Cycling On and Off
Sometimes a furnace will cycle on and off too many times during an hour and it will be necessary to know how to troubleshoot your furnace when it repeatedly cycles on and off. If you have discovered your furnace turning on and off too many times, don’t stress at first.
The fact that it is turning on and then turning off is actually a safety feature you should be thankful is in place. Essentially, what is happening is the unit is coming on and then the safety features are thinking there is something amiss, so it turns the unit off again. Good. Your safety features are working. That is a good thing.
The goal here is to discover why the safety features are kicking in and shutting the system down. If this continues, the safety features may eventually fail and then you have a real problem on your hands.
This particular phenomenon we are thinking about today is called short cycling and consists of a furnace only staying on less than a minute and then shutting down. Since the word “frequent” can be defined as many times as there are people, it is important to understand the ways to troubleshoot this situation.
If your furnace was correctly sized at installation with all factors of your home being considered in the equation, the unit should be cycling on and off between three and eight times per hour. Even with this statement, if your unit is turning on more than eight times, it doesn’t mean it is short cycling. If you are experiencing extreme temperature drops, then the unit may just be performing as expected.
The truth of the matter is that the concern is not with how many times it is cycling on and off, but what is causing the number of times it is cycling. It is a good idea to actually time the number of seconds or minutes the unit is running after it turns on. Time how long it stays off also. This is good information to tell a technician when they arrive.
In order to understand why your furnace may not be operating at peak performance levels, it will be important to understand how the system works. A blower motor is propelling air over the tubes of a heat exchanger. These tubes have fuel in them that heat up and heat your home.
The safety features mentioned before go into play at this point. There is an induced draft motor(IDM) that goes to work blowing all the gas out of the furnace and up the flue. This is necessary because if gas built up in this area, there would be an explosion upon igniting.
There is also an IDM pressure switch or a vacuum switch that makes sure the IDM is operating. If the vacuum switch does not sense the IDM is working, it will shut the furnace down.
If all things are well and the the individual parts are doing their tasks, then the furnace will energize the igniters and create a flame. Fuel will travel into the unit and the heating process will start.
There is also a flame recognition system that will determine if the flame is ignited. This system will also shut the furnace down if the flame does not stay lit.
So, you see there are many safety features built into a furnace.
A high limit switch is a switch that turns your furnace off it it overheats. This is one of the first devices a technician will inspect when evaluating your furnace.
Flue Limit Switch There is also a flue limit switch that will shut the system down if exhaust is not being successfully sent through the ventilation system.
So, What to Do?
Check the air filter. An air filter has two purposes in a furnace. One purpose is to keep the air filtered that you breathe. Another purpose is to keep the internal working parts of the furnace clean so they will function at optimum levels.
If the coils of the heat exchanger become covered with dust or debris, their efficiency will diminish. Also, if filters are clogged, then the high limit switch will go into action sensing the temperature is too high. Result? Complete system shut down.
Check the thermostat. Thermostats are electronic technologies that can fail. So, the fastest way to check it is to remove it from the cycling process. If you have some experience with electrical wiring, take the cover to the thermostat off. Disconnect the red wire and the white wire. Touch the white wire to the red wire and listen for the furnace to cycle on. If the furnace turns on and runs normally, then you may have a bad thermostat. You also want to check your battery while you have it all apart.
Inspect the blower motor because it is possible for the furnace to run while the blower motor is not. If the blower motor is not working, then air is not being blown over the heat exchanger. It will begin to overheat and the high limit switch will do its job by shutting down the unit for safety.
Check the flue for blockage. Most of the time, this will be an animal that has found a warm place to go. If gas is not exhausting up the flue, then check for any kind of blockage.
Inspect the flame sensor. This may involve a thermocouple that will have to be inspected for operational efficiency. You may do this with the use of a multimeter. You will have to do an open test, a closed circuit test, and a resistance test. If you are not familiar with this process, leave it to the HVAC technician.
If, after checking these things your furnace is still cycling to fast, then you may have an oversized unit. The furnace is so large that it will cycle on, heat up the space quickly, and then shut down. Also, it is so efficient that once the area has been heated and the called for temperature has been reached, hot air has not place to go so the safety features shut the unit down.
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