Heating, Air Conditioning, Fridge, HVAC/carrier 9600 series


QUESTION: asking for a friend that is not hip on computers , non-technical and short on funds .
(what are friends for if they can't abuse you once in a while)
i am not familiar with this model but fairly competent around such equipment .
technical paperwork (not the owners manual !) on these is about as hard to find as snake lips .

carrier 9600 series :
flame roll-out switch opening (code 33)
the inducer and main blower are coming up to speed , sound healthy
intake/exhaust of the inducer are clear . outlets and returns of main blower are clear
pulled/thermally checked the roll out switch ... opens at about 230F
the burners ignite reliably and have a blue flame

my friend indicated that the heat exchanger was replaced several years ago sometime after the unit was installed in 2000 (again , grain of salt - non technical type person - show me the paperwork) .

he has also said that this problem has happened in the past ...
the tech shows up and in a very short period of time the problem is fixed .
this has happened "every four years or so" .

the current problem seems to be due to a heat rise in the burner box (measured) over time , not a true "back-flame" . this can happen during a long run or some time after a "short run" .
the burners do not cycle on/off or change to a low flame state during a heat run .
lacking information about the "flame cycling" (if indeed this unit is designed that way) during a "call for heat" is this a normal condition ?

in thinking about the tech previously fixing the problem in a "short period of time" , plus the fact that there is another limit switch wired in series (located on the left side of the furnace , midway down) ...
would this be the temp limit for the exchanger and thus be the part that cycles the flame during a heat run ?
could this part possibly be what the tech changed out so quickly ?
if this switch is the cause , could you provide a part number ?

as i say , i am not familiar with this carrier line and lack a real manual ... kind of frustrating .
i would like to be able to tell my friend the "good news" as well as prepare him for the bad news (in the event of the heat exchangers being "plugged" or "bad order") .

thanks in advance for any information that you may be able to provide .

ANSWER: Larry,
Model numbers etc. on furnaces are changed every year, some two or three times a year, every time something is changed on the assembly line maybe a new screw they change the model or series number, anyway I don't have a manual on your furnace and I can't see it.
A "Roll Out"or limit switch could be any where with in the 24 volt circuit, most limit switches are set for 200F.
Any thing as simple as changing the air filter can solve the problem.
Depending on the BTU input your furnace needs a certain amount of air passing through the system, duct, register, grill, filter sizes?
Also check the blower compartment door switch.

---------- FOLLOW-UP ----------

QUESTION: yep ... i understand about the model changes for trivial reasons as opposed to a manual with a fist-full of errata/addendum/change sheets stuffed in it .
the army was good for the latter in their TMs/FMs .

i will try to figure out which model it is ...
i could find no clear delineation on the furnace , but i do know what model it is not by the lack of a limit switch on the main blower shroud/housing .
the other models in the owners manual do not have this limit switch .
the manual i downloaded shows only three models :
58MVP (this is the above mentioned model)
(the manual (still) at my friends place lists a couple more such as 58MSA , etc .)

during the original analysis , the blower filter was changed and the airflow is unobstructed ...
checked/pulled registers and return grill .
there are no little kids around so i doubt that anything like a stuffed toy or something similar is blocking any of the runs .
i "thumped" the runs and found no dead spots .
figuring that odd things happen and the possibility of an exchanger that is just marginal , i bumped up the heat blower speed to "medium high" (selectable by changing/swapping the motor leads on the controller board . there are two "real" spades : cool and heat , the others are dummies for the unused motor leads) .
i also pulled the inspection plate of the AC exchanger compartment mounted on top of the furnace (just about as clean as a pin in there) and left this off during a heat run ...
man , it is moving some air !
however the slow heat rise in the "fire box" persisted .

until such time as i can get back to my friend's place (25 miles from my place) i hope this additional information may be helpful .
if you feel that it would be better to post this in the "carrier" section , that is ok .
i have been engineering (broadcast/transmitters) and doing industrial/consumer electrical/electronic repair for 30 years and i understand about "asking others" ...
whatever it takes to get the job done is what it is about .

thanks again

Start at the thermostat with the wire connected to the W post, trace the wire all the way to and through the furnace and all the controls on it's way to the gas valve, everything on this circuit is 24 volts, take note of all the limit switches along the way, on a limit switch you can bypass the limit by jumping the 2  connections, most all limit switches have the temperature, name and any model number on the switch it self.
If you know when your burner goes off prematurely you can tell if that's the bad limit if the burner stays on.
When you jump a limit you don't have the safety of the furnace over heating and shutting off. It could be a faulty limit or it could be an actual furnace over heating problem, first you need to determine the area where the limit is, maybe some place your not aware of?
Back in past years Carrier was made by DPB, the D stood for Day & Night furnaces, The P is for Payne furnace, and the B is for Bryent furnaces,  all four brands were made on the same assembly line and are identical except for the color of paint.
When the manufacture ran off a few thousand furnaces on the assembly line he used what ever limit switches he could get the best deal on, You could check a lot of furnaces like yours before you could find an identical control such as a limit switch.

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Jim Barnhart


Fifty + years conditioning the air.


Answer questions about , residential and commercial. Answer questions about sheet metal fabrication. Fifty years plus experience. No answers for oil equipment, No answers for kitchen appliances, No answers for laundry appliances, I don't specialize in one particular area of the HVAC area, I'm more of a General Practitioner, Limited in refrigeration/

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