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drpepper808's avatar
drpepper808
Explorer
Jan 29, 2014

Problem with Suburban SF furnace

Hello everyone, I was hoping that maybe there would be an expert on this forum that could help me.
Currently I am living in my 1975 GMC Motorhome on a six month contract on Charleston SC.
During last night Ice store, my Suburban SF furnace (199?) started to bang and shut off.
The problem was the outside Propane regulator froze up, so I replaced it with my backup, retested the pressure and everything looks good, so I thought.
During the testing pre/post regulator replacement I noticed something with the furnace, this may have been doing it for a while, and I never noticed, but it has me worried.
During the Heat cycle, the Fan turns on, the Igniter triggers, and the platum heats up like normal.
But before the Thermostat triggers the Off Cycle, I notice a “Click” and the gas turning off(I Think), and the air from the vent goes from “VERY HOT” to “HOT”, the it clicks again and the vent puts out “VERY HOT”.
Once the thermostat triggers the Off Cycle, it shuts down normally.
Is that normal???

3 Replies

  • You have indicated why the high temp limit switch is cycling........

    "The person that replace the 1975 unit with this unit really did not know what he was doing. the Vent are to small."

    Ducting too small for the size of furnace will cause excessive heating.....as you have found out (VERY HOT..then HOT air discharge)
    Excessive heat can cause the combustion chamber to crack.......a cracked combustion chamber is a leak...a leak that can allow CO into the inside of RV. CO can KILL.

    Need to increase duct work......add more ducts
    OR change main gas orifice to a smaller size

    Make sure you do NOT have anything blocking air return or in close contact with furnace box.
    Make sure your CO Detector is functioning properly

    Address this problem ASAP!
  • Well that's good. The person that replace the 1975 unit with this unit really did not know what he was doing. the Vent are to small.
    The Very Host is VERY hot. I can not keep my hand on the vent, so I bet it is exceeding 120dgf. It was my plan to re-do last summer but.... lol
    Thanks for your help
    Phil



    wa8yxm wrote:
    There are three Thermostats (Well two and a flame sensor). You know how the flame sensor works, and you know how the indoor thermostat works.

    The one that is bothering you is the high temp cut out.. If the burner is making HOT faster than the blower is blowing cooling air over the heat exchanger... This Thermostat cuts off the burner to keep it from going into meltdown (A very bad thing) and it is working properly.

    Several things can cause this to happen... Some of them are problems, (IE: blockage on one or more air ducts) one is simply the battery running down a bit and thus the blower slowing slightly... But it may well be working as designed.
  • wa8yxm's avatar
    wa8yxm
    Explorer III
    There are three Thermostats (Well two and a flame sensor). You know how the flame sensor works, and you know how the indoor thermostat works.

    The one that is bothering you is the high temp cut out.. If the burner is making HOT faster than the blower is blowing cooling air over the heat exchanger... This Thermostat cuts off the burner to keep it from going into meltdown (A very bad thing) and it is working properly.

    Several things can cause this to happen... Some of them are problems, (IE: blockage on one or more air ducts) one is simply the battery running down a bit and thus the blower slowing slightly... But it may well be working as designed.

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