Old Payne furnace - updraft to downdraft parts swap


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Old 12-12-11, 01:57 PM
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Old Payne furnace - updraft to 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 bore (valve stuck?) 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. http://images.ibsrv.net/ibsrv/res/sr...ilies/wall.gif 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. In my limited experience the valve 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 gas 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 which does not have any visible cracks in spite of possible overheating.

Recently purchased used unit (Payne #2) is Model # 396GAW has a more complex (safer?) control scheme using a door switch, control board with relay, 2 limit switches and a pilot light spark ignition. The somewhat larger 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 switch 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,

"Stick"
 
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Old 12-12-11, 02:24 PM
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NO you can not do this. Sure you technically could, but NO it would not be safe. NO ONE'S life is worth any amount of money, do it correctly by replacing with another upflow furnace.
 
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Old 12-12-11, 03:11 PM
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Sorry. I would recommend against your trying to doctor up the old furnace with an altogether different ignition system.
 
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Old 12-12-11, 05:38 PM
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Thanks for the responses.

I understand your concerns and realize that it's not possible to convince you that I'm not a tongue-chewin' babblin' idiot with a hammer and 16" pipe wrench who has an ambulance chasing attorney for a brother in law, but I'm needing to proceed in some direction in order to get heat into this home pronto. Under most circumstances I would simply go for total replacement too. Divorce, unemployment and foreclosure put me in a position to crash on my friend's couch and I'm not the only one he's extended his kindness to, and it's just not financially feasible.

I have good familiarity and practice with industrial control systems, flame safeguard, electrically operated valves, etc... and I don't see much difference between replacing the failed 2 wire valve with a new model replacement that likely has has superseded numerous times and revisions from the original and requires some additional changes (i.e.location of pilot port) or components (pilot/TC assembly); and installing a used valve, pilot system and controls system from a system that is the exact same size from the same manufacturer, and has more safety features. At first sight, one possible difference is the flow rate of the valve since the used one (#2) supports 4 burners as opposed to the failed one that fed 2 burners, but that seems to be it.

In any case, I'm looking at needing to change out the controls (door switch, transformer, 2 inline thermal switches, pilot and pilot ignition, and valve) or have no heat as there simply isn't money for a new furnace.

Any tips at doing this or additional areas of concern would be appreciated greatly.

Let me be the first to wish you all Happy Holidays.

"Stick"
 
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Old 12-12-11, 07:21 PM
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Sorry, you're on your own as far as I'm concerned.
 
 

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