00:00:10.120 --> 00:00:14.980 Hi, I'm Vance and welcome back to Repair and Replace. 00:00:14.980 --> 00:00:19.420 Furnace is something you never really think about until it stops working, 00:00:19.420 --> 00:00:23.340 but with a little bit of knowledge, it's much easier to troubleshoot problems. 00:00:23.400 --> 00:00:27.400 So today we 're gonna learn how a gas furnace works. 00:00:27.400 --> 00:00:29.680 First we'll cover the basics, 00:00:29.680 --> 00:00:33.640 then we'll dive a little deeper into the sequence of operation. 00:00:33.640 --> 00:00:37.260 So you can understand what each component does. 00:00:37.260 --> 00:00:39.200 Let's begin ! 00:00:42.880 --> 00:00:48.360 Although there are some differences, all gas furnaces work in the same basic way. 00:00:48.360 --> 00:00:52.940 Natural gas is burned to warm up the heat exchanger. 00:00:52.940 --> 00:00:58.000 The exhaust fumes are released through the flue gas vent. 00:00:58.000 --> 00:01:01.480 The blower fan then pushes air over the heat exchanger 00:01:01.480 --> 00:01:04.700 and circulates it around your home. 00:01:04.700 --> 00:01:09.060 The furnace runs until the thermostat detects the right temperature 00:01:09.060 --> 00:01:11.720 and shuts the furnace off. 00:01:11.720 --> 00:01:16.280 Modern furnaces use a draft inducer fan to help exhaust the emissions, 00:01:16.280 --> 00:01:20.900 this force draft also increases the efficiency. 00:01:20.900 --> 00:01:26.120 Condenser furnaces use a dual heat exchanger to condense water vapor , 00:01:26.120 --> 00:01:30.020 this extracts even more heat out of the combustion. 00:01:33.960 --> 00:01:40.060 Furnaces are rated by AFUE or (Annual Fuel Utilization Efficiency). 00:01:40.060 --> 00:01:45.960 This number is a ratio of the fuel burned versus the fuel converted into heat. 00:01:45.960 --> 00:01:50.540 Older furnaces will have an efficiency rating of 70 AFEU or less, 00:01:50.540 --> 00:01:55.060 meaning that only 70 percent of the fuel is turned into usable heat. 00:01:55.060 --> 00:01:58.340 Mid efficiency furnaces are around 80 00:01:58.340 --> 00:02:03.800 and High-efficiency condenser furnaces are generally 90 and above. 00:02:03.800 --> 00:02:07.440 Furnaces can be single stage or dual stage. 00:02:07.440 --> 00:02:10.060 Single stage furnaces run at one speed, 00:02:10.060 --> 00:02:13.500 which means it's either full heat, or no heat. 00:02:13.500 --> 00:02:19.220 Two stage furnaces can run at full speed or a reduced speed when less heat is needed. 00:02:19.220 --> 00:02:24.440 This allows for the furnace to run quieter and may contain a more consistent temperature. 00:02:29.060 --> 00:02:33.140 Modern furnaces have a set of safety switches that shut down the furnace, 00:02:33.140 --> 00:02:35.520 when the system is compromised. 00:02:35.520 --> 00:02:39.580 if a switch is tripped, it will cause the furnace to lockout. 00:02:39.580 --> 00:02:40.980 The furnace will attempt ignition, 00:02:40.980 --> 00:02:45.220 several times before going into a hard lockout for up to an hour. 00:02:45.220 --> 00:02:49.220 Every time a lockout happens the furnace will use flashing lights, 00:02:49.220 --> 00:02:53.340 as codes to identify the source of the shutdown 00:02:53.340 --> 00:02:59.500 These codes are unique to each model so check your furnace manual to see what each code means. 00:03:04.020 --> 00:03:06.440 When the thermostat calls for heat, 00:03:06.440 --> 00:03:09.220 voltage is sent to the control board. 00:03:09.220 --> 00:03:13.520 Next the draft inducer fan pulls fresh air into the burner, 00:03:13.520 --> 00:03:17.240 which allows for clean and efficient combustion. 00:03:17.240 --> 00:03:21.540 When the draft motor reaches full speed the pressure switch will close, 00:03:21.540 --> 00:03:24.340 allowing for the sequence to continue. 00:03:24.340 --> 00:03:27.420 The pressure switch ensures that there is enough airflow 00:03:27.420 --> 00:03:31.060 inside the heat exchanger to safely operate the furnace. 00:03:31.060 --> 00:03:32.840 If the pressure is low, 00:03:32.840 --> 00:03:37.120 the switch will stay open and will prevent the furnace from igniting. 00:03:41.360 --> 00:03:46.620 If the pressure is good, then voltage is sent to the hot surface igniter. 00:03:46.620 --> 00:03:50.400 Some furnaces might use a spark igniter instead. 00:03:50.400 --> 00:03:55.540 After the igniter heats up the gas valve opens and the burners ignite. 00:03:55.540 --> 00:04:00.360 When the flame will continue as long as the safety switches remained closed. 00:04:00.360 --> 00:04:06.540 If the flame isn't detected after 7 seconds the flame sensor will shut off the gas. 00:04:10.800 --> 00:04:12.300 When the burner is lit, 00:04:12.300 --> 00:04:16.920 the blower fan is too late from running to allow the heat exchanger to warm up. 00:04:16.920 --> 00:04:20.720 This prevents the furnace from circulating cold air. 00:04:20.720 --> 00:04:24.660 The blower fan then pushes the air over the heat exchanger 00:04:24.660 --> 00:04:27.340 00:04:27.340 --> 00:04:30.020 The limit switch monitors the temperature of the air 00:04:30.020 --> 00:04:32.080 passing over the heat exchanger 00:04:32.080 --> 00:04:35.940 and will shut off the furnace if the system overheats. 00:04:35.940 --> 00:04:40.220 Any restriction of the airflow might cause the heat exchanger to overheat. 00:04:40.220 --> 00:04:43.480 That's why it's essential to replace the furnace filter 00:04:43.480 --> 00:04:45.400 every three to six months. 00:04:49.360 --> 00:04:53.360 Once the thermostat signals that the temperature of the home is normalized, 00:04:53.360 --> 00:04:56.780 the gas is shut off and combustion stops. 00:04:56.780 --> 00:05:00.000 The blower motor will continue to run for several minutes 00:05:00.000 --> 00:05:02.880 until the furnace is cooled down. 00:05:02.880 --> 00:05:06.080 Finally the unit is placed in standby 00:05:06.080 --> 00:05:08.780 until the thermostat calls for heat again. 00:05:10.060 --> 00:05:14.600 Hopefully, this has given you a better understanding of how a furnace works. 00:05:14.600 --> 00:05:17.160 Now if your furnace is having trouble running, 00:05:17.180 --> 00:05:20.900 then watch our troubleshooting guide for a step-by-step breakdown 00:05:20.900 --> 00:05:24.780 and there's a link to this in the description below. 00:05:24.780 --> 00:05:28.820 If you found this video useful then subscribe to our Channel. 00:05:28.820 --> 00:05:31.540 We're constantly filming new content every week, 00:05:31.540 --> 00:05:34.440 so let us know what repairs you'd like to see next 00:05:34.440 --> 00:05:38.300 and if you need a part for that repair then visit our website. 00:05:38.300 --> 00:05:40.520 We stock thousands of hard-to-find parts, 00:05:40.520 --> 00:05:42.980 and we'll ship it out to you the same day. 00:05:42.980 --> 00:05:44.960 Thanks for watching
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