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Old 12-12-2011, 05:26 PM   #1
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Payne Furnace Pains - Updraft/downdraft parts swap??

Payne 100FAU-12 100k BTU natural gas furnace (Payne #1)
Initial Problem: Furnace gas supply to burners in runaway condition (would not shut down) causing a strong "burnt" odor through house. Thermostat set to coldest temp had no effect, blower still working. Shut down furnace by turning off gas supply valve.
Gas utility called to inspect. When lighting pilot, discovered gas to burners still flowing full so gas man disconnected supply and left, saying furnace probably should be replaced. Subsequent tests indicates that transformer works but the limit switch is likely toast (no continuity between terminals when cold).

Due to severe economic situation we can't afford a new furnace or even a service call. Found a used 100k furnace that had been replaced only for increased efficiency for $50, so sent a friend to pick it up. Call this Payne #2. Upon delivery it was discovered that while it appears to be in excellent condition, it is a downdraft design and the failed 100FAU-12 is an updraft. Single story home was built in 1966 and has only 2 floor located cold air returns.

The failed unit (Payne #1) is so old that I cannot locate manuals or schematic for it. It uses a Unitrol 12005VER gas valve with only 2 electrical connections, and one inline thermal limit, that in my limited experience looks similar to a valve for a hot water heater and requires a pilot to be lit all the time. One other thermal switch is adjustable as to air temp before turning fan on. I assume that this valve is stuck in the open position as gas flowed to burners when the knob was turned to "Pilot" and depressed. This furnace uses 2 individual burners to heat the chamber whiich does not have any visible cracks in spite of possible overheating.

Recently purchased used unit (Payne #2) is Model # 396GAW has a much more complex control scheme using a door switch, control board with relay, 2 limit switches and a pilot light ignitor. The somewhat larget gas valve has 5 electrical terminals vs. the 2 on the original. I also have a readable schematic on the inside of the service door. This unit uses 4 burners.

Realizing that my initial thoughts of swapping the furnaces won't work, I'm left with the idea of swapping the controls and gas valve from #2 into #1. While it may be possible to replace the failed limit swith and the stuck valve in #1, I just don't have the finances to do that. I will need to drill a few holes for controls mounting and for the additional limit switch,

Are there any major problems that I'm not seeing in swapping out the valve and controls?

Adding to the urgency is the fact that there are 7 adults and 2 children currently living in this small home and it's getting rather cold here (Northern CA). We do have a wood stove that is able to keep the place warm enough for sleeping but that's going to get worse as winter sets in.

Thanks in advance for your assistance, experience and suggestions,



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Old 12-12-2011, 08:11 PM   #2
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