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Sorry for the slight delay, I needed to get the actual model # XXXXX inside the unit.
It is NDGE 125PH02
I had a similar problem years ago and called out a repair service. Ultra bad experiance. He diagnosed the problem as the burner limit switch, replaced with a universal 120/240 and didn't break the bridge to convert to 120 and blew componets in the unit. Replaced all blown parts to then diagnose the problem as the draft inducer fan.
The unit appeared to work normally for about a year and the same symtoms reappeared.
I found a replacement fan out of state in business travel and purchased it and stlll have it new in the box, The fan at this time that appeared to cause the problem was removed and inspected. Paint from high heater box temps had flaked off and clogged the vacuum port. I was able to reinstall the same fan and the unit appeared to work normally until late last season and the beginning of this. I do not have a manometer, nor did the orgininal technician use one.
Faster response this time. I am a retired plumber/pipefitter and taught apprenticeship for many years including HVAC so I some knowledge and a reasonable level of common sense. I did your experiment prior to my questions.
Yes, if vacuum motor is jumped the system operates normally. If I remove the jumper, the burner shuts down and the unit distribution fan continues to run. The burner will not re-ignite until I either re-jump the vacuum switch or reconnect the wiring to switch itself. Again, once attached to the switch the unit runs a significant time before starting periods of short cycling until heat demands have been reached. I'm puzzled why the vacuum switch holds for so long, the opens intermentantly. The fan is also pulling enough vacuum to actuate the switch. Why would it drop off after such a signicant timeframe?
I have a small air compressor and blew air through the fan vacuum port prior to startup and it was clear. I also blew air accross the expossed external fan blades to see if the fan turned freely without power. The fan spin like a top, though exact RPM is unknown.
It could be NOGE 125PH02
The vent is metal. 4" outlet increasing to 5-6 inch sheet metal. Approximately 3 to 4 ft of 45 degree rise, then vertical through roof. There is no flue damper or other controls on the exhaust other than the flue exhaust fan in case this extra unasked for information helps.
Perhaps I can borrow a manometer. Did you say it should pull .20" of water column?
Though I'm sure you can not advise this, I'm thinking of operating the system temporarily in the jumped position until I can locate a manometer. The unit is located in my garage, not in the living space and I can keep an eye on the inducer fan to insure it continues to operate.
What about an adjustable vacuum switch available through WW Grainger. If all else fails, it could be set to a slightly lower closed setting and this might hold throughout the cycle.
Just some of my own thoughts. Perplexed!
Thank you for your time and advise!