# Carrier Furnace Error Code 33



## yuri (Nov 29, 2008)

Lack of air flow causing the limit switch to trip. That is a reverse flow furnace I believe. If it is sitting on top of a A/C coil then that coil may be dirty and slowing down the airflow. Make sure you do not use one of those very tight 3M filters as they slow down the air too much. Make sure ALL your vents are open and clean. A MERV 7 or 8 filter from HDepot is good enough and change it MONTHLY or pay more for gas/stress the furnace.


----------



## s6skuzy01 (Oct 12, 2008)

Thanks, we went to the cheaper filters after the first call out to the HVAC mechanic. We're gonna him him another call tomorrow and see, but it seems like once the filter is changed the problem goes away till the next time it gets dirty. Seems the filter fixes the issue but not for long. Could the ac coil still cause that?

All vents are open and unblocked also...


----------



## yuri (Nov 29, 2008)

YES. Just like a blocked radiator causes your car to overheat.


----------



## s6skuzy01 (Oct 12, 2008)

Just checked the AC coil and there was very little dust on it...


----------



## yuri (Nov 29, 2008)

Get your tech to check the temp rise thru the unit and see if it is within the specs on the rating plate. He can also do a meter clock test with the gas meter to make sure it is not overfired or check the manifold pressure with a manometer. If this is all foreign to him then you need a more experienced tech.

Good Luck


----------



## s6skuzy01 (Oct 12, 2008)

*Tech is here*

Did a temp rise test and found:

Without filter - 130
With old filter - 155 and temp sensor tripped

He adjusted the gas and the temp range was withing specs. When they replaced the limit switch, they put one in that allowed for a larger temp range, so they didnt think that was the issue. Also said to use a different filter. Our current filter is a MERV 6.

We'll see if this fixes it...


----------



## beenthere (Oct 11, 2008)

The temp rise is 155°, or the discharge air temp is 155°.

If the rise(difference between return air temp, and discharge air temp) is 130 or 155, something is still majorly wrong.

You want the proper rated limit switch.
And teh problem corrected, not covered up with an unsafe band aid.


----------



## s6skuzy01 (Oct 12, 2008)

Sorry, the discharge temp was 155...


----------



## beenthere (Oct 11, 2008)

If the return temp was 70, that still gives you an 85° rise.
Too much for many furnaces.
Your furnace will have am allowable temp rise rating inside the burner panel.
Look at it, and see what its limits are.

If 85 is above its limit. Then its not fixed right.
And will have a shortened life span.

PS: An A/C coil can look clean on the surface, but be dirty in between the fins, and restrict air flow.


----------

