[noltian]

I have 15 years old Bryant 80% furnance attached to evaporator coil in the attic. Lately, I noticed the furnance works about 4 minutes only and found out LED was blinking displaying code 33.

The furnace fires about 2 minutes only and cut the gas. it keeps cycling in the manner until it reaches the temp set from the thermometer.

I bypassed flame rollout sensors one by one. (I have two) and it didn't change anything. However, furnace works good if I bypassed limit switch. I thought it was the cause and replaced it. Well, new limit switch didn't change anything. The same symptom continues that furnace fires about 2 minutes only.

My fan motor works fine (I replaced 3 years ago myself), I check the vent pipe and it seems fine without any obstruction.

My evaporator coils should be clean as it was cleaned up last summer. I change filter every 2-3 months. I ran the furnace without a filter to see and no change in the symptom was observed.

I watched the flame and it does not look stuttering.

I almost think that's the best I can check myself as I don't know what to check any more. I am guessing heat exchanger may be bad?..I conclude my furnace gets too hot but no clue what would cause to be hot.

Any idea what to check next to get this fixed?
I appreciate any input for the problem I have.

 

[how]

Over heating beyond what you've checked???

Anything restricting the air return intake.(try running with the fan door open and the door switch taped shut)
Anything limiting the heating outlets. (Dirty vent filters or they've been shuttered)
Vent motor not operating at required speed. (failing capacitor?)
Loose main fan cage.
Sound proofing insulation that has come loose within the fan compartment.
Those clean a/c coils not actually being clean.

Do a temp rise check on the furnace. If it checks out OK within the furnace's rating plate, it probably is a cracked exchanger tripping the limit.

 

[roughneck]

What kind of air filters are you using, and are they clean?
Never bypass safety switches, use a meter to check them.

 

[noltian]

What kind of air filters are you using, and are they clean?
Never bypass safety switches, use a meter to check them.

They are clean. I replaced to see and tried to run without the filters as well. I used to use 3M filterate and switched cheaper version last summer.

 

[noltian]

Over heating beyond what you've checked???

Anything restricting the air return intake.(try running with the fan door open and the door switch taped shut)
Anything limiting the heating outlets. (Dirty vent filters or they've been shuttered)
Vent motor not operating at required speed. (failing capacitor?)
Loose main fan cage.
Sound proofing insulation that has come loose within the fan compartment.
Those clean a/c coils not actually being clean.

Do a temp rise check on the furnace. If it checks out OK within the furnace's rating plate, it probably is a cracked exchanger tripping the limit.

What spot do I measure to check the temp rise?. From the input to vent, it is 66 to 98

 

[yuri]

You check the supply temp about 3-4 feet downstream from the furnace in the plenum and subtract the return temp at the filter or just use the temp at the thermostat. There is a allowable temp rise rating on the model # sticker. Usually 30-70 deg F or close. Sounds like you are way too hot. Check your motor capacitor to see if it is weak or replace it. Need a meter that checks microfarads. Cheaper to buy a capacitor for $10. It needs to match the new motor specs. Some use 5 mfd others use 7.5 or 10. Yours may be too small if you used the original. The specs are on the motor label or box.

 

[noltian]

You check the supply temp about 3-4 feet downstream from the furnace in the plenum and subtract the return temp at the filter or just use the temp at the thermostat. There is a allowable temp rise rating on the model # sticker. Usually 30-70 deg F or close. Sounds like you are way too hot. Check your motor capacitor to see if it is weak or replace it. Need a meter that checks microfarads. Cheaper to buy a capacitor for $10. It needs to match the new motor specs. Some use 5 mfd others use 7.5 or 10. Yours may be too small if you used the original. The specs are on the motor label or box.

Thank you for reply.

Replaced the capactor for the same spec 10uf/370vac. It ran a minute longer than before

Ran without fan door. It ran 4 minute longer

I am not sure if that means anything

Could it be they fan motor too weak? How can I check?

If the heat exchanger is bad, is there a way to check?

I appreciate for your help

 

[beenthere]

Your A/C coil may be a lot dirtier then you think.

 

[Ozarka88]

Does the furnace have a secondary exchanger. A plugged up secondaries could cause the CO to go up. If the CO is above 200 ppm with no other underlying factors, the HX is plugging up and needs to be replaced.

Feel the top of the furnace when it's running. It should be warm, but not hot.

Remove the hoses from the trap.There should only be clear "water". If there is brown sludge, then the secondary heat exchanger is probably plugged. Also, the hoses should should feel firm, not flimsy and soft. This is also a good indicator that the heat exchanger is plugged.

May have to pull fan motor and inspect the heat exchangers. You should only see smooth metal. If it's plugged, and corroded through, you will see small brownish red balls or globs on the outside of the fins. Rust in the condensate is likely from the exchanger as well. What was the heat rise?

 

[ben's plumbing]

this may help....

 

[noltian]

Does the furnace have a secondary exchanger. A plugged up secondaries could cause the CO to go up. If the CO is above 200 ppm with no other underlying factors, the HX is plugging up and needs to be replaced.

Feel the top of the furnace when it's running. It should be warm, but not hot.

Remove the hoses from the trap.There should only be clear "water". If there is brown sludge, then the secondary heat exchanger is probably plugged. Also, the hoses should should feel firm, not flimsy and soft. This is also a good indicator that the heat exchanger is plugged.

May have to pull fan motor and inspect the heat exchangers. You should only see smooth metal. If it's plugged, and corroded through, you will see small brownish red balls or globs on the outside of the fins. Rust in the condensate is likely from the exchanger as well. What was the heat rise?

I don't think mine has 2nd heat exchanger. Mine is 80 percent Bryant.
I am not sure if mine is hot when I felt it. I have another unit and temp was the same, 101 degree when it was running. Heat rise is from 66 to 100

you lost me with the word " trap and hose". Is there a hose in the furnance?.

I am thinking of giving up DIY. It seems it is beyond what I can do myself. I will try to clean up evap coil and see the heat exchanger myself with your instruction though.

Thanks again for reply.

 

[noltian]

the model number is 373lav069115 bryant

 

[roughneck]

That temperature rise is too high, at 100*. Why is it so high? What's gas pressure, and what speed is the fan running?

 

[noltian]

That temperature rise is too high, at 100*. Why is it so high? What's gas pressure, and what speed is the fan running?

It is setup as medium high. What temp is it supposed to be?

 

[noltian]

I meant to write 66_89 for heat rise. Near 100 is surface temp of furnance

 

[roughneck]

Depends. What's duct static, on both the supply and return? What does the evaporator coil look like, and is there a large pressure drop across it? What is the gas pressure set to? Is the blower running up to speed? Is it shutting down mid cycle?
Some of these tests will involve working with live natural gas lines and live electricity.

 

[roughneck]

That temp rise is still too high. It will say the maximum allowable temp rise on the data tag on the equipment.

 

[Ozarka88]

I don't think mine has 2nd heat exchanger. Mine is 80 percent Bryant.
I am not sure if mine is hot when I felt it. I have another unit and temp was the same, 101 degree when it was running. Heat rise is from 66 to 100

you lost me with the word " trap and hose". Is there a hose in the furnance?.

I am thinking of giving up DIY. It seems it is beyond what I can do myself. I will try to clean up evap coil and see the heat exchanger myself with your instruction though.

Thanks again for reply.

A standard-efficiency furnace — which has a metal exhaust pipe should not have condensation. Otherwise, the condensate trap collects exhaust byproducts in vapor form into a small container connected to a drain tube or pipe. Some traps can be removed and replaced. The condensate is acidic so it's advised to wear some type of hand barrier. A good pro would be able to capture your CO readings. It's worth it.