As it proceeds through its sequence of operations, a gas furnace heater must reach and maintain a heating state without shutting off before heating the desired space to the designated set point. Normally, one would make sure that the thermostat is switched to “heat” and raise the set point to a level above the ambient temperature, and then the furnace will fire up and start blowing warm air within about a minute. Yet sometimes this either never happens, or the process is halted before reaching the desired temperature. We will explore some of the common problems encountered with a gas furnace in both residential and commercial applications, as well as the furnace repair services likely to be performed.
Residential and commercial furnace functionality is largely the same. A gas furnace essentially uses a thermostat to control when a set of flames should be produced and blown into the heating space. Along the way there are components acting in conjunction to produce this simple function, and then there are multiple safety mechanisms which only allow the process to continue under a very specific range of operating conditions, with power being cut and gas valve closing as soon as the parameters of safe operation are not met.
Once it receives the instruction to start from the thermostat, the furnace will first activate the draft inducer motor. This motor draws the products of combustion out of the firebox and through a flue. It must work for the sequence to continue, and its function is proved by a vacuum switch that will only let electricity through if there is a vacuum produced by the inducer. Once that is shown to be working well, the igniter or spark generator will start. Then a gas valve will open. While it is open, the machine must verify that it is being ignited. This is done through a flame sensor. If the flame sensor does not heat up in time to prove that the fire is on and gas being burned completely, then the furnace will shut down. Assuming the gas has been properly ignited, the furnace will continue operating.
After a short while, generally less than 30 seconds, the blower fan will kick in and blow air through the duct around the hot firebox and into the heating space. The furnace will keep doing this until the thermostat reaches its set point, unless one of the safeties stops proving correct function. The common problem at this stage occurs when a temperature limit switch around the furnace goes above its designated temperature range. This means the unit is overheating. At that point, the unit will completely shut down (the fan will continue for about a minute and a half and then shut down as well).
Sometimes, the beginning sequence does not make it through. This could be because the ignition has failed, because the gas valve did not open, or because the vacuum switch was never closed. Or perhaps there was never a call for heat registered, or the control board may have registered but improperly processed the call. Yet if the burners start and continue, but the fan never comes on, then there may be a problem with the fan not blowing. This, often times, is remedied by replacing the blower motor. One of the most common problems requiring furnace repair is overheating. Several repairs can be done to make the furnace operate within its temperature range.
If your furnace doesn’t work, please contact a furnace repair specialist immediately so they can help you stay warm and safe. Repairs are often simple, and a service technician can help you get up and running again after diagnosing the problem with your furnace heater.