RPJ Furnace won't ignite

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Old 12-06-07, 05:45 PM
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RPJ Furnace won't ignite

I have an RPG furnace where the burners do not ignite. The furnace is receiving a heat signal and the inducer motor is running. The inducer pressure sensor has 2 leads to C and NO. It also has two nipples with clear pvc lines-one runs to the inducer motor and the other runs to the sealed burner box. I suspected that the inducer sensor was not performing properly so i jumped the 2 leads and the burner fires and runs as expected. I also checked the switch on the sensor and it works correctly when enough vacuum is pulled to trigger the switch. When connected to the air lines, however, the diaphragm only moves a little and not enough to trigger the switch. When i disconnect the line going to the burner box, the pressure switch will sometimes function correctly. When it does function properly the burner fires and runs as expected. The minute I hook in the burner air line, however, the burner immediately cuts out. I seem to be getting enough negative pressure from the inducer motor, but the line going to the burner box does not seem to provide any pressure or vacuum. I checked the draft inducer exhaust line with the furnace running and there does not seem to be any obstruction in the PVC pipe: the air flow is quite strong but I did notice that it is very moist. The burner box exhaust also seems to have good air exhaust as well, though the air is dry and flows slightly less than the inducer draft line. How do i check the inducer pressure diaphragm to see if it is working correctly? how much vacuum should the inducer motor and burner box pull in order for the diaphragm to fully engage? Is there something else I should be investigating? Thanks.
 
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Old 12-06-07, 07:48 PM
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Is this a 90% furnace with two PVC pipe going outside?

What happens if you undo the pipe on the burner box? If it works, then your intake pipe is blocked some where.
 
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Old 12-07-07, 06:21 AM
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Yes it is a 90% with 2 PVC pipes going to the outside. The pipe connected to the burner box is sealed in place, so I didn't want to break the seal yet. As an alternative, I removed the burner box cover and the furnace fired correctly. I'll check to see if I have a blocked pipe tonight.
 
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Old 12-07-07, 10:36 AM
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The intake pipe shouldn't be glue in place... if so, or don't have a clamp around it, get a coupling, and cut the pipe to clear out the path, and when done, put the coupling in between the pipes.
 
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Old 12-07-07, 04:08 PM
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Verified that both PVC pipes going to the outside of the house are obstruction free. Everything is pointing to a bad diaphragm in the pressure sensor (the switch itself works fine). How can I check the pressure sensor before I spend $ 150 to replace it? What negative pressure should trigger the switch properly (the sensor has 1.78" W.CPF. inked on the housing). Thanks.
 
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Old 12-07-07, 05:07 PM
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You are going to need a water manometer to read pressure, an avg home owner isn't going to have one.... Unless you can make one?? I've never seen one made..
 
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