What would cause a furnace to shut off prematurely?

Question

I've got a Carrier Weathermaker 8000 furnace that shuts off about 90 seconds or so after ignition, started doing this recently. I get flashing code "33" which indicates:

The code indicates:

33 LIMIT OR FLAME ROLL-OUT SWITCH IS OPEN - If open longer than three minutes, code Changes to #13. Check for:

• Defective blower motor or start capacitor.
• Dirty filter or restricted duct system.
• Loose blower wheel.
• Defective switch or connections.
• Inadequate Combustion air supply Flame Roll-out Switch or fuse link.
• Open Flame Roll-out switch,or fuse link. Manual reset or replace.

I've found the manual here, although it doesn't really help me.

Leading up to this, I hadn't checked/changed the filter in a while. It wasn't on my mind because I usually hear my furnace straining or so on.. when I did go check it, I realized that was because the filter had collapsed and been pulled down the air intake.... so yes it was filthy and also I basically ran without a filter for who knows how long.

First things first, I cleaned all the dust out of the inside and blew air through all the components.

I've been able to check the large rectangular Flame Roll-out Switch and the 2 smaller switches, they all are closed when checked with a multimeter for continuity, and just for kicks I pressed the button on the 2 smaller switches to give them a manual reset.

It appears that I've got a no-kidding problems with the unit becoming too hot, not a false positive. Although I am not positive about this.

It seems like my next step would be to check the capacitor to see if it would be causing low voltage to the blower motor, but I don't know how to check that? Also it could be the blower motor being 'defective' - how do I know that or check that without simply trying a replacement part?

So please, if someone can tell me how to check the capacitor, how to check the motor, and what to do next, it would be very appreciated!

Answer 1

Check the high limit switch. If the furnace is allowed to continuously over heat, It's likely the high limit switch has burned out.

Make sure that the outlet vents are clean, and clear of debris. You need good air flow both in, and out of the system. also make sure the heat exchanger is clean, so it can transfer heat properly to the passing air.

If the thermostat has an option for Fan, switch it to Fan and make sure the fan starts. If the fan does not turn on, you should contact a trained HVAC tech. Capacitors can be deadly if not handled properly. You could accidentally discharge the capacitor into yourself, leading to electrocution. Or short the capacitor, leading to an exploding capacitor. In either case, it could be deadly.

Answer 2

I solved the premature shut-off problem by simply changing the air filter. The filter appeared clean but was completely clogged by white mineral deposits from my ultrasonic humidifier.

Answer 3

Usually the evaporator coil is obstructed/dirty, this reduces the air flow. Pleated air filters that are dirty, closed registers, a broken heat ex-changer or a bad limit.

Answer 4

If you have a central air "A" coil, it is likely now plugged with dirt which is causing the high temp limit shut down. You will need to call a furnace man to clean the dirt from the "A" coil.