HVAC Questions? Ask an HVAC Expert for Answers ASAP
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If the switch is not popped up then this isn't the one that is open. It is most likely the main limit that is open. This limit is an auto reset limit meaning when the temperature lowers to it's reset temp it should close and allow the furnace to come back on. Some times however if the sensor has a lot of dust and soot on it it will not reset so you have to pull the limit clean the sensor and try again.
This limit sense the temperature inside the furnace and if there is an airflow issue this limit will trip because the heat inside is building up to high. When the blower finally pushes this excess heat out and lowers the temperature inside to the limits reset temperature it closes and the burners are allowed to come back on.
Things that cause this airflow issue is a dirty filter, dirty coil, malfunctioning blower motor that isn't pushing air or enough air, blocked return runs, blocked supply runs, a combination of all these things, or simply a bad limit.
It is possible that the control board has an internal fault in it but there is a way to test for this.
First find the limit I am talking about. Here is a picture of what one looks like.
To bypass this limit and see if the limit is open or if the problem is actually in the control board you first turn power off to the furnace, take the two wires hooked to this limit off and hook the two wires together, turn power back on to the furnace.
If the furnace runs as normal with the limit bypassed then the limit is open and you need to pull it out and clean the sensor disc. If the furnace still gives you the 33 blink code with this limit bypassed then it's the control board (or one of the roll out limits is actually bad, you can test the roll out limits the same way by simply hooking the two wires together thus bypassing the. all of these limits are hooked in a loop so if any one of them is open the board will flash 33)
Do not use your furnace with any of these bypassed this is only for testings. Using your system with these limits not in use can cause your home to set on fire.
I have 2 furnaces of the same model for 2 different zones. I swapped out the limit switch and this has solved the problem in the nonfunctioning furnace. The limit switch that I removed and that is not working remains open when tested with an ohm meter. It is supposed to be a switch that automatically resets. Is there a reason why this switch may have gone bad that would indicate any other ptoential problems with the furnace? i.e. - could this be a secondary problem?
They will do this if it has tripped many times over the time of it's life or if the sensor gets real dirty. Try cleaning the sensor disc off and tapping it on the side of a wall or railing or something to see if you can jar it loose.
The sensor trips because of these airflow issues like I talked about and it is suppose to auto reset when the temperature drops back down to the reset temperature on the limit or if you cycle high voltage to the unit. If it continues to not reset then you will need a new one they are not very expensive.
Thanks Matthew. I will see if I can get it to reset, and if not, there is a Payne dealer about 10 miles from me.