Trane XE78 "pressure switch problem"

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  #1  
Old 10-14-03, 04:40 PM
deturney
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Trane XE78 "pressure switch problem"

The furnace has been running fine recently, but only used occasionally due to fluctuating weather. Today I turned it on and it did not start to blow hot air as usual so I investigated and found the circuit board blinking 3 times continually indicating a problem with the pressure switch. I located the switch and removed it. When I suck on the inlet to it, it closes easily with very little pressure. What pressure (vacuum?) is this monitoring? Could a dirty filter be the culprit? I have yet to figure out where the filter housing is since this thing is in a very cramped crawl space and there is no obvious filter location. Any advice as to my next move would be appreciated.
Dave
 
  #2  
Old 10-14-03, 05:12 PM
bigjohn
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The pressure switch has 2 positions. We could call them "ready to start" and "combustion air is available". When your thermostat calls for heat, the first thing that happens is the combustion air fan starts. This switches the pressure switch proving that there is combustion air flow. At the end of the heatng cycle when the combustion air fan shuts off, the switch returns to the ready to start position. The ready to start position is necessary in case the switch were to stick in the air available position which could potentially give a false indication that the combustion air flow is ok when it is not. So your problem is either a malfunctioning or marginal switch or something is wrong with the combustion air flow. Can you tell if the comustion air motor is running? Sometimes it is, but the little blower wheel is loose on the motor shaft or it has broken apart. Also, check the flue for blockage. If all that is ok, you may have to replace the switch. The air filter doesn't involve the pressure switch. Maybe the filter is inside the return air grille?
 
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Old 10-14-03, 05:23 PM
deturney
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Trane XE78 "pressure switch problem"

BigJohn,
Thanks for the reply. The combustion air motor is functioning, I will have to check further to determine if the wheel is loose on the shaft, or if the flue is blocked. Unfortunately I can't get back to it until tomorrow, late afternoon. What is the best way to check for a blocked flue? I assume I need to remove the motor and make sure the wheel (fan?) is tight on the shaft.
Dave
 
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Old 10-14-03, 05:56 PM
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could very possibly be

rust or debris in the inducer housing clogging the air passageway. can you blow into the tubing back into the furnace?
 
  #5  
Old 10-14-03, 06:15 PM
deturney
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yes.......

I will also try that, the tubing seems clear where it protrudes and attaches to the switch but I suppose it could be blocked at the other end where it is not visible.
Thanks
 
  #6  
Old 10-14-03, 07:06 PM
H
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The culprit

a common one which is the more likely is theres a bird that thinks this would make a great nest....OR bees think the same thing....one way to tell without going outside is to disconnect the exhaust at the furnace and with the gas **** off see if the switch closes with a meter...if so, go looking for the blockage. does this one by chance have two pancake style pressure switches on the top left corner?
 
  #7  
Old 10-14-03, 08:00 PM
deturney
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No....

The unit is mounted on it's side in a basement/crawlspace. After removing the access panels the top of the unit is to my left and the single pressure switch mounts on what is now the bottom (left side) a little beyond half-way along the bottom as it sits. Only one switch as far as I can tell. I will know more tomorrow when I can get back to it.
Thanks
 
  #8  
Old 10-15-03, 04:42 PM
deturney
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It now works!

Recap: furnace stops working, combustion motor spins-up and I can hear airflow but mains don't light. I check furnace and the board LED indicates a problem with the pressure sensor. Three helpful people give me many things to check: motor spins but not fan, blocked flue, blocked tube leading to pressure sensor, bad pressure sensor. One thing I feared was a bad board-also a possibility, right? So, I have the pressure sensor out and take it to work today and test it with a continuity tester and it seems to work just fine. I leave work early to take advantage of the daylight and immediately climb up the roof and check the chimney (which is the flue vent exit) and it is clear as far as I can see in broad daylight. Back to the furnace, I do the easiest thing first and blow through the pressure sensor tube back into the furnace and there appears to be no obstruction at all. I decide I need to have another check point before tearing anything else apart, so I reattach the pressure sensor and it's wires and tube. When I try the furnace this time it fires up and runs just fine. It has now been through 4 cycles and is still working just fine. All I did was remove and test the pressure sensor and blow back through the tube that attaches to it into the furnace, then hooked it up and the furnace is now working. So, I suppose it could malfunction again. The board or sensor itself could be intermittently malfunctioning, or the tube may get blocked again. This crawlspace/basement is very damp this year due to lots of rain. Any thoughts guys? By the way, Thanks again to all three of you who took the time to offer assistance.
Dave
 
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Old 10-15-03, 06:38 PM
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sometimes.....

they just stick, or there is water in the tube, and leaving it loose drained it, i doubt it is the board.
 
  #10  
Old 10-15-03, 07:21 PM
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ventilation

This is key in a crawl space, humidity is a killer on any electrical component. It sounds like the condensate drain was blocked. Next time hook a vac on the tube that goes to the combustion chamber....there just might be a flake in there that blocks the air flow that sets the sensor off. This is a differential pressure switch....it may have just hung up, hit it with WD 40. Hope all works out fine for you....you'll at least know where to go next time it stops.
 
  #11  
Old 10-15-03, 07:32 PM
deturney
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The furnace is.....

around 10 years old. Most electronic components fail early in their life don't they? It is cycling fine now and I have a greater understanding of the logical sequence required to make this thing operate. I also have a lot more confidence in my understanding of the repair process thanks to you guys!
 
 

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