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Unusual Suburban NT-SP30 problem


AZCowboy
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I have a Suburban Model NT-30SP furnace installed in my 5th-wheel. This unit is way out of warranty. It is 
controlled using a LUX programmable thermostat. I am experiencing an intermittent problem when the furnace 
starts the ignition sequence. Every once in awhile here is what happens: 
The thermostat energizes signalling the furnace to ignite. There is a delay of several seconds and normally 
the fan starts; however, every once in awhile there is a "POP" immediately before the fan starts. From that 
point on everything appears to be normal. This does not happen every time the furnace starts and I have not 
noticed it happening two or more time in a row. This problem is not recreate-able at will.

Any assistance you might be able to offer me would be greatly appreciated.
Edited by AZCowboy
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I downloaded and read all of the pertinent information and in theory what is happening can not happen. The "pop" definitely appears to be gas related. The entire propane system has been leak/pressure tested and is secure. IF the problem was re-creatable at will, a solution would probably be found; however, it is not. Subuurban will not offer any assistance because of "liability" concerns. As typical, take your RV to an authorized service center; which is impossible. I guess I am the only one since the dawn of creation to experience this problem. Thank you for your help.

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How old is this unit. We need to know what what type power board is on it. And the age should give us some idea.

  Also try turning off all gas appliances for 10 hours or so. And at the same time turn all propane bottles off. Then try to ignite a burner on the range and see if there is any propane for the flame. That flame must be burning immediately.

 

 Thinking,    Vern

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Vern - The unit is in a 1989 Komfort 32' 5th wheel. However, this RV was not really put into use until August of 2001. I was "on the road" for 12 years with WorKamper so the furnace was not used very much until I had to hang up the keys. Don't believe that it doesn't get cold in SE Arizona. The furnace has gotten a workout over the past 6 years. The only thing replaced on the furnace was the fan motor about 4 years ago.

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The normal process is for the fan to start and run for a time to purge any remainder of gas from the combustion chamber before the ignition sequence starts. The same motor turns both the blower for distributing heat and the one supplying combustion air. If the pop is happening before the fan starts, that would indicate a failure of the purge cycle to take place. This leads me to suspect the igniter circuit board. It normally has that delay built into it. 

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