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3 Amp Fuse Blows Sporadically
in Gas Heating
I have an older Bryant gas furnace and a Lennox central AC unit and have an issue where the furnace or AC will start up and run normally for anywhere from 20 minutes to several hours and then the 3 amp fuse will blow. I originally suspected a bad relay on the board, so I installed a new control board and was very careful with the wiring. I also tested the tstat wiring and transformer and everything checked out fine. When I was using the furnace, the inducer came on and the heat cycle worked perfectly, but after the blower came on, the fuse would blow...again at unpredictable intervals. The same thing is happening now with the AC, so I am starting to suspect something with the blower motor. Is it possible that bad windings in the motor could be causing a short that is blowing the fuse? Any suggestions would be greatly appreciated. This has been beyond frustrating!
No... the motor winding on the blower motor is line voltage (110v or 220v) and the fuse is on the low voltage circuit.
That fuse is to protect the transformer from letting the smoke out. Look at the green wire from the thermostat to be in direct contact with the common somewhere. Also, look at the fan circuit thru the circuit board.
Picture of the circuit board with the fuse and or the model number of the furnace might help.
For example: I had a similar situation with a bad transformer. I replaced the transformer for a customer in September (cooling season was over and the heater would not work). The heat ran fine for the entire winter. Come June and the AC did not work. I found that the fuse I added to the circuit failed (good thing I added that fuse). I replace the fuse and tested the furnace and everything was fine. I turned on the AC and after a 5 minute thermostat delay, the compressor contactor powered up for about a minute or less, then the fuse blew again. I traced the problem to the thermostat wire that went to the outdoor unit. A weed-wacker had pealed the insulation off the wire and when the condenser fan moved the air thru the condensing unit, the wire fluttered in the breeze. Eventually, within less than 1 minute, the uninsulated portion of the thermostat wire to the condenser touched together and a dead short on that control circuit blew the fuse.
Since my failed wire was the one that operated the condenser, the problem never happened in the winter. Since your problem is happening when the blower is in operation, you should look at that low voltage circuit that operates the blower relay.Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics1
Is it the same tstat you always had?0
What type of fan control relay do you have, Old Style (Pre 1990s) or the newer style control on a circuit board?
Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics0
invest in what they call a "lil popper. basically a re-settable fuse. this way you can test the system without blowing thru fuses.
was there any other components added to the system on the low voltage side? just because it blows when the blower motor comes on doesn't mean the its the blower relay. it just means that you have exceeded the allowable amperage draw on the low voltage circuit thru the board when the blower relay is energized. if its a short it will trip immediately because it will have no load to limit the amp draw but if its an overload it will be a slow blow.
as has been stated your transformer could be weak. make sure you have good connections to transformer0
do you have an ammeter? a transformer failure that would blow the secondary fuse is unlikely. An ammeter would sort out if it is near capacity or if you have a chaffed wire or loose strand or something that is intermittently shorting.0
I assumed the bryant furnace was the air handler for the lennox split system. Is this the case or are they separate systems?0
im assuming its a furnace with a-coil on top. why would you have a separate air handler?0
Hello everyone and thank you for all of the feedback. I'll now need to spend some time to follow the suggestions you've provided. In terms of some of the above questions:
@EdTheHeaterMan: I have the newer style control board
@pedmec: I had purchased the lil popper a few weeks ago and am currently using it while I troubleshoot...great little device!
@mattmia2 and @pedmec: The tstat is nearly two years old and it checks out fine. I do have an ammeter and will try some additional checks. You are correct that the furnace is the air handler...furnace with the coil on top0
Th reason for the t-stat question was did you recently change it to a model that draws more current. It appears the answer is no. My suspicion is on a chaffed or loose end of a wire somewhere. Also maybe a splice somewhere that lost its wire nut or other insulation.0
Quick Update - although I have not had time to perform additional troubleshooting, I wanted to mention that the AC worked through the night and into today without the lil popper tripping. @pedmec mentioned the possibility of a weak transformer, but I am wondering how the unit can function perfectly for nearly 24 hours or more if a component is bad. Who knows, an hour from now or perhaps this evening, the fuse will blow. I guess my question at this point is what component could be on its way out and causing this totally unpredictable functioning? I did not touch any wires or do anything since my original post.0
Chaffed wire shorting to the cabinet or something else.0
As Matt says......blower vibration causing intermittent grounding of wire.
Often where cables pass thru the furnace cabinet.
Or sometimes metal clips holding wires in corners of cabinet will cut insulation.0
bad contactor in the condenser. coil could be failing. or the contactor itself is falling apart. but it would be in a/c only. is it failing in heat?or just a/c only.0
fan and ac if i recall from above. re-reading the original post, looks like any time the blower is on, so it could be any of the control wires that is chaffed or missing some other insulation or has a protruding strand that the vibration is causing to short.0
Thanks again for the feedback folks. To answer @pedmec, the issue appeared late this past heating season and again when I started using AC. I am starting to think that the blower vibration theory may be it. During the heating season, the fuse only blew when the blower motor came on. Everything else in the heating cycle worked. I looked back at my notes from the heating season and I had jumped R & G on the control board to isolate the blower with all other tsat wires disconnected from the board. The blower ran, but blew the fuse after a few minutes. It did not always blow immediately however. I am going to trace the wires more carefully to see if there is any source of a short, but so far today, everything has worked without an issue.0
Can you take a pic of the wiring diagram and post it?0
If r and g are jumped on the furnace the blower should run continuously.
You only need r and w for a heat call, a timer or thermostat on the furnace controls the blower in heating.
On a cool call the thermostat closes r-g for the blower and r-y for cooling. The r-y also makes the furnace run high blower on most modern furnaces and r-g only runs low blower(on older furnaces r-g was high fan and r-y only connected to the contactor in the condenser).
It is possible that when the blower is running even on a heat call some terminal that is connected to an open contact on the thermostat becomes energized depending on how the logic in the furnace works.0
If you disconnect all the tstat wires, including the AC cable, and jumper the R-G as you did and still blow the fuse, then the problem is not in those tstat wires.
Taking a picture of wire placement first is a good idea.0
does the fuse have a burnt spot in the middle of the fuse? or is just melted away?
the only problem with the chaffed thermostat wires is that he jumped them out and the fuse still blew.
is the ifc board you purchased an oem board? did you get it from a Bryant distributor. meaning, not using the old one to match up with the new one. using the furnace model numbers to purchase a new one?
can we get a model number of the furnace. what size fuse is blowing. 3 amp or 5 amp.0
There is other 24vac control wiring besides the t-stat wire. Look at the standoffs for the control board, make sure they aren't broken or missing and the board isn't touching the chassis of the furnace, make sure there isn't a scrap of wire or sheet metal or a screw or something behind or on the board.0
Also keep in mind one side of the transformer's 24 Volt secondary may be grounded and the other side may go through the 3 Amp fuse. The fused 24 Volts may go to other places inside and outside of the Furnace, like safety sensors, dampers, etc. If one of these wires is chafed, or inadvertently crushed the vibration from the blower may just be agitating the situation. The associated blower control circuitry may not have the actual defect. Also an unrelated (to the blower) sensor that is connected to the fused 24 Volts (and maybe otherwise working fine) may cause a intermittent short to ground. A meter or a low current 24 Volt test light would help to see if fused 24 Volts is found elsewhere. A deep visual inspection with good lighting, tap test applicable sensors may help. Any corrosion ?National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System1
Hello all - with a family member in the hospital, I have not had time to troubleshoot and since my last post, the unit has been working flawlessly for 6 days. Just now, I saw that the popper was tripped and all of a sudden, when I reset it, the unit tripped immediately (at least a dozen times) with the thermostat calling for cooling. I opened the cabinet up, taped the power switch and check transformer voltage at the board. Shows 26.5V. I reset the popper again and now, everything is working!! This is driving me nuts. I have a very good light and have checked every visible wire and cannot see any damage that would cause a short.0
It can be very difficult to see damage. You would think not, but... try wiggling wires.
Also be aware that it may not be somewhere obvious. Mice have been known to chew insulation in the darndest places...Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0
I believe you swapped out the controller, so the 2nd controller should not be suspect, but 'like' electronics devices often have 'like' defects, keep this in mind, sad but true.
I am not a big fan of 'lil popper' type troubleshooting, except it may save fuses. If I were there, and I was tasked to find your problem knowing the history you have posted here, this is what I would do.
Its is called the Dim Bulb (or Dim Lamp) tester (some videos on YouTube). Probably in this case (2) two old school #1156 automotive Back Up lamps Connection In Series . The two 12 Volt lamps 'connection in series' gives you the 24 Volts and the #1156 gives a current limit of about 2.1 Amps (close to 2/3 of a 3 Amp fuse). The lamps can be a substitute for the original fuse or the 'lil popper'. Since you have the 'lil popper' I probably would put the series lamps in series with the non-grounded side of the secondary side of the 24 VAC transformer. Have the lamps outside the furnace where they can be viewer from as many vantage points as possible. If you get fancy you could use an extension cord too if needed to extend the lamps to other places (NOT using 120 VAC).
With things working normally the lamps may be dim and may fluctuate a bit as the controller board does its thing. When a serious overload happens (the kind that will pop the 'lil popper' or blow the original fuse) the lamps will light up bright. This gives you instantaneous information and it instantaneously resets if the fault goes away. Way better than the 'lil popper' method. The down side it having the appropriate lamps for every occasion.
Once the Dim Lamp test is set up ever so gently wiggle any wires that are any where near anything metal. Or even two wires that may be rubbing to each other. Tap and wiggle any sensors that may provide a ground path to the fused 24 Volt source.
Surveying the system (AC Voltmeter, low current 24 Volt test lamp, wiring diagram) for (see my previous post above) all Fused 24 Volt power destinations. With the Dim Lamp test it should be easy to locate the defect if it is a intermittently grounded fused 24 Volt source. I initially hesitate to disconnect or move anything since agitation can make the problem go away for a long time until some settling may bring it back again.
Once convinced the problem is not in the furnace housing I would expand outwards. If the thermostat wire is rubbing on a pipe or duct or any grounded metal your problem could be there. If your thermostat wiring includes the 'C' wire it could be a staple that has compromised the insulation on two wires, a rogue nail in a wall. As anything been done to the dwelling prior to this situation?
I have seen equipment with symptoms like you describe, the defect is often wires crushed between two pieces of metal or a very small insulation rub through. If a wire is chafed where it goes around a metal corner or the like you have to look very closely if does not take much to pop a 3 Amp over-current device.
The dim lamp can be moved to other parts of the system to prove a point. Like if you suspect the wires to the compressor / condenser unit? Connect the Dim Lamp tester to the fused 24 volt wire (inside the Furnace) that goes to the compressor / condenser unit. The 'lil popper' can be the control board's over-current device in this scenario.National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System2
disconnect the 2 conductor wires feeding the condenser. see if it doesn't pop with it disconnected then it might be a frayed 2 conductor or something is happening in the condensing unit (bad contactor coil).0
Just want to pause to say thank you to all who have been patient with me as I try to nail down the root cause of this. There were requests to see the wiring diagram, which I have attached. The Furnace is a 39-year-old Bryant 397HAW036080 and my AC unit is a 29-year-old Lennox HS26-311-1P. YouTube and the Internet have enabled me to keep both units running, but I may have met my match with the current problem. Given the age of this system, I'd say I've gotten my money's worth.
As mentioned originally, this issue has occurred during the past heating and current cooling seasons and despite inspecting the wires, jiggling wires, etc., I cannot find any obvious issue that would cause a short. The unit will now run for 30 minutes to several hours and then the fuse blows. It is now happening right in the middle of a cooling cycle. Absolutely no predictability. I shut everything down, wait 10-15 minutes, power back up and set the tsat to call for cooling. Compressor comes on, blower comes on, runs for a while, then the popper trips.
The dim bulb option sounds interesting, but let's say I try that, and wiggling wires does not produce clues. The brightened bulb tells me a problem is occurring, but I think I would be right back to the mystery of root cause. As one person commented, maybe the new board I purchased is bad. It is an OEM part.
Given the age of these units, I suppose I should expect recommendations from you to replace the entire HVAC system...lol. I'm pretty persistent, perhaps to a fault, so if I happen to figure this out, I'll post an update. Thanks again for your advice and guidance on this!
I'd like to pause for a moment to thank everyone for their patience and suggestions as I try to work through the issue. I have attached the wiring diagram that was requested. I have certainly gotten my money's worth from this system, as the furnace is a 39-year-old Bryant 397HAW036080 and the AC is a 29-year-old Lennox HS26-311-1P. YouTube and the Internet have enabled me to keep everything running very well, but I may have met my match with this issue.
I have inspected every wire I can see and wiggled each wire while the unit is running, with no results. The popper is now tripping 20 minutes to several hours after the cooling cycle starts and the house never reaches the tstat temperature setting. This comes after everything worked perfectly for 6 straight days.
Since I hadn't mentioned the exact age of the system originally, I suppose the next advice I'll receive is to replace it...lol. That said, I am persistent - perhaps to a fault - so if I happen to miraculously find the cause of this, I'll let you know. Thanks again for all of your guidance on this...I really appreciate it!
Just discovered something new...during cooling cycle, the outside compressor and fan unit shuts off without the popper tripping. During the time it is off, the furnace blower stays on. A few minutes later, the compressor comes back on, and everything runs fine until the popper trips. Not sure if that helps narrow things down.0
Did you try with the control wires for the AC, at the furnace, disconnected?
(as mentioned twice above)
With the AC cycling for no reason I would suspect the cable going outside.
Mice like to chew on those.
Usually the 24 volts runs to the AC contactor only, but you may have high and/or low pressure switches out there.0
the wiring diagram pic is from the original board which you don't have anymore so its kinda not relevant although it would be close. you should have a new wiring diagram that came with the new ifc. its made of sticky paper so you can stick it to the door or over the original.
as mentioned before you need to disconnect the condenser from the ifc so you can see if that is causing your problem.
model number of the condenser will help if it is determined that the condenser is causing the tripped fuse. would need to know what we are dealing with0
Sadly the wiring diagram does not show the electrical location of the 3 Amp ATO style fuse. In fact the only fuse device on the 24 VAC side of the transformer is device 11C Link, Fusible (over temperature) which may be a special heat sensitive wire that surrounds the heat exchanger assembly, this may have rubbed through or is crushed somewhere (blower vibration induced failure, sound familiar?). I would also suspect device 7H1 Switch, limit SPST - NC, it possibly could be shorting to its mounting. I would be also curious about the wires connected to R, H, GH and the devices they go to The wires could be rubbed through at some other grounded metal (water pipes, AC compressor / condenser unit housing, etc). All suspicions are in the priority order they are listed in.
The 24 VAC side of the transformer at Terminal SEC 2 is grounded at Note 1, near or at the Spark Generator mounting.
Can you post a picture of the back side (solder side) of the old control board you swapped out? Both sides would be better for accuracy (I have a picture off the web of the component side, but yours may be a bit different). It would help me better understand where the 3 Amp ATO style fuse is in the circuit and what loads it protects.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0
Blower55 said:Just discovered something new...during cooling cycle, the outside compressor and fan unit shuts off without the popper tripping. During the time it is off, the furnace blower stays on. A few minutes later, the compressor comes back on, and everything runs fine until the popper trips. Not sure if that helps narrow things down.
Something is going on there if the condenser shut down while there's still a call from the thermostat. Find out if that is wiring or low pressure related. My guess is the wiring to the condenser or the contactor. And it's just a guess because like @ChrisJ mentioned, amp it out. Without that you're flying blind.
it blew with an a/c call. you need to determine whether its at the condenser or not.0
So far we have determined that @Blower55 has a fuse that is not on the wiring diagram he posted. I wonder if his camera will be able to take a picture of the actual circuit board. The camera was so close to it when he took the diagram picture. I wonder if they could a picture like this
Then we could look at a close up like this to actually see the fuse location
Then we might be able to see if his control board is one of these common ones.
Or we can just tell @Blower55 to look for bad wiring again.
You see, this is easy for many of us to understand, however there are those that look at these wiring diagrams and see the Hieroglyphic diagram.
The term open and short circuit are not fully understood by most people. I have even had an electrician's helper try to explain an open circuit by saying it was a short circuit.
So here is the wiring diagram tho OP supplied with 3 different "Short Circuits" that could blow the circuit breaker.
Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics0
Folks...another update...I arrived home at around 8pm this evening and the house was at 80 degrees, AC unit was not working, and popper was tripped. I shut off the breaker at my main electrical panel, increased the thermostat temp setting so that there would be no call for cooling, and reset the popper. Waited around 15 minutes, switched the breaker on and lowered the tstat temp so that there would be a call for cooling. It is now 4.5 hours later and the AC has been running continuously without any issue. I'd also like to remind everyone that the blown fuse issue started during the heating season, so this problem is not limited to the AC unit.0
@EdTheHeaterMan, I love the colored annotations. It may help some folks. Using your annotations I doubt it is #2 the (orange circuit) since the 3 Amp over-current device trips out in A/C mode too. If I have been following along accurately the problem started when the furnace was used for heat and continued as the weather changed and A/C was needed. So the defect is common to both modes. Also I don't think it is your #1 scenario since the board was changed and with changing the board the problem did not go away (different board and all the wires to the board were all moved or disturbed). My best guess so far is the 11C Fuse Link (rub through to ground) in your red circuit.
I was hoping @Blower55 still has the old board for a photo opportunity being it is out of the furnace and easy to do. With a good picture(s) the associated fuse protected circuitry would probably be obvious to me, enhancing defective circuit focus by better understanding all of what the fuse protects.
To me troubleshooting is an Art, a Science and Logic. The Art is devising the simplest quickest directional tests to focus in on the defect, the Science is the administration of the tests so they are accurate and valid, Logic helps quickly exclude non-issues and parts that are proven irrelevant, no use wasting time on the irrelevant.
The Lennox HS26 series has pressure switches and delay timers so the compressor / condenser cycling may be a normal symptom of low ambient temperature, low or high refrigerant charge or a plugged up condenser core... I think it is unrelated to the OP issue.
Connecting an ammeter, to me, (as much as I do like ammeters) probably will only reveal three things two of which we already know exist. (1) normal current, (2) fault current (if it will fail while you are watching, don't blink), (3) an abnormally high running current (just over 3 Amps), like if the blower control relay coil is partially shorted. However I think that relay was changed when the board was changed.
If your not testing, you are guessing, which is mostly what is going on here so far. I still maintain that the Dim Bulb test is the best troubleshooting method for this situation. Its cheap, easy, and can provide best direction to the fault. A Tech with a $600 Fluke multimeter is just that, a Tech with a $600 Fluke multimeter, it does not necessarily make him a better troubleshooter. The 'lil popper' is a great tool but its limitations waste time and provides no new troubleshooting direction without additional strategies.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0
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