why does furnace turn on and off

Does your furnace s cycle on and off frequently? That s what we call short cycling. It can wear down your furnace, possibly damaging it. So, what s causing it? There are different possible causes. We ll walk you through each of them and what you need to do to fix them. Dirty air filter A dirty furnace filter can cause your furnace to overheat and shut down. How? The dirty filter decreases airflow over your furnace s heat exchanger (the part that actually heats the air). With little air to exchange heat to, the heat exchanger overheats, causing a limit switch to shut the gas valve. No gas = no flames. Later, the furnace will try to turn on again, overheat, then shut down again. Solution :. Dirty flame sensor The flame sensor s job is to confirm that, when the gas valve is open, a flame is actually present in the furnace. If a flame isn t there, the flame sensor shuts the gas valve to prevent dangerous levels of gas from building up in your home. However, if the flame sensor gets
coated in carbon or rust, it won t register the flame. So it shuts the gas valve. Again, no gas = no flames. Later, the furnace tries to turn on again and quickly shuts down because of the flame sensor. Solution: A professional needs to clean the flame sensor. Improperly placed thermostat Is your thermostat placed near an air vent or a window that has sunshine coming through?

If so, those heat sources can trick your thermostat into thinking that your home s air is warmer than it actually is, shutting off the furnace prematurely. Solution : Make sure your thermostat is located on an interior wall away from windows and vents. Other issues Both of these issues can cause the furnace to start and stop frequently. Both issues need a professional s touch to fix. Want your furnace fixed today? with Ragsdale Heating, Air and Plumbing if you live in the metro Atlanta area. Does this sound familiar? Your furnace turns on, runs for a few minutes and then shuts off. Soon after, it turns back on and the process repeats. What s the problem? Typically, a furnace keeps turning off and on (or short cycling ) like this because of one of these problems: QUICK DIY FIXES: Open all your air supply vents (yes, even in rooms you don t use) Clean air filter (left) and dirty air filter (right) An open air supply vent If these DIY fixes didn t solve the problem, you most likely need to. Now, if you want to learn more about these issues and why they cause the furnace to short cycle, read on. A common reason a furnace short cycles is because the heat exchanger, the part that heats your air, is getting too hot due to a lack of airflow. Once the heat exchanger overheats, it trips the high limit switch, which shuts the furnace down to protect it.

If the heat exchanger (highlighted above) gets too hot, the furnace shuts down. If the high limit switch goes bad, the furnace won t shut down and the heat exchanger will crack. Dirty air filter: If the filter gets too dirty, the blower will struggle to pull in enough air over the heat exchanger. Closing air supply vents: Closing too many air vents can slow down certain blower types and therefore reduce airflow. Dirty blower wheel: Over time, the blower wheel collects dirt, slowing it down and therefore limiting how much air it can deliver over the heat exchanger. Contact a technician to clean the blower if needed. With this in mind, you can see why our initial DIY fixes include changing the air filter and opening the supply vents. Related article:. If the furnace tries to turn on multiple times before stopping for awhile, the problem is probably a soot-covered flame sensor rod. So what is the flame sensor rod? And why does it being covered in soot cause short cycling? Well, the flame sensor rod is a safety device designed to detect a flame in your furnace burners. If it does not detect a flame, it shuts the gas valve. Now, normally, this is a good thing for you because if it didn t shut the gas valve, your furnace could fill your home with gas. And one well-lit match could cause an explosion. But if the flame rod gets covered in soot, it can t accurately tell if there s a flame or not, so it shuts the gas valve, even if there is a flame.

And no gas means no flame, which means no heat for you. Solution: Clean the flame sensor rod. Since this isn t an easy DIY job, we suggest calling a professional. Note: If you have an older furnace with a pilot light, the problem is similar, except the flame sensor is over the pilot light, not the furnace burners. Has this short cycling problem been ailing you for years? Or did you just get a new furnace? If you said Yes to either of the above, the problem may be that the furnace isn t sized properly for your home. You see, furnaces are like shoes: They have to be sized just right or you ll have problems. For example, an oversized furnace will heat the home too quickly, causing it to turn off and on too often. If this is your problem, you have a few options depending on how old your furnace is: If it s brand new: Contact your installer about this issue so they can fix it. Hopefully you hired a contractor who gave you a labor warranty on their work. If it s old (15+ years): Consider getting a new furnace. Make sure that your contractor performs a Manual J heat Load Calculation as part of their free estimate. This calculation helps the contractor find the furnace size you need. Need more furnace advice? Talk to an expert.

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