First, we will look at gas furnaces. You should check for dirt, rust, etc. inside the furnace and clean as necessary. You should also check the vent pipes to make sure they are not blocked and are in good physical condition. You should clean or replace the air filters. At this time it is a good idea to ensure that the blower wheel is clean and to oil the motor if required.
**NOTE** Most newer furnace blower motors do not require oiling.
If you have a condensing furnace, you should check to make sure the condensate drain is not clogged. You should also check your thermostat. If it is not a digital unit, ensure that it is perfectly level. This would be a good time for thermostat replacement. Normally, replacing a mechanical thermostat with a digital unit, will save about 5-10% on the heating bills. Finally, you should run the furnace through a normal heating cycle to check for proper operation. During this cycle, you should check the temperature rise across the furnace.
**NOTE** The furnace should have been running for ten minutes before you measure operating temperatures.
To do so, you should measure the return air temperature (temperature of air entering the furnace) and the supply air temperature (temperature of air leaving the furnace). Then, you should subtract the return air temperature from the supply air temperature. You will find a recommended temperature rise range typically about 35-65 degrees)on the label plate for the furnace. Compare the value you measured with the label plate. If the temperature rise you measured is below the minimum range for your furnace, the blower heating speed can normally be reduced. If the temperature rise you measured is above the high range for your furnace, the blower heating speed can be increased.