If your gas furnace initiates the start cycle but doesn't fire up, it's likely because of a faulty, damaged, or dirty ignitor/sensor. This is a very common problem — you can try to clean the sensor and replace the ignitor.
Defective Igniter or Pilot Light
If your furnace has been working without any problems up to now and suddenly will not ignite, there's a good chance that either the electronic ignition system has failed or that the standing pilot lights that comes before the ignition process has gone out.
When you turn the furnace on but no heat comes out, there may be an obstruction. Dirty air filters restrict airflow, which can make it feel like not much heated air is coming out of your vents. Check the furnace filter and replace it if its surface is covered with contaminants.
Most likely, your furnace is making a clicking noise and not providing heat because your furnace's spark igniter is trying to ignite your pilot light or burners but is failing to do so. In other words, there's an ignition system malfunction that needs to be fixed.
If your thermostat is off but heat is still on, you are probably experiencing some kind of "short" or connection issue. Most often with this problem is that a wire is touching something it should not be touching. Another possibility is that the gas valve is stuck open. This is rare, but it can happen. If.
The cause of your furnace not blowing hot air could be the thermostat configuration, a dirty air filter, the high limit switch, a clogged condensate line, insufficient fuel, leaky ducts, ignition problems, a blocked burner, flame sensor issues, gas valve malfunctions, a cracked heat exchanger, or a clogged coil.
A propane furnace or oil furnace blowing cold air might indicate a low or empty fuel supply. And, a clogged oil filter can also cause an oil furnace to turn on and blow cold air. If you suspect a fuel supply issue with your natural gas furnace, it's best to contact a qualified professional.
Short cycling is a common sign of a bad flame sensor. The sensor thinks that there isn't a flame, so it shuts off the gas as a safety mechanism. Except, there is a flame and without the gas to supply it, it dies. This is a frustrating problem, but an HVAC technician can fix it.
You can reset the ignition on your furnace simply. To do this you must cut the power to the furnace first by turning off the circuit that powers it. Once the power is off you can remove the burner door to reveal the ignitor. Turn this to “off” and leave it off for at least 5 minutes.
Try checking the circuit breaker box or replacing the batteries. Bad wiring: The thermostat may be unresponsive if the wiring has gone bad. Remove the cover and check inside for loose, detached, or corroded wires. If you spot any problems, contact an HVAC technician for assistance.
In most newer systems, the button is often directly next to the blower motor, while in older systems, it may be harder to spot as it is sometimes underneath or behind the motor. The reset button is often made of yellow or red plastic and marked with a large “R ” to make it easier to spot.
While furnaces can stop operating for complicated reasons, home owners should check a few usual suspects first behind a lack of hot blowing air: Electrical problems causing a loss of power. Blocked air vents and ducts restricting airflow. Improperly configured thermostat settings.
There is a work-around to waiting. If there is no safety risk (like a gas leak), a furnace can be reset to bypass this wait. Turn off the furnace's power switch and wait 10 seconds before turning it back on.
Check Your Furnace Air Filter
Next, check your heater's air filter, as this can cause a furnace to stop blowing air. Your filter could be so saturated with dust and debris. In that case, even though the furnace is actually blowing air, your clogged filter is blocking it from making it into your ducts.
The Short Answer Is: Warning signs of a bad furnace ignitor include the furnace not starting or heating, a noticeable clicking sound without ignition, or frequent short cycling where the furnace turns on and off rapidly. Additionally, if the ignitor appears cracked or discolored, it may need replacement.
Most furnace flame sensors cost less than $20, but if you opt to have a professional HVAC technician make the repair, you can expect to pay between $75 and $250. Need help replacing your flame sensor?
If it's broken or malfunctioning, it may fail to detect the flame and shut off the gas supply as a precaution, even if the burners are actually lit. This can result in your furnace starting the ignition process but failing to stay lit.
Flame sensors have an average lifespan of about five years. Therefore, you can expect to need to replace it at least twice before replacing your entire heating system.
A dirty or malfunctioning igniter can prevent your furnace from lighting. The technician will clean the pilot assembly if you have an older furnace with a standing pilot light. For furnaces with electronic ignition, they will check the igniter for any signs of wear or damage.
Clogged or Dirty Air Filters
A dirty or clogged air filter can restrict airflow, causing your furnace to overheat and shut off the burners. In such cases, the blower fan will continue running, but no warm air will be distributed throughout your home.
A furnace blowing cold air could be the result of a leak in your ducting. Over time, the changing temperatures wear on all sorts of things, including your ductwork. In fact, it's rather normal for ductwork to develop leaks over time. When ductwork has a leak, cold air will enter a duct and blow into your home.
1. Will a furnace run without a flame sensor? No, a furnace will not operate without a flame sensor.