Top 4 Reasons Why a Gas Furnace Short Cycles! HVACR Service Call Troubleshooting Tips!

hey guys this is Craig McClatchy with AC service tech and today what we'll going over are the four main reasons why a gas furnace short cycles during operation so short cycling means that the gas furnace is running through its full sequence of operation for heat it's providing heat in the building and then it's shutting off before the thermostat is satisfied so then it's immediately turning back on again and trying to heat the building again and then shutting off and then turning back on again so we're gonna be going over the four main causes of that in order to troubleshoot a furnace you need to know the sequence of operation for heat and in order to start the sequence of operation you have to have 24 volts on the W right here on the control board so our sense 24 volts to the thermostat and inside the thermostat R and W touch then over at the control board you have 24 volts on W and that starts the sequence of operation so what happens is in this furnace this is a 90% efficient furnace you have the inducer motor turns on first and then the pressure switch closes then the hot surface igniter turns cherry-red then the gas valve blows the full gas flow then the control board senses the flame through the flame ratification process then there is a blower on delay so it's waiting for the heat exchanger to heat up back here then after that the blower motor turns on so that's the full sequence of operation for Heat and so what we're talking about is all of those things are happening and then all of a sudden the unit shuts off the first reason for a furnace to short cycle is the thermal limit is tripping so this is a thermal limit there's multiple different types these are the ones that are found on this type of furnace you can also have these type as well but they are a little bi-metal disk on the inside here that's going to trip due to heat it's going to pop and it's going to open up the electrical connections on the back of the thermal limit and that's going to send the signal to the furnace to stop the ignition process stop the flame process but it's going to allow the blower motor to cool down the heat exchanger so on this one right here you have L 200 minus 40 so what that means is that if this which is back in the heat exchanger area back back in here if it gets up to a temperature that is 200 degrees then this is going to open up and it's not going to close back down again until it gets to 160 so these are normally closed electrical safety switches so they may look like this it may look like this they could extend in further and look like this they could be a part of a fan limit switch like this so on this the third see there's one two three on here and this on this dial it could be up at 180 it could be 200 it could be on some of these types of switches it could be 220 it just depends on the furnace but there's several things that could be affecting that and one is the blower speed so the blower motor might be running at too low of a blower speed and it's not able to keep up with the heat that's being generated by the flames so in that case the heat exchanger is heating up heating up heating up and then finally this limit switch trips and and that's what's happening so that at least allows the furnace to run a little bit and then through its full sequence of operation then it shuts off during the main the main run time so that's one cause and the blower motor affects the thermal limit switch so you may have to check the blower speed and increase the blower speed so that your temperature rise which you can take a temperature measurement in your return duct and your supply duct so you're checking your temperature differential you want to make sure that that's stable it's not just increasing so in a single speed gas furnace you may have say 70 degrees over on the return and then 120 on the supply which is 50 degrees and so you want to make sure that that's not rising so that could be an issue you could be over firing here you could have a water column pressure on the output of this gas valve it might not be set correctly so maybe you're over firing the system one problem could be that the thermal limit switch itself is the problem so say it's weak over time and you're actually reading a temperature in the supply which will be over here maybe you're reading a temperature of 100 degrees and this normally closed switch is actually opening up and this switch says it's not supposed to open up until it gets up to 180 degrees well then you know that this switch is the actual problem so so that could be the problem this switch itself another reason that the gas furnace could be short cycling is the pressure switch so just because the error code on the control board is is signaling that there's a problem at the pressure switch it does not mean that the pressure switch is the actual problem it could be a problem affecting the pressure switch so you could have your condensate backing up into the inducer motor housing so remember that this is a 90% efficient furnace so you have the extra efficiency being taken from the water that's created during the flame process and so then it's draining the water out through this tube right here and so and you have this pressure switch tube connecting back here and if the water level rises in this inducer motor housing it's going to shut this pressure switch off so this pressure switch is proving that the inducer motors running and that there's no there's no issues while the furnace is running but you could also have something like the exhaust pipe right here could be closing off so maybe there's a bird's nest in it or something like that you could have exhaust pipe like this so if you don't have it pitched properly what's happening is this furnace is extracting so much heat that you have water basically your your gas is condensing end up in this exhaust pipe and then it's trickling back to the furnace and that's why this exhaust pipe has to go a quarter-inch of pitch perfect as it moves away from the furnace so what happens is your actual water that's condensed will trickle back down into the furnace again so say you have a 15 or 20 foot run and I've seen this quite a bit where instead of pitched upwards you're pitched downwards and all of a sudden your water is all building up inside the exhaust pipe and filling up and then there's a little pathway for the exhaust to go through and so then it runs for a little bit and then shuts off runs for a little bit shuts off because it's pushing through this little pathway so that's an issue so you want to make sure that these are supported every four foot and that you're going a quarter to pitch per foot upwards as you move away from the furnace now you also see that you could have a problem right here where you your intake is blocked you see that the pressure switch tube is connected to your intake right here your combustion chamber and if you have a problem with your intake right here you could have the pressure switch tripping because of that so that's another issue your inducer motor itself may not be running you could have maybe some of the fins on the on the blower or the inducer wheel in here are broken and so that could be a problem so you could also have your actual pressure switch itself is the problem so the pressure switch is reading a vacuum so what happens is there will normally be a water column reading on it and in this case on this one it's one point seven four inch water column on this one it says small as 0.1 eight-inch water column so remember that this is a very very small reading and you don't want to be sucking on these tubes with your mouth because you and you can't control how much how much pressure you're exerting on that tube so so that you want to be very careful with so what I want you to know is it's twenty seven point six water column for one PSI so it's really really not a lot so you could have any type of pressure switch it look like this sometimes and these will have normally open and normally closed connections you could have multiple different types but what I want to say is that you can check this pressure switch right here with your multimeter and you should T in a manometer while you're reading this so before you turn your furnace on what you want to do is you're going to either test this pressure switch with a resistance value across the two terminals with the electrical wires off but in order to keep this completely running the whole time and the full sequence of operation happening you're going to want to put one alligator clip on the common terminal over at the control board or on the ground of the furnace and the other terminal on the tap that is not normally supplied with 24 volts so say you have 24 volts coming in here and it's not coming across here until the inducer motor is fully running then your what you're gonna do is are going to have that alligator clip on there and this one on the ground and then you're gonna be able to read 24 volts when this closes as well you're gonna have your water column manometer tea tin into this tube so you can see if there's any hiccups running so it's maybe your you're reading the correct water column level and then all of a sudden there's a hiccup and the the water column level lowers then you can see oh that's when it's happening that's when the problems happening so you want to have both of these tools readily available and then as well you can also test this pressure switch by itself you could use a tool such as this this is SDM n6 and this is a dual water comm manometer it also has a pump built into it and so you could turn the power off to the furnace and test the pressure switch by itself so independent of the actual furnace running so you'd take these two wires off you take the two tubes off and you just be using this tool with the internal pump in order to test that pressure switch so that takes a lot of guesswork out of that pressure switch if the press which is the actual problem or not so I have a video using this tool link down description section below problem number three could be the control board itself and so I have a video on the sequence of operation and also the control board itself down description section below but to troubleshoot a control board you really need to know how the system works so for instance when you have your 24 volt signal on the W you want to know when you're you're sending your 120 volts to your hot surface igniter when you're sending 120 volts to your inducer motor when you're sending power to your blower motor and what happens is a lot of times these relays anytime you see one of these black covers that's a relay and the contacts can become pitted and so you're not sending the correct amount of voltage or you're not sending voltage at all you have like a high resistance across the contacts when they're closing and your output voltage all of a sudden doesn't come out you know you could have a a problem where the the terminal right here was soldered onto the board app and so that could be an issue with this control board you could also have a wire somewhere along in here so say it's to the inducer motor and the the little clip right here is bad so for instance I have one right here and so this this right here you can see it's actually falling apart right here and this happens a lot on out package units but it can also happen on a indoor furnace where the the terminal is just not tight enough on the Spade connectors so that could be an issue where you're sending voltage and then all sudden you're not sending voltage and it just has to deal with maybe the vibration of the furnace or something like that so you want to be able to check your connection so you turn your furnace off you check you to your electrical connections and you can tighten them down you can tighten each of the Spade connectors down with your your wire strippers and cutters so you can go ahead and tighten this down and then you can push it back into the Spade connector again so so that's something that you want to check for it could just be a loose connection and all of a sudden you are no longer accenting that voltage I also wanted to show you what these relays look like underneath the black cases so I pulled the black case off of here and there's just this little tiny connection right here so you can imagine your blower motor getting powered through a tiny little relay like this and these connections can just get pitted so here's another relay right here it's very small and so these these are the contacts right here that's it you know so this could be part of the problem so you just need to measure your output voltage you got to know the sequence of operation to know when the Control board's supposed to be sending the voltage out on each of these terminals another problem that could be occurring in a furnace is the flame proofing process could be interrupted so for instance if you had the hot surface igniter turned cherry-red and then your gas flows coming across here one thing on a outdoor package unit is these little spots right here it could become rusted shut and the gas isn't making it across here so sometimes it's making it crossed all the way over to the flame rod and other times it's only making it over to here so your furnace may be running sometimes and not other times and what happens in that case is you actually need to take these burners out and clean them out and put them back in or purchase new ones because each time that you're cleaning these out your your kind of the chambers are opening up more because there's less metal there so you want to adjust these make sure they're all safely in there again but this right here is a direct ignition so that means that it's blowing the full gas flow across all this stuff if you just had one centralized assembly such as this right here then what you're having is you have the flame rod and the hot surface igniter all in one area and this is a lot safer when you have a full the recognition like this but you could be having some type of problem such as maybe the flame rod itself is it's fully covered with a carbon dust and you need to clean that off and so you need to turn the furnace power off take the flame sensor out from the back which is this is the back back here and you can take it out and clean it with unsoaked steel wool in order to get that that black carbon dust off of it because that may be impeding the the voltage from making it into the flame so there's AC voltage coming from the control board over to this flame rod and then from the flame rod into the flame and then it's rectifying the voltage and you have a DC microwave signal being sent back over to the control board and that's the flame proofing process for this and that's called flame rectification and we have several videos on that down description section below another flame proofing component is a thermal pile or a thermocouple and these are used on furnaces that don't have line voltage they don't have AC voltage in order to control the ignition process though they may have 120 volts going to a blower motor but these are typically used on furnaces that don't require power so they're usually older furnaces or freestanding stoves or something like that but what happens is these can go bad over time and what you're doing is you're involving this thermo-pile or thermocouple with a flame and you're generating DC millivolts in order to power the gas valve so the first thing is you have to have enough millivolts to have the pilot flame continuing to be lit so you're opening up that first chamber you're actually holding open the first chamber within the gas valve but maybe you're not supplying enough DC millivolts to open up the full gas flow when your thermostats powering for heat so that could be an issue and we have multiple videos on the thermal pal and the thermocouple down description section below so knowing the sequence of operation for the furnace that you're working on is extremely important so you want to know looking at in this case it's a 90% efficient furnace but this one has a hot surface igniter and also a sealed combustion chamber you may have one that does not have a sealed combustion chamber it's just open and maybe it's a spark ignition so it could be a pilot ignition or it could be a direct ignition such as this one that I showed you earlier so this is a direct ignition where it goes across you you could have an 80% efficient furnace where you don't have to worry about the condensate so you really need to know the furnace that you're working on in order to diagnose it so all I can say is that we have multiple videos on these down description section below and when you're at a furnace you can look at the wiring diagram in order to determine what's supposed to happen to the furnace that you're working on so there's many reasons why a gas furnace would short cycle and if you have different experiences I'd love to hear from you down in the comment section below I'd also like to have other people learn from your experiences down there as well and from my experience I would say that the pressure switch is probably the most common problem so it's a pressure switch related problem but I hope this video helped you you can also check out some free resources over at AC service tech comm for HVAC our technicians and if you want to help support this HVAC our training channel click here if you want to subscribe click here and if you want to see another HVAC our training video click right here hope you do it yourself and we'll see you next time at AC service tech Channel

Channel: AC Service Tech LLC

Views: 314,563

Keywords: acservicetech, repair, maintenance, how to, fix, hvac, hvacr, how do you, test, ac, service, tech, check, measure, read, superheat, subcooling, refrigerant, procedure, voltage, charging, temp, furnace, heater, heat, short, cycle, pressure, switch, sensor, electrical, 24v, inducer, motor, gas, valve, combustion, sequence, of operation, control, board, wire, thermostat, problem, no, bad, call, technician, course, class, school, exhaust, pvc, drain, clog, blower, fan

Id: Gqoqa1nrWDc

Length: 16min 56sec (1016 seconds)

Published: Sat Nov 30 2019