In the middle of the heating season, the last thing you need is for your furnace to stop working. There are many reasons for a furnace not to turn on. Some require a professional to diagnose and fix; others can be addressed without a technician’s help. A furnace problem can originate from a variety of sources:
Your furnace relies on feedback from the thermostat to turn on. Make sure the thermostat is set to “heat” and set the temperature high enough for the furnace to kick in. If it’s set on heating mode, switch the fan “on” to get the furnace to blow air. Otherwise, check that your thermostat has power or displays an error code (for programmable units).
Furnaces run on electrical power. If the unit won’t turn on, check that its service switch is “on”; An electrical power switch may be on or near the furnace, or an emergency shut-off switch in the furnace room or basement may be improperly set. Also, the circuit breaker serving the furnace may have tripped. Check the main electrical panel to see if it needs to be reset.
Other power-related causes include an open inspection door (if your furnace has a safety switch), a damaged power cord, or electrical shorts on the control board.
If the pilot light is out, try to light it, but make sure the flame reaches the bottom of the thermocouple switch. Check the assembly or sensor for dirt, and clean it, if the pilot light goes out too quickly. A faulty ignition sensor can also cause a problem.
The reservoir can fill up, but you can drain the reservoir to resolve the problem. If that doesn’t work, look for a blockage in the condensate pipe. Aside from a clogged drain, the pan’s pump may need to be cleaned. But when the float switch is stuck in the “up” position, a new pump will likely be required.
Check the unit’s inspection window to see whether the light is flashing green. If not, or it is red, the blower motor may have failed. The good thing is you don’t need to know much about the blower motor, or where it is, to check whether it’s working or not.
A clogged air filter will prevent your furnace from turning on. Furnaces have a safety feature for this to prevent them from overheating. Replace any dirty air filter. If you check and clean/replace your filter often, your furnace should never malfunction due to poor air quality.
Check that the gas valve is fully open. The furnace won’t turn on if the gas supply is insufficient, and leaving the valve only partially turned for safety reasons can cause a malfunction. Make sure the main gas line is turned on. But if you smell gas, evacuate your home immediately and call an emergency technician.
Modern furnaces have switches that can affect performance. On high-efficiency units, look for whether the condensate pan overflow switch has been triggered. Removing standing water under the air handler should reset the switch. The flue gas spill switch on a newer gas-fired furnace, designed to prevent leakage, can cut power if flue gases aren’t properly venting. Call a professional to address this problem.