Every winter season homeowners have issues with their furnace staying lit, thus not getting heat and remaining cold for an extended period. We know how it goes. You go to bed comfortably and wake up in the middle of the night to a bitter chill. It is inevitable and frustrating for people plagued with this type of issue. The question, "Why won't my furnace stay lit?" becomes a nightmare and haunts them as they huddle around a space heater for warmth. For these people, we have put together a list detailing reasons why the furnace will not stay on and produce heat during those freezing winter nights.
Probably the most common issue when dealing with a furnace that will not stay lit or ignite consistently is a dirty flame sensor. The flame sensor acts as a communication vehicle with the circuit board and tells it when a flame is lit inside the furnace. When the sensor gets rusty or dirty, it cannot properly detect the flame and causes the furnace to malfunction. A clear indicator of a dirty flame sensor is when the furnace quickly cuts off two or three times a day during the normal heating cycle. The furnace attempts to turn back on and start a cycle again, but shuts off moments later. Most households that do not have annual maintenance performed in over a year or have pets will often run into this problem until a qualified service technician cleans the part. It is not a major issue, just a pesky one.
Another cause of this issue would be lack of air flow causing the furnace to overheat. The furnace has a control set to turn off the burners when the heat exchanger reaches a certain temperature. In most cases, the blower to the furnace will continue to run so it can cool down. Once it reaches the set cool down temperature, the burners should automatically reignite. If this continues to happen over time, the high limit control may malfunction and not allow the furnace to reset. At that point, it needs replacing by a qualified HVAC technician. The most common cause of an air flow restriction is simply a dirty filter. It is recommended homeowners visibly check their filters on a monthly basis. If the filter appears to be clogged or blocking air flow, then it must be replaced. In more extreme weather, homeowners are urged to check the filter more often due to longer furnace cycles. If the filter still appears relatively new upon checking, then there is no need to immediately replace it.
Other air flow restrictions, such as a dirty “A” coil, lack of return air, inadequate duct work, dirty blower wheel must be repair by a qualified service technician. This must be determined through static pressure measurement performed by the technician.
Each furnace requires a certain amount of combustion air to allow the flames to breathe properly. There is a method requiring total BTU’s and square footage of space feeding the gas appliances. If the space is not large enough, then it can cause the furnace to malfunction. The problem can usually be remedied by opening the furnace room door or adding vents to the room to allow more air into the space. In other occasions, air can be added from the exterior if it is accessible. The issue is that unconditioned air pushes into the home even when the system is not running. However, different types of damper systems can eliminate this type of issue. This is normally not an issue with a condensing furnace that has two PVC pipes to the exterior. These systems bring their own combustion air from the outside to feed the flames, thus they do not require combustion air from inside the home. If it only has one PVC pipe (exhaust only), then the furnace room requires combustion air. When you have a problem with a condensing furnace not staying lit, you may want to check the PVC pipe on the outside of the home to check for an occlusion (birds nest, kids toy, wasp nest…etc.).
The pressure switch is a round shaped control mounted near the top of the furnace close to the draft inducer fan. Normally, there is a tube on the pressure switch coming off the face of the furnace or draft inducer. From time to time, this tube can become blocked by a spider’s nest or debris and cause the pressure switch to work improperly. This problem could indicate a major issue with the furnace or could simply be a bad pressure switch.
If all the above options are not the issue, then there may be a problem with the circuit board inside the furnace. A qualified technician should check the furnace as it is unsafe for homeowners to perform this task. Many circuit boards have an LED code that flashes during a furnace malfunction. Typically, it can be viewed through a sight glass about the size of a quarter on the lower door of the furnace. The code should give an indications of what is causing the furnace to lock out and not heat your home.