How a Furnace Works - Furnace Sequence of Operation

hey guys jay here with word of advice tv and today i want to go over furnace sequence of operation and why you should know that and before we take these doors off we're going to need the doors off before you do that if your furnace isn't working the first thing you want to do is look into your sight glass which is going to be on the bottom door of the furnace right here you should have a sight glass most of the newer furnaces will have them and if that's blinking some kind of an error code my furnace power switch is off right now so you're not going to see a light but if that will be blinking some error code remember what that code is before you take the doors off so i'm going to go ahead and take my doors off and show you why i just said that so here's my furnace power switch right it's off right now usually on most furnaces there will be another switch right here called a door switch on the bottom door and once you take that door off the switch automatically pops open and interrupts the power and if you had some kind of an error code blinking on your control board for example i'll turn my power back on so you can see my light my lights right there blinking and you can see that through the sight glass usually if you had some kind of an error code and you flipped your furnace power switch off or you took the bottom door off that error code will clear out and if it's an intermittent problem then it might be a little hard to track down but if you have a code then you know where to start and as you can see i just have mine taped up or you can just if you have a strong magnet you can just plop a magnet on top of it when you're troubleshooting or if you want the furnace to be running while you have the doors off otherwise this switch when it's popped open that'll interrupt the power and everything will be blank and in most furnaces the explanation of those error codes will be on the back door of either the top furnace door or the bottom door on mine all i have is a schematic there's no explanation of the error codes and the top door all all that is is just grates as you can see so there's nothing on the back of that on this particular furnace the error codes are actually right on the control board itself and different furnaces will have different error codes and different diagnostic charts but for the most part the error code names will be pretty similar from furnace to furnace all right enough with the side tracking back to the main topic which is the furnace sequence of operation and before we dive into that just want to briefly explain what the components are this here is the gas valve it looks electronic gas valve so you know once power is applied to it gas valve opens up and lets the gas through these two are the pressure switches i have a two stage furnace therefore i have two pressure switches one for the low stage and one for the high stage i have an inducer motor or a venting motor and it's also a two-stage inducer motor usually if it's a one stage it'll have just two wires coming out of it it'll usually be a black and a white mine as you can see has a black white and red that usually indicates that this is a two-stage motor then here we have the burners the igniter you can't really see actually let me just take this little cover panel off real quick so you can see the burners better okay now we can actually see the igniter a little better there's the igniter here's my burners these are inchop burners most of the newer furnaces will have this style this is what they look like as you can see mine's a little dirty if you wanted to clean it up while you're in there real quick all you have to do is just get a brass brush if you really want to be diligent you can also blow them out with some compressed air as you can see that looks a lot better right off the bat but anyways that's not what this video is all about so let's put this burner back in and then we got the flame sensor on the other side see that little steel rod right there that's the flame sensor here's the main control board or the circuit board and then here we have the blower housing and all these wires right here they go to the blower motor so the furnace sequence of operation is basically the order of how the furnace starts up so you have power coming in from your breaker panel to your electrical junction box power goes down into the control board and you always have power just sitting at your control board and this control board sends power to your thermostat and when there is a call for heating so for example if i go to my thermostat set it to heat and increase the temperature which is a call for heat my heat turns on the thermostat closes the switch up there and it sends power back down to the control board saying that there's a call for heat or a need for heat and then the control board starts the process of heating up the house so the way it works is power to the thermostat back down from the thermostat the very first thing that will turn on is your inducer motor your inducer motor turns on and that's basically just sucking air through the burners and then out your exhaust pipe right here inducer motor turns on these pressure switches they're hooked up by a hose in the back here to the inducer motor housing here and their purpose is to prove that there is indeed a draft if there's like a bird nest or something these pressure switches will not close they're normally open switches which means the pressure switches are open when they're off but when this inducer motor turns on those pressure switches close and they let the circuit through once the control board senses that the pressure switch closed it then sends a signal to the igniter for it to start to glow the igniter starts to glow and then the control board sends power to the gas valve opening it up gas comes through lights all the burners and then on the other side we have the flame sensor and that's just there to prove that there actually is a flame once the control board realizes that there is a flame it leaves the burners on if the flame sensor is not sensing any flame then the burners will shut off after about three seconds but anyways the burners turn on and about 20 or 30 seconds later depending on what kind of furnace you have your blower motor will come on and now let's turn the switch on i have my thermostat calling for heat upstairs and see the furnace in action so the thermostat is calling for heat the very first thing that should energize is this motor the motor turns on you can't physically see it but the pressure switch will close and as you can see my igniter is starting to glow really bright yellow or bright orange and then you're gonna hear that gas valve click the burner's light and now the furnace is going to be preheating for a while once it warms up the heat exchanger enough then the blower motor will come on and start distributing the air and the reason for that is if the furnace is cold if the blower fan comes on right away you're gonna feel cold air coming out of the vents instead of warm air so it kind of does a little preheat before it turns on the blower mode and once the thermostat is satisfied and what i mean by that is let's say you set it to 74 and it finally gets to 74. everything just goes backwards from there so the gas valve gets de-energized the burners turn off the blower motor stays on for like 30 seconds to cool off the furnace then everything just shuts off and the furnace goes into standby mode well and that is the furnace sequence of operation and now i just want to simulate a few problems why is it important to know the sequence of operation is depending on where the sequence is interrupted that's where you should be looking so for example if your igniter is glowing but the gas is not going through you know that the problem revolves around the gas valve either it's not getting power it's not getting gas or the gas valve is bad or if your inducer motor is running but your igniter does not start to glow that means one of two things either your igniter is bad or for some reason your pressure switch is not closing so let's just turn the gas off to the furnace quick i'll let it cool off for just a little bit before i hit the power switch as you can see my furnace is already trying to restart the igniter is glowing and you're going to hear the gas valve click but nothing happens because my gas is turned off right here so i'll turn this back on now and flip the power switch off so let's do an example scenario let's say that for some reason here's my pressure switch hose let's say for some reason there was some gunk that built up in here some rust or scaling or whatever this port was plugged therefore my pressure switch was not sensing any draft so i'll leave that unplugged and see what happens so my thermostat is calling for heating the inducer motor turns on and that's it nothing else happens my igniter is not glowing my gas valve is not trying to open and my control board here is blinking three red blinks so one two three pause one two three pause and it'll keep blinking that over and over and over again and according to my diagnostic charts that means we have a pressure switch problem if i hook that hose back up or in the real world i clear the bird nest out or cleaned up that little pressure switch port now my igniter should start to glow right away the sequence of operation continues let's say my flame sensor is extremely dirty and it's not sensing any flame i'll simulate that just by unplugging it as you can see the burners just went right off my igniter starts to glow again to try again and usually furnaces will try anywhere from three to five times to reignite over and over and if they keep failing it'll go into a soft block out which means for three hours the furnace will sit there and do nothing so my flame sensor is still unplugged so boom flame comes right off okay and let's just try one more scenario now we know our sequence of operation we can watch it let's say my igniter is bad i'll simulate this just simply by unplugging mine so the inducer motor comes on our pressure switch closes and according to our sequence of operation the igniter should start to glow or your sparks start to spark but that's not happening and you'll hear the gas valve click so the gas valve opened up for five seconds and then closed back up that's the duty of the flame sensor it's a safety if there's gas but no flame flame sensor shuts the gas valve off so if you hear your gas valve opening up but your igniter never started to glow then you know that either your igniter is bad or it's simply not getting power from the control board well guys and that's all i wanted to show you another thing i want to mention is that most furnaces will have an owner's manual or installer's manual buy the furnace somewhere maybe you have like a little pocket where it's in or you know where your books are i'm not the original owner of my house so i don't have those books unfortunately but if you have that installer's manual handy that can be very helpful for troubleshooting the manual will have a detailed sequence of operation it's a little more complicated than how i explained it and it'll also have the diagnostic error chart in there and other helpful material that you could use so if you have that book i recommend looking through it as you will probably learn a lot from it and since i don't have my book i actually looked up the sequence of operation from a lennox 80 furnace online i just want to hop over to my computer screen quick and just go over that with you quick so you know how to read that alrighty so i went to manualslib.com which pretty much means manuals library and they have a bunch of manuals on this website and i pulled up a lennox 80 furnace right here and i found the heating sequence of operation for this furnace so let's just go over it quick it's a little bit more complicated than how i explained it i did a very simple version of this so right here we have two columns one is the normal heating mode and the other one is the abnormal heating mode so for example let's start from the top it goes from top to bottom to begin with the control has main power on so the furnace has power going to it and the burners are off and if for some reason the sequence of operation goes out of order right here in every single box it tells you what the furnace will do step by step so for example if for some reason the burners were on the gas valve would turn off the inducer would come on the indoor blower would turn on and sequence holds until no flame is sensed or the main power is interrupted and for every single abnormal scenario it tells you in these boxes right here what error codes will be flashing there's a lot of information crammed into little boxes so they do abbreviate a lot you'll probably notice that if you read through it but anyways let's go back to our normal heating mode and just go through from top to bottom and just see how this furnace runs so with this furnace the very first thing it'll do and this is most furnaces is they will do a control self check to make sure that all the limits are closed the pressure switch is open the flame rollout is closed and the led on the control board is blinking a slow flash rate which means that the furnace is idle or in standby mode and once the thermostat calls for heat the led light will start to blink fast and then the furnace does a quick pre-check to see if the pressure switch is open it's a normally open switch but sometimes they get stuck closed if it is stuck closed then the furnace will not proceed and the igniter will not start to glow so for example if the pressure switch is not open says no right here tells you right here that the gas valve the inducer and the blower will turn off and sequence holds until pressure switch closes or thermostat resets control and this particular linux model has two diagnostic lights that's why it says one is off and two is flashing so once it does a check and make sure that the pressure switch is open the inducer motor will get energized and it'll come on the pressure switch will close and the inducer motor will do a pre-purge for 15 seconds and at that point there will be a trial for ignition which just means that your hot surface igniter will start to glow or if you have a spark igniter it'll start to spark and your main gas valve will open and light the burner the burners will stay on for 10 seconds to see if the flame sensor will sense the flame if the control board gets the signal that the flame is on it'll continue to run and pre-heat the furnace and then it'll energize the blower motor after it's been preheating for 45 seconds and then while the furnace is heating the burners are on the limits all the limits need to stay closed and the pressure switch needs to stay closed as well if that does not happen for example let's say your furnace is overheating and your limit trips it tells you right here on the right side no what will happen so the gas valve will turn off the inducer will turn off and the blower will stay on until that limit recloses and it will be showing your error code of led1 flashing and led2 steady on but in a normal heating cycle both of these will stay closed until the thermostat opens up or until it's satisfied your house warms up and then the led on your control board will once again start to blink a slow flash rate instead of the fast one the inducer motor will turn off and your blower motor will stay on to cool off the furnace for 60 90 120 180 or 240 seconds depending on how your control board is set up as you can see that is a lot more involved than how i explained it so the install manual has a lot more information in it it also has more information on the diagnostics or the error codes for example if it's blinking led alternating slow flash alternating slow flash for both lights it tells you what actions or corrective actions you need to take to fix that problem so these manuals are full of good information so if you have one around your furnace or if you know where your manual is pull it up and look through it or at least look through a table of contents to see if you have something there that will help you with the diagnostics well guys and that is all i had for the furnace sequence of operation i hope you found this video helpful thank you so much for watching don't forget to mash that like button on the way out and we'll see you next time [Music] you

Channel: Word of Advice TV

Views: 741,676

Keywords: sequence of operation, furnace sequence of operation, furnace troubleshooting, furnace not working what to do, furnace repair do it yourself, how a furnace works, furnace blowing cold air, furnace not igniting, furnace not staying on, word of advice tv, gas furnace sequence of operation, sequence of operation gas furnace, how furnace works, furnace, how does a furnace work, how a gas furnace works, how gas furnace works, The Most Common Reason Why A Gas Furnace Won't Turn On

Id: AQD_MedIklI

Length: 17min 50sec (1070 seconds)

Published: Sun Nov 11 2018