Temperature rise is the difference of air temperature between the conditioned return air and the heated supply air sent from the furnace.
Temperature rise is part of the initial setup of a furnace. Before taking a temperature rise reading, you will need to make sure you have a clean filter, proper gas pressure, combustion test, and correct furnace/duct/return air size.
The temperature rise is listed on the furnace data-plate and will be similar to the one listed here.
The example data-plate above states a required temperature rise of 40°F – 70°F. Meaning the temperature rise cannot fall outside said range. We generally aim for the middle range. Doing so will give the customer some wiggle room for when the filter gets dirty and airflow is reduced.
Before taking any readings, allow the furnace to get into a steady run state for about 10 minutes. The supply air temperature should be made a couple of feet from the furnace plenum takeoff to avoid heat radiating off the heat exchanger and giving you an artificial, and also very high-temperature rise.
The return air temperature is subtracted from the supply air, and the result is the temperature rise.
Example: Furnace data-plate calls for a 40°F – 70°F temperature rise.
Readings collected: Return air is 66°F and supply air is 120°F
120 minus 66 results in a temperature rise of 54°F
When taking a temperature rise, the goal is to be somewhere in the middle of the requirement. If you’re too low, you risk creating condensation on the heat exchanger if you happen to fall below the low end of the scale. You will also be sending cool air through the structure which makes for uncomfortable living conditions. The typical reason for this is too high of a blower speed or an underfired system. Lower the blower speed if you’re moving too much air or adjust gas pressure if you have an underfired system.
Being too high on the temperature rise scale is also not a good thing either. Being high on the temperature rise can cause the furnace to overheat and shut-off on high-limit causing problems with furnace components and the heat exchanger itself over time. A high-temperature rise is usually caused by overfiring the appliance or not moving enough air across the heat exchanger. Check gas pressure to correct overfiring the appliance. If you’re not moving enough air, increase blower speed if needed, clean/change filter, and check for sufficiently sized ductwork for lack of airflow issues.