Technicians often hear how critical airflow is in the cooling season, but we sometimes forget that it is just as important in heating as well. Just as poor airflow can be a major source of problems in the cooling season, poor airflow in the heating season can cause inefficient operation, high utility cost, and system failures.
Gas and Oil Furnaces
Gas and oil furnaces will operate with a higher than normal temperature rise if the airflow through the furnace is inadequate. While the air leaving the registers will be toasty, this can actually result in less heat being delivered into the house. The limit switch on a furnace with inadequate airflow will typically cycle the burners on and off - reducing the amount of heat being produced. The cycling causes poorer operating efficiency, increasing the cost of operating the furnace.
With electric furnaces and strip heaters, poor airflow will eventually result in open fusible links, bad thermal limits, and open heat strips. Like the gas and oil furnaces, the thermal limits may cycle the strips on and off, reducing the amount of heat.
Heat pumps with poor indoor airflow will operate at excessive head pressures because the indoor coil is the condenser in the heating cycle. This reduces system capacity by increasing the compression ratio while the compressor is operating. Eventually, the compressor will cut off on the high pressure switch. Usually, the high pressure switch will need to be manually reset, meaning no compressor heat until the service tech arrives. Systems without a high pressure switch will shut off on the compressor internal overload. Once that happens, the compressor may not come back on for a couple of hours. Either way, the compressor is taking a beating and the system is using more electricity to produce less heat.
So if you run into furnaces that are cycling on the limit, electric strips with open fusible links, or heat pumps with tripped high pressure switches – check out the system airflow. Remember those are all symptoms – not the root cause. Fix the cause of the problem – poor airflow.