# TEMPERATURE RISE MEASUREMENT IN A FOSSIL-FUEL FURNACE

StepExpected Result/Action
1. Drill a hole in the return air duct near the furnace as shown in Figure
SP-13-2. Drill a second hole in the supply air duct out of the line of sight of
the heat exchangers so that radiant heat does not affect the readings. This is
especially important with straight duct runs.
Holes drilled in the supply and return air ducts in preparation for measurement.
2. Turn on the power and gas to the unit. Allow the furnace to run about ten
minutes to stabilize the temperatures. Set the thermostat high enough to
allow the furnace to run continuously during the measurement.
Furnace on and temperatures stabilized.
3. Using the electronic thermometer, measure and record the return and supply
air temperatures. Make sure the thermometer readings have stabilized for
the measurement.
Supply and return air temperatures measured and recorded.
For our example assume:
Supply temperature = 128° F
Return temperature = 7 0 ° F
4. Using the measured values of supply and return air temperatures, calculate
the temperature rise using the formula:
Supply Temperature - Return Temperature = Temperature Rise
Temperature rise is calculated.
For our example assume:
128° F - 7 0 ° F = 5 8 ° F
5. Compare the measured temperature rise to the temperature rise range on
the furnace information plate.
If the temperature rise is within the range shown on the furnace information
plate, no adjustment of airflow is required. Ideally, however, the rise should be
slightly above the midpoint of the range. For example, with a temperature rise
range of 40° F to 70° F, the midpoint is 55° F.
For our example assume:
The temperature rise of 5 8 ° F falls slightly above the midpoint (55° F) of the
range of 4 0 ° F to 7 0 ° Fshown on the furnace information plate. This indicates
If the temperature rise is too low, decrease the blower speed to decrease the
airflow. If the temperature rise is too high, increase the blower speed to increase
the airflow. Follow the furnace manufacturer's instructions for increasing or
decreasing the blower speed.

Purpose – This procedure describes how to measure the temperature rise in fossil-fuel furnaces. Temperature rise is the difference between the return air temperature entering the furnace, and the supply air temperature leaving the furnace. The amount of temperature rise gives an indication of whether adequate air is flowing across the furnace heat exchangers. However, it cannot determine air quantity. Temperature rise measurements are normally made when a furnace is initially installed and when troubleshooting furnaces.

Temperature Rise
The correct temperature rise range for a particular furnace can be found on the furnace information plate (Figure SP-13-1). The correct amount of temperature rise is critical in highefficiency furnaces. If too much air passes over the heat exchangers, condensing can take place in the heat exchangers or vent, causing corrosion and failure. If too little air passes over the heat exchangers, the resultant overheating may cause premature failure of the heat exchangers. If the temperature rise is too low or too high, the furnace’s blower speed must be changed to bring the rise into the desired range. Before making temperature rise measurements, make sure that:
• The furnace is fired at its full rated input.
• The furnace air filter is clean.
• All supply and return registers are open and unrestricted.
• If equipped with a bypass humidifier, the damper in the bypass duct must be closed.

Temperature Rise Measurement

The method for measuring temperature rise is provided in the detailed procedure at the end of this section and briefly outlined here.

Temperature measurement holes are drilled in the return air duct near the furnace and in the supply air duct out o f the line of sight of the heat exchangers (Figure SP-13-2). The furnace is turned on and o p e rate d for ab o u t ten minutes to allow the temperatures to stabilize.

The supply and return air temperatures are measured with an accurate thermometer, then the temperature rise is calculated by subtracting the return air temperature from the supply air temperature. Ideally, the temperature rise should be slightly above the midpoint of the range shown on the furnace information plate. If the temperature rise is too low, decrease the blower speed to reduce the airflow; if too high, increase the blower speed to increase the airflow.