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Furnace Acting Up Again, Any Thoughts Helpfull


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#1 farmer john OFFLINE  

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Posted January 29, 2014 - 01:08 PM

hi guys; well its gotten really cold again -20s again and the wind is brutal, about 3 weeks ago my furnace would not fire up after much trial and error and a lot of cursing and praying we figured out what was wrong, ice in the direct vent exhaust ( code 3) fixed this then got code 4, high limit switch, well there was none to be had so we watched a video and it said to bang it hard and it might reset, well it did and the furnace ran like a charm till last night (now my tractors are cold) ok i could not resist the gt referance, ha ha ha !!! i had ordered a new limit switch and had it handy and installed it but the furnace fan runs all the time and code 4, in one of the videos i saw them jump the high limit, so i did this and the furnace is running correctly, turning off and on and when in idle mode i have a green light, i am thinking that my new high limit switch (160-30) is fudged and like the last time one could be days away. i plan on leaving the limit jumped till the new one comes in but i worry about co2, gas and blowing the place up, this is a 1996 direct vent 80% mid furnace. i think im good but i have been wrong before, any thoughts thanks farmer john


Edited by farmer john, January 29, 2014 - 01:11 PM.

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#2 hamman ONLINE  

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Posted January 29, 2014 - 01:35 PM

You mention that it is a High Efficency furnace. is this your home furnace or shop furnace or both?  I would check the condensate drain. I don't jnow how you have it drained but it could be backed up. What brand of furnace is it? The high limit you have is a 160* - 30* differential. You say the fan (?) runs all the time. Is that the circulating fan or the combustion blower? The codes on the door should give you a referance as to the problem. If it works after "jumping out" the high limit it is probably bad. By the way the high limit is a SAFETY for your furnace. That is designed to shut your burner off in case there is a mafunction of the circulating blower or other parts to keep it from overheating and causing a potential fire or something.    JM2CW.   Roger.


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#3 jms180 OFFLINE  

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Posted January 29, 2014 - 01:36 PM

don't know the arrangement of the furnace heat exchanger tube. I would make sure heat exchanger tubes are not rusted or broken causing heat to come out the side of tubes tripping the high heat limit. I would also have a co2 monitor inside home.

Edited by jms180, January 29, 2014 - 01:44 PM.

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#4 farmer john OFFLINE  

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Posted January 29, 2014 - 02:01 PM

thanks very much, i do have an o2 sensor, i will get my friends camera an see we can see any cracks in the heat exchangers



#5 jms180 OFFLINE  

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Posted January 29, 2014 - 02:19 PM

thanks very much, i do have an o2 sensor, i will get my friends camera an see we can see any cracks in the heat exchangers

Maybe use flashlight to shine inside burner tube look for cracks also
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#6 dthomp17 OFFLINE  

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Posted January 29, 2014 - 04:51 PM

Sometimes you can have a cracked heat exchanger and you can't actually see the crack or it may be real hard to find. A good friend of mine, who does furnace installations and repairs, told me that a good way to check for a cracked exchanger is turn on the blower without lighting the furnace. Hold a lighted candle close to the exchanger and move it around slowly. If there's a leak, the candle flame will show it and may even go out.
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#7 DougT ONLINE  

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Posted January 29, 2014 - 05:03 PM

Any chance the terminals on the new switch could be a little different? I'm not familar with your furnace but if this switch is the same basic thing as mine, it would have one side control the heat blower on and off and the other side to shut the burner down if it gets too hot like Roger said. If you have to jumper the high limit to get it to start cold, that tells me that the high limit side of the switch should be normally closed and if it gets too hot it should go open. You could check this on your switch with an ohm meter and a cold furnace. I'd say either the switch is bad or wrong design.


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#8 HowardsMF155 OFFLINE  

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Posted January 29, 2014 - 05:18 PM

Sometimes you can have a cracked heat exchanger and you can't actually see the crack or it may be real hard to find. A good friend of mine, who does furnace installations and repairs, told me that a good way to check for a cracked exchanger is turn on the blower without lighting the furnace. Hold a lighted candle close to the exchanger and move it around slowly. If there's a leak, the candle flame will show it and may even go out.

I think that is one of the better methods short of tearing the unit apart to physically check for a crack.  If your unit is a 96 and hasn't had the heat exchanger replaced yet, I would be fairly certain that is the root cause.  One other thing that can make you tear your hair out is when the board goes bad and starts giving you false error codes.  

 

I would be very concerned about a high limit switch that keeps going out.  If jumping it out keeps things running, you need to investigate why it keeps tripping.  Older models used to be re-settable  I suspect the fan that runs constantly is a warning feature, the furnace is trying to tell you something is wrong and get you to check the board.

 

Ignore this part, I thought you had a high efficiency furnace, then reread your original post and saw it was a standard efficiency unit.

Check your condensate drains as has been suggested, also trace those condensate lines into the furnace.  Some units have two spots they drain from, one being the heat exchanger, and the other is located on the fan housing.

 

I had a unit giving me intermittant trouble one time and I pulled the draft fan out.  Found the original installer had dropped a small screwdriver into the fan cavity.


Edited by HowardsMF155, January 29, 2014 - 05:26 PM.

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#9 DougT ONLINE  

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Posted January 29, 2014 - 05:41 PM

If the furnace won't start cold without jumping the switch, how could the heat exchanger be affecting it?



#10 Sawdust OFFLINE  

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Posted January 29, 2014 - 05:45 PM

I had to unclogged my FIL's furnace a few weeks ago showing code yellow. It was his vent iced up outside preventing enough air to escape. I shut it down for a few minutes after I cleaned it out & let it cycle through then it was fine. Hope you get it figured out. 



#11 farmer john OFFLINE  

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Posted January 29, 2014 - 06:14 PM

thanks for your thoughts, i am still stumped, the furnace is working now the way it should but i dont know how????????? i went out and bought a new co2/propane/nat gas detector for by the bedrooms and i will put the old one downstairs. we put a camera inside the furnace and saw a few points of concern but its late and the stores are closed, i will go and get a smoke bomb from the furnace supply tomorrow, that should tell me if the heat exchanger may be cracked, when we pointed a laser type heat detector we found one area of the box considerably hotter a small crack might heat the box over the limits, we also put a co2 detector inside the hot air duct directly over the furnace with no alarm, ps this is a luxair PBLU-LD2ON140A its a 140,000 btu it seems odd that when we had the last real cold snap i had issues also, if i find a leak in the heat exchanger i will put in a little bigger furnace possibly a 160,000 btu, yeah its a big house 3750sq ft one floor with full basement in case someone thinks its overkill



#12 bja105 OFFLINE  

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Posted January 29, 2014 - 07:25 PM

Don't leave the limit jumped.  Never jump a safety control!  That one is a fire hazard.

This is one time you should have called a pro.  It would be fixed by now.

 

Don't use pleated filters.  That's my psychic furnace man guess as to why your limit failed.  If it got hot enough often enough to kill the limit, I would worry about the heat exchanger. 






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