I have a Carrier 58PAV natural gas furnace in my basement that keeps cycling with an error code of 33.

1. The burners ignite just fine when calling for heat, and they run for 5 minutes. No error code.
2. Then the burners stop. I get a code 33. The blower runs for 5 minutes.
3. And after this, the cycle repeats itself. The burners run for 5 minutes, the code goes away, etc., etc.

According to my research, there might be an air flow problem preventing heat from dissipating away from the heat exchanger, which triggers the burner shut-down.

I tried the following:

- Checked filter: dirty. Removed filter: no change.
- Checked exhaust pipe going into flu: was loose. Put it back in correctly: no change.
- Checked registers: all open. Can definitely feel a strong flow of air especially on 1st floor. 2nd floor air is very weak (always has been).
- Removed humidifier which is installed on side of A-coil in order to bypass the duct work, and let all the air escape into the basement. It worked, code didn't come back and burners stayed lit for as long as I tried this.

So my conclusion is that there is an air flow issue that is preventing heat dissipation at the furnace. My house was remodeled and the contractors installed flexible ducts everywhere to replace the traditional ducts. I think about 8 of them. Unfortunately some of these ducts were undersized in order to fit in walls and I have a mix of 6 inch and 4 inch ducts.

Could this ductwork be the cause for my trouble? I think this condition has been present since the remodel 2 years ago, but went unnoticed. I'm unsure that the condition was present before the remodel, but it very well may have since I remember the house was always hard to heat anyway.

What is the fix? Obviously I can't go back and change the ductwork now.... That leaves me with few options. I was thinking of maybe increasing the blower speed (if possible) to push air through the small ducts and hopefully get more dissipation. Or maybe running an additional duct directly inside the basement (there is none at this time. I think there was 1 before). I'm afraid that if I run a duct inside the basement the air will take the path of least resistance and I will end up with almost nothing everywhere else.

Thank you for your help...

 

UPDATE: Can't edit the first message anymore. I think I found what the problem is... Sort of. I tested running the furnace with the blower door off and it actually ran strong for 30 minutes until I got tired of watching. Then I installed the door back, it shut down after 2 minutes. So this would seem to indicate an issue with the return air? Blower not getting enough air which prevents air dissipation? I guess my testing with the humidifier taken out was a false positive because I may have had that door open at the time. The return ducts where not touched during the remodel so I'm starting to think I've always had this issue since buying the house and never paid attention to the code or the short cycle...

Would cutting a new hole on the other side of the blower and installing a filter holder be an acceptable solution? It would draw air from the basement. Not the best but I work with what I have... Not feeling like running new returns and I don't even see how I could anyway! I'm planning on testing this concept by replacing the blower door by a filter (attached with foil tape) just to see if this could help.

 

What are the dimensions on the return air duct? Your total inlet duct cross section should be 337 square inches (23 1/4" x 14 1/2"). You could cut a hole on the other side but you will want it small. The Owner's manual is here: http://www.alltemp-hvac.com/58pa-15si.pdf