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My Carrier furnace works fine without the air filter in place on the return. With the filter in place it works for a while and then stops with a complaint about "limit, or flame rollout is open" (Carrier unit error 33).

What are some next steps I can carry out to narrow down the problem?

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What type of filter are you using? Some of the higher-end filters can really restrict airflow and might cause this. Have you tried a different type of filter? –  Steven Jan 18 '12 at 14:08
7  
If air flow is restricted the heat is not dissipated quickly enough, and the high limit switch will kick in as the furnace overheats. Make sure the filter is clean, and the appropriate density for your furnace. If a different filter does not fix the issue, it may be caused by an under powered blower motor. –  Tester101 Jan 18 '12 at 14:34
    
I usually use the cheap filters, but I tried a more expensive one and that did not help. I may try one of those really cheap ones (fiberglass?) that look as though they only filter the largest particles. –  rlandster Jan 19 '12 at 0:22
    
How does one test that the blower has sufficient power? –  rlandster Dec 16 '12 at 1:12
    
The problem turned out to be that the gas feed was set too high causing the furnace to overheat. The technician reduced the gas flow and the problem went away and the furnace has worked perfectly ever since. –  rlandster Feb 24 at 0:36

1 Answer 1

As Tester101 mentioned (and he really should've made it an answer, because it's at least a partial one), the filter could restrict airflow to the point where your furnace overheats. Hence the 'Limit' error.

But wait! Trying a cheaper filter, or running without a filter, isn't a good answer. You'll shortly find that the furnace won't run at all, with or without a filter.

What this probably means is one of two things, both of which are not DIY in my book. The first is that your blower unit is going, or is clogged with dust/dirt/pet hair/warm fuzzies/etc. This would require a qualified HVAC professional to replace. The second is that you may have an A/C unit, and the coils are clogged. The second is somewhat DIY-able, but I can't give you directions because I don't know your unit. Mine slides out on kind of a tray, which lets me blow the coils off with compressed air or coil cleaner (which is available as an aerosol at the hardware or home improvement store near you.)

Not tending to this risks a furnace fire. That's why it stops. Get it taken care of!

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