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Discussion Starter · #1 ·
Ok, so my furnace for the upstairs of the house has been acting out the last few weeks. We only moved in in October so don't really know much about the set up, but 2 HE furnaces, no idea on the maintenance history and the main floor one runs very smooth. Its an ICP N9MP2100F14C2 furnace that is around 10 years old.

Anyway, upstairs one is overheating very quickly, will fire up and run for 1 minute or so then the limit switch shuts her down, I get 4 flashes on the Board (Limit switch open) then 30 seconds later it resets and off it goes again, it does eventually reach the temp that the thermostat is calling for but only after running numerous short cycles.

Here is what I have thrown at it so far (in my normal fashion), starting with cheapest first:

- New air filter (not a super expensive one which I find tend to restrict airflow)
- New flame sensor
- New ignitor
- New limit switch
- 2 x new roll out switches
- New pressure switch
- New thermostat
- New main control board
- New draft inducer blower

Now I can feel that the furnace is definitely overheating the main vent up to the air outlets upstairs, just above the furnace is very hot to the touch and the vent pipe is warmer on this one too (when compared to the other one that works fine)

I have checked the venting for blockages, nothing there, checked that the air intakes upstairs are all uncovered and even tried running with the bottom door off/no filter to increase the airflow.

Here are my last 2 options for what I think is wrong:
- damaged heat exchanger
- Fan motor on main fan at bottom, but this does run and no funny noises or funny smells from it

Anyone else have any ideas before a reach my last resort and call out a tech :wink2:

I know I could have had one out and probably had this fixed for less than the all the parts I have bought so far but hey, I'm self isolating and what else is a man supposed to do...
 

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To get overheating after only one minute, the blower motor is either not turning on or running far too slow.

Here is what I have thrown at it so far (in my normal fashion), starting with cheapest first:

- New air filter (not a super expensive one which I find tend to restrict airflow)
- New flame sensor
- New ignitor
- New limit switch
- 2 x new roll out switches
- New pressure switch
- New thermostat
- New main control board
- New draft inducer blower
You don't troubleshoot by changing out parts especially those that have nothing to do with the problem.

That must be hundreds of dollars worth of parts - the money could have been spent on a service call and repair and at least you would have had a working furnace by now.
 
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Discussion Starter · #3 ·
To get overheating after only one minute, the blower motor is either not turning on or running far too slow.



You don't troubleshoot by changing out parts especially those that have nothing to do with the problem.

That must be hundreds of dollars worth of parts - the money could have been spent on a service call and repair and at least you would have had a working furnace by now.
Thanks for you help 12345, I do acknowledge that fact in my last sentence and thanks for you confirmation that it is the blower motor that is the likely culprit.
 

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Did you replace all those parts chasing the limit problem?!?!
Please, NEVER just start changing parts. As you’ve found out it quickly lightens your wallet and leaves you with equipment that’s still broken.
DO NOT just change the blower. Test it and troubleshoot the issue with your meter.
 

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Thanks for you help 12345, I do acknowledge that fact in my last sentence and thanks for you confirmation that it is the blower motor that is the likely culprit.
I never said that the blower motor is the likely culprit, I meant blower likely isn't coming on and or is but is not getting up to speed.

Please don't change the motor without verifying it's bad first!
 

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Discussion Starter · #6 ·
Thanks Roughneck, appreciate the feedback. I will get a buddy of mine who is dab hand with a meter to come take a look at the blower motor.

While I have been looking at this it seems my blower on the main floor furnace is now on constantly even when the thermostat is not calling for heat. I checked the fuse on the control board, removed the thermostat from the wall to see if that was the issue and gave the relay and limit switch a quick tap but it still runs, no codes showing on the board..
 

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Discussion Starter · #7 ·
I never said that the blower motor is the likely culprit, I meant blower likely isn't coming on and or is but is not getting up to speed.

Please don't change the motor without verifying it's bad first!
I'm with you, I did not mean I was going to switch it out right away (although I guess with how I have thrown parts at it so far that might think this was my next step) I merely meant that that is where I will focus my attention.

The blower is coming on btw when heat is called for
 

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The first step is to make sure the high limit is actually opening by checking with a meter.
 

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Any updates with your equipment?
We don’t want you to buy anymore unnecessary parts. Any questions on diagnosing the problems?
 

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Discussion Starter · #12 ·
Lol, no worries the parts store is closed today:biggrin2:

Just waiting on my buddy with his meter to come give me hand

Thanks
 

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Discussion Starter · #14 ·
No not as yet, my friend is self isolating as he was not feeling to good, hopefully it won't be too much longer before he is able to come round with his meter and take a look at the main fan.
 
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