Carrier furnace 58sta070 code33

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Old 02-28-15, 01:55 PM
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Carrier furnace 58sta070 code33

My furnace is short cycling. When it does it shows the code 33 limit circuit fault. I've changed my limit switch and flame sensor to make sure there was no fault there. The burner keeps turning off, the code blinks and the blower keeps working. Then the system shuts down and restarts the heat cycle with the exhaust fan first. There is a audible hum from my intake. I've cleaned my blower fan recently and the filter is new. Do I now need to clean the system including the ducts to make this problem go away? Could this be a blow motor going bad?

Thanks for the help.
 
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Old 02-28-15, 02:04 PM
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Has the filter been changed regularly? If not the AC coil might be dirty.
 
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Old 02-28-15, 02:44 PM
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It has. I. Opened the top door to access the coil and it appears clean. When I took out the filter just now and left the top door open the furnace finished it's cycle.
 
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Old 02-28-15, 04:53 PM
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That would seem to indicate that you do have an airflow problem. Check and make sure you don't have too many registers closed down or blocked returns.

Depending on how your furnace and coil is set up.... it can be hard to see the side of the coil where the air enters. That's the side that gets plugged.
 
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Old 02-28-15, 09:08 PM
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Could a bad blower motor cause this problem? The bottom of the AC coil, where the air pushes through is up against the heater and nearly inaccessible without taking apart the exhaust. Any thoughts on easier ways to clean it than dismantled the entire furnace? None of the registers or intakes are blocked at all. I just took out the blower fan again to inspect and it is completely clean.
 
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Old 03-01-15, 08:58 AM
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If you have the blower out.... you may be able to look up thru the heat exchanger and see the bottom of the coil.

My furnace is set up the same way..... coil is buried.... no access door. I'd have to yank my coil out to clean it.

By a bad blower motor.... do you mean where it doesn't always run or is running too slow ?
Usually if the blower is running it's ok and at the right speed. How is the airflow out of the registers...... strong..... weak ?

Did you put in a basic fiberglass filter ? Some of the high efficiency-high end replacements are too restrictive for a basic furnace.
 
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Old 03-01-15, 09:37 PM
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I've been using an ordinary pleated filter for about 3 years. Before that I was using the basic cheap filter and didn't know any better. Everything seems to be fairly clean now I will probably try to take back Out the blower motor and look up through the system.it's cold here so I've got to put that back in all the time.

by a bad blower motor I mean that I've been having a strange hum coming from the intake. I had a guy out to try to figure out what the problem was and he never was able to but I suspect that it has something to do with the blower motor since that's the closest thing to the intake.I wonder whether or not the motor is going bad and it is having a hard time pushing.maybe it pushes at the same speed regardless until the motor actually fails just don't know.if that is the case I assume that the air conditioner coil must be the reason why it is restricted.it doesn't seem like there's any difference in the amount of air coming out of the registers but I could it just not notice a difference from before.

is there anything else that could be causing the problems such as the pressure switch?
 
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Old 03-02-15, 07:06 AM
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A dirty evaporator coil is the most likely cause.
Other possible causes include a dirty blower wheel, crushed ductwork, internal duct insulation loose and blocking airflow, improperly sized ductwork and too many closed dampers or registers.
 
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