperature for heating appliances – checking the fan limit switch ratings and measuring the temperature manually.
Fan Limit Switch Ratings
The fan limit switch is a component of the furnace system that determines when the fan blower system comes on and when the fan goes off. Additionally, it occasionally determines when the furnace shuts down – typically when there’s overheating.
Blower fan switches have ratings on them that indicate the supply air temperature under different circumstances. The three main settings are as follows;
- FAN OFF: This rating indicates supply air temperature when the fan is not running. In many furnaces, the value is set at 90°F. So, if the furnace is running but the fan is off, then the temperature coming out at the supply vents is likely around 90°F. It can be slightly more or slightly higher.
- FAN ON: This value indicates the average temperature of supply air when the furnace is on, and the fan is running. In most cases, the fan will only start running when the supply air is at least 20°F higher than in the FAN OFF position. So, expect the supply air with the FAN ON to be around 110°F.
- FAN LIMIT OFF: Finally, the fan-limit-off setting is the factory preset temperature at which the limit switch automatically trips off to shut down the furnace. It’s usually triggered when overheating is detected. Most manufacturers set the limit at 200°F.
Measuring Supply Air Temperature Manually
Since temperatures fluctuate significantly from one second to the next and one application to another, sometimes it’s best to measure the temperatures manually to determine whether your supply and return temperatures are within range.
All you need for the process is a masking tape and perhaps a masking tape. Also, wait until the conditions are suitable for temperature measurement. The best time to measure is on a day when outside temperatures are above 60°F or lower. Allow the furnace to run for at least 15 minutes before you begin.
Once all conditions are met
- Begin by placing the thermometer at the return vent. Leave it for five minutes and record the reading.
- Move the thermometer to the return vent and do the same. Wait for five minutes and record the temperature.
- Find the difference by subtracting the return temperature value from the supply temperature value. The difference is Delta T.
- If you’re interested in the average supply and return temperature values for the entire house, measure the return values for all rooms, add them, and divide the sum by the number of rooms. Then, do the same for supply temperatures.
Common Issues and Solutions
A furnace that produces too little heat may leave you in the cold or result in uneven heating. The following are common causes and what you can do about each.
When the Temperature is Too Low
You should be worried if your furnace supply temperature is below 100°F. It means that the furnace isn’t producing enough heat. The lower the value, the worse the problem. Common causes include;
Wrong thermostat setting
You should set the thermostat between 68°F and 72°F during winter. If it’s set lower than that, it will produce cooler air in compliance with the settings.
Solution: Ensure the thermostat is set to the correct temperature, ideally above 68°F for the entire winter season.
Leaky air ducts
Leaky ductwork can lose most of the heat within the air while absorbing cooler air from outside the house. This often leads to lower supply air temperatures.
Solution: Check whether there’s a leak in your duct system and fix it promptly. You’ll likely need professional assistance.
The furnace is overwhelmed
The furnace can be overwhelmed when it’s freezing outside. This is especially common in very cold states up north, notably in homes with older furnaces.
Solution: If your furnace is overwhelmed, the only solution is to get back up or replace it. A supplementary electric or gas cooker will do the trick until you’re ready to purchase a new furnace.
When the Furnace Overheats
Overheating is even more dangerous than under-heating as it can cause an explosion, which puts you at significant risk. Overheating can also damage the heat exchanger and the entire furnace. The following are common causes and solutions;
Poor air circulation
One of the most common causes of furnace overheating is poor air circulation. Restricted airflow causes internal components of the furnace to become too hot. This usually happens if there’s dirt buildup or a blockage.
Solution: The only remedy here is to ensure regular maintenance. Keep the filters, fan, and entire furnace clean at all times.
The thermostat determines when your furnace comes on and when it goes out. It also directly determines the temperature of air coming out at the vents. Therefore, if the thermostat fails, the supply air can be hotter than ideal.
Solution: Check that the thermostat is located in the correct position and set to the right temperature. If both location and positioning are accurate, then the thermostat could be broken.
Malfunctioned temperature limit switch
The furnace has several mechanisms built in to regulate temperature, specifically to prevent overheating. These include the fan limit switch and the temperature limit switch, also known as the furnace plenum thermostat. If any of these switches fail, you may experience overheating.
Solution: The best solution here is to call your HVAC services provider as checking, diagnosing, and fixing the limit switches requires technical know-how.
Supply temperature when heating with the furnace should be 130°F to 140°F at the furnace and about 110°F to 120°F at the supply vent. If your supply air temperature is outside this range, there’s a problem. Fortunately, most furnace under and overheating issues can be diagnosed and fixed with ease.