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Furnace Starts Then Stops - Troubleshooting _ Repair and Replace
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00:00:12.160 --> 00:00:16.060 Hi, I'm Vance and welcome to Repair and Replace. 00:00:16.060 --> 00:00:20.580 If your furnace starts, then stops, and does this several times, 00:00:20.580 --> 00:00:24.580 then it's likely a safety switch has tripped or has failed. 00:00:24.580 --> 00:00:28.720 In this episode, we'll look at the causes of short cycling. 00:00:28.720 --> 00:00:32.440 We'll briefly cover each repair, but if you need more information 00:00:32.440 --> 00:00:36.300 then watch the videos linked in the description. 00:00:36.300 --> 00:00:40.980 To troubleshoot short cycling, we're going to look at a few specific scenarios, 00:00:40.980 --> 00:00:48.180 1: the furnace shuts off before ignition, 2: the furnace ignites but the flames won't stay lit, 00:00:48.180 --> 00:00:54.700 and 3: the furnace ignites, blows warm air but shuts off after a minute. 00:00:54.700 --> 00:00:56.280 Lets begin. 00:00:59.940 --> 00:01:04.060 Modern furnaces have a set of safety switches that shut down the furnace 00:01:04.060 --> 00:01:06.760 when it detects an unsafe condition. 00:01:06.760 --> 00:01:15.380 This includes the pressure switch, flame sensor, flame roll out switch, and limit switch. 00:01:15.380 --> 00:01:19.360 If a switch is tripped, it will cause the furnace to lockout. 00:01:19.360 --> 00:01:22.940 It will cycle on and attempt ignition several times 00:01:22.940 --> 00:01:27.420 before going into a hard lockout for up to an hour. 00:01:27.420 --> 00:01:32.340 Every time a lockout happens, the furnace will use flashing lights as codes 00:01:32.340 --> 00:01:35.060 to identify the source of the shutdown. 00:01:35.060 --> 00:01:42.180 These codes are unique to each model, so check your furnace manual to see what each code means. 00:01:42.180 --> 00:01:48.160 Even without any furnace codes, you can still troubleshoot the problem by observing the furnace. 00:01:48.160 --> 00:01:53.159 Keep in mind, if you are going to troubleshoot your furnace, always be safe and turn off 00:01:53.160 --> 00:01:57.340 the power and gas to your furnace before doing any repairs. 00:02:01.340 --> 00:02:06.000 If the draft inducer fan runs, but the furnace shuts off before igniting, 00:02:06.000 --> 00:02:09.080 it could be a problem with the pressure switch. 00:02:09.080 --> 00:02:14.480 When the thermostat calls for heat, the draft inducer pulls fresh air into the burner. 00:02:14.480 --> 00:02:19.580 This clears the burner of exhaust gases and allows for safe combustion 00:02:19.580 --> 00:02:22.560 When the pressure is low, the switch will stay open 00:02:22.560 --> 00:02:26.040 and will prevent the furnace from igniting. 00:02:26.040 --> 00:02:31.660 Try to identify anything that might cause a change of pressure inside the heat exchanger. 00:02:31.660 --> 00:02:37.780 Check to see if there's any obstructions in the flue vent or in the inducer motor port. 00:02:37.780 --> 00:02:44.320 Next check to see if there are any blockages or water in the pressure switch tubing. 00:02:44.320 --> 00:02:50.160 If the airflow is good, then the pressure switch might be faulty. 00:02:50.160 --> 00:02:57.960 Alternatively, if the burners still don't ignite, then the furnace ignitor might of failed. 00:02:57.960 --> 00:03:01.220 The ignitor can be tested for continuity with a multimeter 00:03:01.220 --> 00:03:05.560 to see if a continuous electrical path is present. 00:03:05.560 --> 00:03:10.840 If there is no continuity, then replace the furnace ignitor. 00:03:15.040 --> 00:03:20.880 If the burners shut off shortly after igniting, then the flame sensor could be dirty or defective. 00:03:20.880 --> 00:03:24.700 When the airflow is good, the hot surface ignitor will heat up, 00:03:24.700 --> 00:03:28.400 the gas valve will open and the burners will ignite. 00:03:28.400 --> 00:03:32.120 The flame sensor monitors the burners, and will shut the furnace off 00:03:32.120 --> 00:03:36.040 if there's no flame present after 7 seconds. 00:03:36.049 --> 00:03:42.739 If the flame sensor is dirty, the furnace might shut off even when there's a flame. 00:03:42.740 --> 00:03:48.680 Remove the sensor and clean it with a soft abrasive pad. 00:03:48.680 --> 00:03:55.540 Now if the burner still doesn't stay on, then replace the flame sensor. 00:03:59.560 --> 00:04:05.340 After ignition, the blower fan will run and circulate the warm air around your home. 00:04:05.340 --> 00:04:08.400 If the furnace ignites, the blower fan runs, 00:04:08.400 --> 00:04:11.040 but the furnace shuts off before the house warms up, 00:04:11.040 --> 00:04:16.160 then the filter could be dirty, or the limit switch may be faulty. 00:04:16.160 --> 00:04:20.500 The limit switch monitors the temperature of the air passing over the heat exchanger 00:04:20.500 --> 00:04:24.460 and will shut off the furnace if the system overheats. 00:04:24.460 --> 00:04:29.000 Any restriction of airflow might cause the heat exchanger to overheat. 00:04:29.000 --> 00:04:34.580 This is why it's essential to replace the furnace filter every 3 - 6 months. 00:04:34.580 --> 00:04:38.820 If you've replaced the filter, and the furnace still continues to shut off, 00:04:38.820 --> 00:04:42.460 then check the limit switch. 00:04:42.460 --> 00:04:45.420 The limit switch can be tested for continuity. 00:04:45.420 --> 00:04:51.000 If the switch has continuity, the multimeter will show zero ohms. 00:04:51.000 --> 00:04:55.100 Even if the limit switch has continuity, it might still be faulty. 00:04:55.100 --> 00:05:00.600 As a limit switch fails, it can start shutting the furnace off at lower and lower temperatures. 00:05:00.600 --> 00:05:04.260 So if the limit switch has failed or is starting to fail, 00:05:04.260 --> 00:05:08.180 then it's best to replace it. 00:05:09.340 --> 00:05:13.160 If you liked this and want to see more tutorials and informational videos, 00:05:13.160 --> 00:05:15.040 then subscribe to our channel. 00:05:15.040 --> 00:05:18.540 And if you need help, you can call or visit an AMRE location 00:05:18.540 --> 00:05:21.040 to talk with our knowledgeable staff. 00:05:21.040 --> 00:05:22.460 Thanks for watching.
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