Heating, Air Conditioning, Fridge, HVAC/Furnace not igniting


We went on vacation and turned off thermostat, upon return it was 50 degrees in home so turned on unit.  Hour or so later realized it was not warming up, fan running but only circulating air.  Tried resetting per instructions on inside of unit.  Read some similar issues online and tested two switches, the high limit switch and the bottom door switch and both showed continuity.  
Unit is a Heil DC90, model no. NTGM075EGA3, serial no. L993466132
During reading other answers, someone described the pressure switch and it having '-' hose connected to the blower and the '+' connected to the combustion box.  Well, mine has it in reverse.  (my brother-in-law) was over this spring to help diagnose why a/c unit froze up and ended up switching these hoses I guess) Well it was as simple as dirty filter on that one.  
Now, my wife has said she has used the furnace a couple of times this fall and felt as if it has worked even with the hoses switched.  Is that possible?  And has it  finally killed that pressure switch?  I tried putting the hoses back the right way and still cannot get pilot to kick in.
Final question...when I pulled the hose off the negative side of switch, a small plastic nail shaped device came out of hose (it has a small opening at nail head like the eye of a needle).  Does this stay on the negative side or does it transfer to the positive side with the hose?

The neg or low side of the switch has to go to the blower housing.  The other end goes to the combustion chamber side or is left open to the inside of the unit.  It is hard to say without proper tools if the switch is not working or there is a problem causing the switch not to close.  Most control boards have flashing lights on them that blink a code to indicate why it is not working.  You need to determine if the switch is opening and closing using a meter.  

I am not sure about the piece that came out of the hose.  I would have to look at it to see what it looks like and what the function of it is.


Heating, Air Conditioning, Fridge, HVAC

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Craig HVAC Expert


I have been in the HVAC field for the past 18 years. I can help with most HVAC questions. I work on commerical buildings for the most part, and have yet to find anything I could not troubleshoot and repair, when repairable. I work on small 1 ton units to a 2500 ton chiller. Troubleshoot air flow, elect, and control problems. I attend regular classes to keep up with the latest and greatest.


I work on commerical sites, hospitals, gov't buildings. I can troubleshoot just about anything in the HVAC business.

5 years trade school, VFD training classes, Liebert factory training, some York and Trane factory training.

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