How to Find a Short (HVAC)

hey guys J here with word of advice TV and in this video I want to talk about low voltage shorts so if you come to a unit or maybe it's at your house and the 3 amp fuse is blown you replace the fuse and it just blows again that means you have a low voltage short somewhere and that's what this video is going to be all about to track down that short to see what is causing this fuse to blow usually there will be a little burnt mark in the middle of the fuse word blue some furnaces will not have a fuse they'll have a little 3 amp breaker instead I believe it was Lennox that was doing that for a while they I think they still do that's where I got this little breaker I pulled off an extra that it didn't need and then some furnaces like mine for example they don't have a 3 amp fuse at all so I spliced one in so I'll show you how to do that as well and the purpose of the 3 amp fuse is mainly to protect the wires because when there's too much current going through the wires they start melting so for example this little alligator jumper that I have I've seen this before where there's a short and I jumper thermostat for example I've seen this little jumper wire become a snake you know you turn on the power to the furnace and this thing just starts to coil up and twist and you see the insulation melting on it so the purpose of the 3 amp fuse is to stop that if there's over current too much current that view is blows and prevents anything from melting so primarily therefore the wires but it ends up protecting the components as well if you don't have a 3 amp fuse inside of your furnace or your air handler you definitely should splice one in because if there's no fuse and there's a low voltage short then your control board your transformer and your thermostat are probably all gonna get taken out if you do get a short somewhere there's two ways that you can try to track down a short one way you will definitely need a meter for and that's to check resistance when checking resistance the power should be off and whatever you're checking the wires that you're checking the wires should be disconnected the other way is to leave the power on and you can do you can really narrow it down without a meter but in order to pinpoint you probably will need a meter but you could probably get away without one too but I'll show you how that works so let's begin I'll just say my furnace apart quick I already have the power off I have the power switch off to my press for now so here's my Control Board I'll just put some more light here and because I have so much wires here and I don't really want to disconnect them I have two more Control Board examples right here as well that we will touch on instead of this one so I don't have to take mine out but remember how I mentioned that my control board did not have a low voltage fuse or three amp fuse I have a 5 amp fuse in mine actually but the way I put this one in is very simple if you look at your transformer in my case my transformers right here that transformer will have a primary side which is 120 volts and a secondary side which is 24 volts primary side is always going to be not always but almost always primary side is going to be a white wire and a black wire that's your 120 volts 120 world hot and this white one is the neutral and two other ones typically red and blue is your 24 volts usually the red is the hot and then the blue is the secondary common and if you're not sure which wires are which you can also look on the transformer itself a lot of times it'll be labeled which one is the common which one is the hot or you can trace the wires see where they go try to figure it out that way or you can look at the wiring diagram which is from your furnace usually on the back of a furnace door and try to figure out which wires are which using the wiring diagram and I don't want to spend too much time on this but basically on the wiring diagram if you look at the legend it says TR and th TR is the common side and th is the hot side here's our transformer that's the symbol for the transformer you got a blue wire coming out of it and you got a red wire coming out of it and if you trace them down th right here is the and TR is our blue wire so right here th is our hot side and that is the red wire and even though I have a symbol for a 5 amp fuse right here my control board actually did not have this 5 amp fuse I had to splice it in myself so if we look back on our control board here's that blue on red wire and they go into this big molex plug right here a bunch of wires and this molex plug is signified right here there's the TR which is the blue wire right here and there's th which is the red wire right here I won't go into that either all control boards are different so I don't want to specifically focus on that one but anyways the two wires coming from the secondary side of the transformer the red one is my hot all I did is I cut this red wire in half and I put two female connectors on it so let's just unplug this to show you what that would look like so literally this used to be one piece of wire I cut it in half and crimped on these connectors and then the fuse has the mail spade connectors you just plug it right in and now you have a fuse in line with your low voltage so if something goes wrong instead of everything going out your transformer your board your thermostat only the fuse blows so that's one style where the board doesn't have a fuse at all or maybe it does have a fuse but it's kind of dangling like this sometimes it's fast into something mine is just kind of hanging out in the middle of all my wires another style is this 3 amp little circuit breaker I believe Lennox is the one that uses this and generally the breaker will not be on the control board itself it will be mounted to the bracket that holds the control board so for example like right here somewhere by the transformer you'll see a little breaker there'll typically be a little hole for it and if it's tripped the breaker will pop out you can press on it to reset it and it resets but it works the same as a fuse except that one's resettable which is kind of nice the 3 amp breaker so I kind of like the three out break or more and the last style which is the most common actually is where the three amp fuse is on the control board itself so if we look at this board for example the spot for the fuse is right here so if you come down to your furnace you take the doors off you just kind of have to take a close look at the board and look for our fuse like this a lot of times it'll be purple or this color right here to violet it can be some other color but generally it'll be that color maybe yellow or orange and right here see how it says secondary that's where the secondary hot wire from the transformer would plug in and so basically right here and right here this is my fuse this is a secondary hot side if we look at the underside of the control board that circuit is connected the hot side of the transformer and one side of the fuse so that would be the same as a splice and the wire like this okay so let's get into how to chase down a short let's start with the power on I'll flip my furnace switch back on and there's a little light right there most control boards will have an LED light or a troubleshooting light that should be on if the fuse is not tripped if the power is on and the light is off a lot of times that means your little fuse is tripped okay so with the power on the first thing I like to check is to make sure that the secondary side of the transformer is actually outputting the correct voltage which should be somewhere between 24 to 29 volts and when you take the bottom door off of your furnace or your air handler of course there's gonna be a door switch like this and you have to tape the door switch with either some electrical tape put a strong magnet on it put some sheet metal tape on it but basically this door switch needs to be closed otherwise all the power is shut off to the furnace so once you have the door switch closed the power is on you have your meter set to voltage I'd like to just verify and make sure that I have 24 volts so I would put one meter lead on one side and one meter other and if we look at my meter I have around 28 volts so we do have the proper voltage going in and at the moment the thermostat is set to OFF so the control board is just in standby mode so if I come to a furnace and three amp fuse is blown or the five amp fuse is blown the first thing I do if I'm going to check it with the power on I first turn the power off to the furnace I unplug the current fuse from the control board or from the wires that it's connected to and I put in a three amp breaker like I have right here to see what I typically use this was just a three amp breaker with two wires connected to it one side has a female connector the other side has a male connector so I have a three amp breaker like this that I use instead of a fuse or they have special tools for exactly this purpose for tracking down a short they have a little mini popper that's also a three amp breaker and they have a short pro tool that you could get that has a little light on it if the light goes out that means there's a short but this is what I use it's a little 3 amp breaker so instead of the fuse or of course you could just get a box of fuses and get like 10 of them and every time it blows just keep replacing it but I like to use this because it's resettable and then for example you would plug in your 3 amp breaker or your mini popper or whatever you're using in place of that fuse so now if there's a short somewhere if I'm jumping things and I'm checking stuff does breaker will trip and I can just manually reset it instead of having to replace a fuse every time it blows so here's a typical air-conditioner and furnace setup this is not a heat pump this is just straight AC this is what it would look like this is basically the wires coming from your thermostat typically it'll be five wires they go into this thermostat strip on the control board and then you have another two wires don't go out to your air conditioner outside and those two wires that go out to the air conditioner ultimately end up powering up the contactor coil which is inside of the condenser unit and once the thermostat calls for cooling does plunger falls in and allows the power to go through and they won't go into basic thermostat wiring in this video because I have another video where I describe that in detail if you want to know more about wiring just check out that video to begin with it's good to rule out the thermostat so if you go upstairs or wherever your thermostat is and take the thermostat off the wall then take all the wires then go to that thermostat you could take a picture of them or label them and then pull them all out so they're just open like this and I have another video where I talked about how to bypass a thermostat at the control board and at the thermostat itself so I won't talk about too much details on that either but your read is going to be your power wire the 24 volts coming from your control board and yellow is typically cooling w is heating and G is fan so after you replace their little 3 amp fuse or maybe you put in your little circuit breaker like this you disconnect the whole the thermostat wires and then you bypass the thermostat by hooking up one jumper cable to R which is powered and the other one to heat and if your furnace is off at that point you go downstairs and you turn the furnace back on if the air conditioner comes on and does not blow the fuse then you know that the thermostat is the problem that's what's causing the fuse to blow if they're checking the heating if it's the furnace season you would jumper r2w which would bypass the thermostat if your furnace comes on without blowing the fuse then you know that the thermostat is the problem and if you jumper r2g that will turn on the blower fan inside of your furnace once again if this turns on no problem and doesn't blow the fuse then you know that once again there's no problem with the G wire and the thermostat is internally defective and that's what's causing the short so that's how you rule out the thermostat to make sure that it's not the thermostat that's causing the fuse to and usually it'll be pretty obvious I mean if it's the AC season chances are something's wrong with the AC side of it and if it's the heating season and then chances are something will be wrong with the heating or the furnace side of things it's pretty rare that the fan is the cause of the problem so in summary this test is only to rule out the thermostat actually you don't even have to take the wires off of the thermal stats so if you just take the face of the thermostat off you can just jump right right away without taking the wires out if you have little magnetic jumpers like this that works even better they just stick right to the screws that hold down the thermostat wires so for example if you turn your thermostat to heating and the fuse blows turn the thermostat off go downstairs replace the fuse come back to the thermostat take the thermostat face off the wall and then you will jumper r2w which is the same thing as setting your thermostat to heat and with the face of the thermostat off the furnace turns on without blowing the fuse then you know that the thermostat is bad if you do it this way and the fuse blows anyway then you know the thermostat has nothing to do with it the thermostats fine something's wrong with the 24 volt side of the heating circuit and that's tripping the fuse after you ruled out the thermostat next thing I do is take off all the wires from the control board thermostat strip you of course wanna label them or take a picture if you're not sure where each wire goes and then take all the wires off and while you're doing this you want to have the furnace power off because if you touch the red wire to pretty much any of these it'll short out and blow the fuse if you have a fuse in there so after you have these wires taken out kind of just spread them out so they're not touching each other and they're not touching anything metal after you have all the wires off I would test it one more time so I would turn the power on with all the wires disconnected and see if the fuse blows if the fuse does blow that might mean that your control board is internally shorted somewhere you might need to replace the control board I don't see that very often at all that the control board is the cause of the fuse blowing and after you verified that the fuse does not blow with all the wires off of it you can turn the power back off you can stick the red wire which is power into our tighten it down and then I like to put in all the components one at a time and see if they blow the fuse so I would put in G which is fan turn on the furnace make sure it doesn't blow I would put in W which is Heat turn on the furnace make sure it doesn't blow put it in Y which is cooling turn the furnace back on see if it blows and then C is common that one we could have even left in this one's not a hot wire so this shouldn't ever cause any kind of shorts the common wire unless it's touching one of the hot legs so all of this you'd be doing with the thermostat setting set to OFF and the fan on automatic and the purpose of this test with everything off is to see if these wires going through your wall and then the wires going outside is to make sure that none of these wires are internally shorted so if the insulation is off of the wire for example if the common wire if the insulation somewhere inside of your walls is gone and it's touching your red wire which is powered which is also bare I mean the only scenario I can think of most of the time is if somebody was I don't know driving a nail through the wall and the nail went through the wire and if somehow these two shorted to each other that would blow the fuse so this test right here with the thermostat off pulling all the wires off and then putting them on one at a time is to check and make sure that none of these wires have missing insulation and are touching each other inside of your walls after you got your thermostat wires hooked up you can go ahead and hook up your air conditioning wires and this should not trip the fuse because really the only time there should be power going to this ywír is if the thermostat is calling for cooling but your thermostat is off but these as well you can put on one at a time just to make sure nothing trips put all your wires back in and you can screw them all down so after you verify that you're getting about 24 volts from the transformer you know your thermostats not the one causing the problem none of these are blowing the fuse with the power off on the thermostat just set to off you can proceed to the next step and for this step you can either go up to your thermostat and for each thing just turn it on to heat then try cooling then try fan but if you don't want to run back and forth you can just jump her to thermostat at the control board so of course our is our power I can jumper r2g which is our blower fan and with the furnace power on that should turn my blower motor on so for example let's try that on my furnace right now and see if the blower motor will come on so I turned the power switch back on and when you're jumping you actually want the power switch off so let's just turn the power switch back off I'll jumper r2g and that should turn on my blower fan there it goes [Music] turn the power back off and then if I would jumper r2y that would turn my air conditioning on and r2w would turn my furnace on so I would check these one at a time to see which one of these will blow the fuse so if you check it RG the fuse doesn't blow or the breaker doesn't trip you can move on to the next one the next one is our two w if you jumper are two w the breaker doesn't trip either and then you jumper R to Y and all of a sudden your little three amp circuit breaker pops in that case you know that the cooling part of it is what's causing the three amp fuse to trip so if the fuse blows when you have power going from R to Y that means that your problem is going to be either the wire itself going outside to your condenser unit or something inside of the condenser unit maybe there's bare wires mice chewed it up or the contactor coil could be bad and that's what's causing it to short out so if your fuses blowing when your air conditioner turns on I would go outside and before I do anything else I would visually inspect the wire make sure there's no breaks in it there's no bare spots where it's touching anything metal then I would also open up the electrical section make sure that the wires are not broken or chewed or anything like that inside of the unit itself as well and if you don't see anything obvious the first thing I do is I like to narrow out the possibility that there's a short inside of the wire itself where the white and red wire are somehow touching each other maybe inside of your wall somewhere I'd like to rule out that possibility and it's very simple to do that all you have to do is disconnect the two wires coming from your transformer or basically from your control board inside of the furnace from the two wires that go to your contactor coil with those two wires disconnected and not touching anything I would turn the power back on turn your air conditioner on or just jumper it at the control board r2y if the fuse does not blow with everything disconnected that means that the wire is good if the fuse or if the three amp breaker trips what the wires disconnected then you know that the problem actually lies in the wire itself that runs from your control board to the outside unit so you either have to try to find where it's shorting out or you'll just have to simply replace this whole piece of wire and a lot of units are wired very simply where the two wires coming from the control board will go straight to the contactor coil but there are times where only one side like a white for example will go to the contactor coil and by the way if you're wondering if it matters if the red wire goes to the side of the contactor coil or just sight it does not matter these two wires they can go either way so let's say one wire goes into the contactor coil but if you have a bunch of safety switches like the low pressure switch the high pressure switch and maybe like a fan to a board or something then this wire the red wire it'll go into a different loop so for example this wire will go into a wire nut that goes into the low pressure switch then comes out of there it goes into the high pressure switch comes out of there goes into the control board and only from there it comes back and goes into the contactor coil so if your fuse is not blowing with the wires disconnected but with them connected it is that could mean that your contactor coil is bad or one of those components one of those safety features is causing the short if you do have all those extra components wired in series with your contactor coil the easiest way is to just disconnect the wires from them and just all those components like the low pressure switch high pressure switch the control board on them to ground and make sure that they have no resistance between ground if they do that means that are shorted out and you would do that similarly how you would mount a contactor coil so for example here we have two contactor coils right one is a two pole contactor this one right here it has a cover over two two poles this is a single pole contactor it just has the one plunger and the coil is right here sometimes this coil gets shorted together and if it gets to that point that will blow your 3 amp fuse so if we check our two contactors a normal resistance on the contactor is about 10 to 20 ohms so with the wires disconnected you'd put one lead on one side and one lead on the other and we're getting a read of 17.8 owns so this contactor coil is good if we measure our other one we're getting around 10 ohms on this one so this contactor coil is good as well and I've noticed from experience that anything under 8 ohms may start to trip that fuse so it should be above 8 ohms and if you're seeing 0 ohms that means this contactor coil is completely shorted together and that will blow your fuse every time and to check if a component is shorted to ground like let's say if this contactor is shorted to ground or maybe the compressor delay board inside of the condenser unit all you would do is disconnect power disconnect wires and then if I go from one side of the contactor coil and then let's say to the screw that holds this contactor to the housing of the unit if I put the other lid on the screw you should be reading o L open line like that if you have resistance that means that that component is shorted to ground then you can go to the other side of the contactor coil check it to ground as well you should have a well there as well and that's how you would check the other components as well and if your 3 amp fuses blowing when you jumper our 2 W or when your furnace is on that means that chances are one of your safety switches is shorted to ground so maybe it's a pressure switch or a high limit switch a flame rollout switch the first thing I would do is just visually inspect all the wires going to your safety switches make sure that there's no bare spots there's no kinks spots maybe some wire is pinched in between two metals or maybe over the years from the vibration if a wire was touching the metal part of the furnace or especially where it's getting hot maybe that wire rubbed a hole in the insulation and is starting to short out so look for any visual damages if you don't find any at that point you would need a meter you can set it to a continuity or a resistance and with the power off to the furnace you would disconnect the wires and just check from each terminal to ground which is basically any screw that screws into the furnace and you should have oh L from every switch to ground if you have continuity from any of the switches to ground then you know that that switch is bad and is shorting to ground I remember this one - that kept ripping my 3-amp fuse and for a long time I couldn't find what was tripping it finally I narrowed it down to the high limit switch right there and if I went from one terminal to ground with the wires disconnected I actually had continuity from this switch to ground so I took that switch out and the two prongs that hold the high limit switch inside of the heat exchanger the two prongs going in there they were actually one of them was touching the walls of the cabinet of the furnace and it was shorting out so right when the furnace would call for heating that thing would blow that 3 amp fuse right away alright and lastly I just want to go over ohms how to try to track down a short using the ohm or checking for resistance as I mentioned previously when you're measuring resistance you want to have the power off if you have the power on and you try to check resistance with your leads chances are you're gonna burn your meter out and you'll need a new meter so you want to have the power off and the wires that you're checking disconnected so for example let's say this was my thermostat wires going to my control board I would want to pull all of them off and just leave the common wires on so the common coming from the condenser unit and the common going to your thermostat both of those would stay on and the rest of the wires would come off and then I'll do this example on my actual control board so I pulled off a couple thermostat wires from my control board I'll put one meter lead on my common with the common wires connected to it and with the other meter lead I'll just check my other wire for example the G which is fan so G to fan should be oh L open line there should be no continuity otherwise that'll be a direct short so my fan wire is not shorted out and for example is just check Y as well the yellow wire that one is oh al as well so that one's not shorted out and let's say one of them was shorted to ground so I'll go ahead and simulate a wire shorted to ground by touching it to the metal so my fan wire is now touching metal and if I check it now I'm getting point five point four ohms many times it'll just be zero point zero if you're getting resistance everything should be o L if you're getting resistance between common and one of the hot legs one of the hot thermostat wires that means that wire is shorted to ground well guys and that's how you check which wire is shorting out using the resistance reading on your meter I prefer to use the power method which I spent more time on and that's actually all I had for this video how to track down a low voltage short I hope you found this video useful if you have any other comments or suggestions if especially if you have experience dealing with low voltage shorts yourself it'd be awesome if you can share your experience with us in the comments below thank you so much for watching this video don't forget to mash that like button on the way out and we'll see you next time and if you're still here and not in the comments section below let me show you one more little tip so let's say this won't involve blowing fuse most of the time let's say that you suspect that you have a broken wire inside of your walls a broken thermostat wire somewhere inside of your walls and you really have no good way of visually checking it an easy way to check which wire is broken is to go down to your furnace the furnace control board pull off all the wires pull off all the wires from your thermostat as well and then at your thermostat take any two wires especially the ones you're suspecting let's say your heat is not turning on which would be the white go ahead and wire nut the white wire along with G or the green wire together so with these two wires wired together this is essentially just one piece of wire now and if you go down to your control board with the wires disconnected if you set your meter to continuity and check green to white you should have continuity we hear an audible beep so that means we do have continuity if there was no beep that means that either the green wire or the white wire has a break in it to narrow it down you can just wire not green and red together check it and then why you're not white and red together and check it and whichever one does not do a beep that's the wire that's broken so hopefully this will help you out someday [Music] you

Channel: Word of Advice TV

Views: 387,589

Rating: 4.9166565 out of 5

Keywords: how to find a short, how to find a short in a wire, how to find a short to ground, how to find a short with a multimeter, how to find a short hvac, how to find a low voltage short, how to find a low voltage short hvac, how to find short in furnace, how to find short in air conditioner, 3 amp fuse keeps blowing in my furnace, 3 amp fuse blowing, 3 amp fuse furnace keeps blowing, 3 amp fuse air conditioner, 3 amp fuse, low voltage short, word of advice tv

Id: 4e4wsruGS0s

Length: 32min 5sec (1925 seconds)

Published: Wed Aug 14 2019