Limit circuit fault but house keeps warm


Old 10-31-17, 09:30 AM
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Limit circuit fault but house keeps warm

I read all of the messages I could find on the forum about this problem before posting.

Please see below where I post my analysis and what I have tried so far. Please let me know if you have any ideas. Thank you in advance for your input.

I have a 10 year old carrier 4 burner natural gas furnace. Model number 58MTA080-F-12112.

It is giving off code 33 which is limit circuit fault. The funny thing is that the house still heats up. I ran the furnace all winter last year like this. Very rarely do I get a lock out. Last night I got a 14 code ignition lock out. I reset the furnace and went to sleep. When I woke up the temperature was at 68, the setting of the thermostat. The unit was still running.

I have been trying to follow the reasons for this code failure listed on the furance door.

1. Defective blower or start capacity.
> I can hear the blower start up fine. I can manually run the blower from the thermostat. I do not hear any grinding noise.

2. Lose blower wheel
> I admit I have not opened up the blower compartment. But I do not hear any squeaking.

3. Defective induction motor or start capacity. The fan for the burner unit starts up fine. The flywheel turns freely. I do not hear any squeaking.

4. Inadequate combustion air supply.
> I checked the air input from outside. Nothing is blocking it. I reached my hand in the vents (it is a high capacity furnace which gets air and vents out the side of the house). I could not feel any nests or anything in the conduit.
> I checked all the hot air vents inside the house and they are open and unblocked. I checked the cold air returns and none of them are blocked. When I run just the blower I can feel air moving in and out of these vents

5. Flame roll out switch.
> I can see the reset on the side where the burner chamber is. But wouldn't that stop combustion? The burners light OK.

6. Dirty or restricted air duct system
> I replaced the filter last night. I just use the cheap fiberglass filters, not a HEPA filter, etc because I have seen where these better filters turn off the system due to difficulty with getting air movement.
> I pulled the door the the evaporator thinking that it could be dirty. I is surprisingly clean. The coils have no dirt on them.

7. The limit switch itself
> I have yet to check this. I assume if it was stuck closed the furnace would not start. If it is stuck open why would the furnace shut down with a limit circuit fault?

This morning I cleaned the flame sensor. I cleaned the sensor because I got a 14 ignition lock out last night.

I figure the burner unit is getting to hot so the system is shutting itself down. Then the system resets itself, the burner chamber cools down, and the system starts again. Wash and repeat.

This doesn't seem like the most efficient way to heat a house, but it seems to be working. My concern is I will be out of town for work, the system will fail, and my family will want to attack me.

A friend of mine who is a HAC guy and pretty talented suggested changing a dip switch on the circuit board so the blower runs at a higher speed. This is to cool the combustion chamber faster.

Any ideas on what to try next? Thank you very much for your time.

Last edited by rhornback; 10-31-17 at 11:10 AM.
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Old 10-31-17, 10:17 AM
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You posted the model number for an evaporator coil not a furnace.

That evaporator coil can be very dirty on the furnace side and still appear clean on the side that you see when you remove the panel.

I would measure the temperature entering the furnace and the temperature a few feet away from the furnace inside of the air ducts.
Subtract the return air temperature from the supply air temperature to get your heat rise.
Compare this number to the rated heat rise listed on the furnace nomenclature. This tag is in the furnace burner section.

Verify that you are not exceeding the rated heat rise of your model. (It will probably say 30 to 60)

The external static pressure should also be measured.
Old 10-31-17, 10:55 AM
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Hi Houston, thank you for your reply.

The furnace model is Carrier Furnace Model 58MTA080-F-12112. I will also update above.

I have been taking pictures along the way that I will post. Maybe some of them clarify the condition of the unit.

The heat rise says 40-70 on the high heat and 30-60 as you indicate on the low heat. I attached a picture below.

I will go to Menards tonight and see if I can purchase a digital thermostat measurement tool.
Attached Images      

Last edited by rhornback; 10-31-17 at 11:23 AM.
Old 10-31-17, 12:51 PM
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A secondary heat exchanger issue may exist here...
Old 11-01-17, 08:20 AM
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Hi Houston,

I called Carrier. 1-800-227-7437 if anyone ever need the telephone number. I gave them my serial number.

They told me that there is a 20 year warranty for parts only for the primary and secondary heat exchanger.

My understanding is the labor for heat exchanger can be high. Many people choose to put in a new furnace instead.

The idea of increasing the blower motor speed makes sense to me. I think some of the blowers have 3 speeds. So if I can increase the speed when the furnace calls for low heat that more air might below out of the combustion chamber and cause fewer limit circuit faults.

Should I still check the heat rise? I purchased the tool at Menards last night.

Thank you in advance for your ideas.
Old 11-01-17, 08:50 AM
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The blower speed is increased by changing motor taps on the control board.

Based on the failure of the secondary heat exchanger.... increasing the blower speed would only mask the problem. The clogging from the coating would still continue and worsen the blockage.

Houston can comment further but I would recommend a heat exchanger inspection.

Manual..... hvac partners/docs/1009/public/09/58mta-1si.pdf
Old 11-01-17, 06:47 PM
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Just a quick update. I have been educating myself on how a furnace works. Where is the heat exchanger, secondary heat exchanger. Which pipe outside is for air intake and exhaust, etc. Lots of fun.

I was concerned that there might be rust or a crack in the heat exchanger. Especially the secondary heat exchanger. So I purchased a consumer grade carbon monoxide detector. It is suppose to measure ppm. I put it on top of the furnace today and it is showing 0. So assuming the detector is accurate I am not getting carbon monoxide from the inside.

Tomorrow I want to try outside where the exhaust is. It was raining today so I did not try it because I do not want to get it wet and maybe damage it.

Tomorrow I also want to pop-off the combustion chamber cover and watch while the burners run. If something was wrong with the primary heat exchanger I would expect to see some fluttering of the burner flames. I know that is not a given but I do not have the tools the pros have so I am doing the best I can.

What is the possibility that the unit is oversized for the house?

Thanks in advance for reading this and your ideas.

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