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Originally Posted by pacnw

the premise is that the 3.5"NG setting is providing MORE gas than the 11"LP setting and causing soot production?

Volume for volume, LP contains about 2.44 times more btu content than NG, so the same or similar volume of LP through the NG orifices, even at 3.5", would result in an input rate much higher than the furnace's input rating. It would definitely be over-fired and burning rich. At 11" the the problem would be worse. I've seen both cases, 3.5" and 11" without the orifice replacement, and they're both bad situations. Some Carrier units use 3.5" for LP, with only an orifice change-out required for conversion. The LP orifices are smaller even though the manifold pressure isn't changed. HTH.

i ran the IDM and blew most out of the house... but the first time i didn't do that and it was a huge mess in the attic.

Im betting you will be back next year to clean it again. Had a friend that blew a furnace out in his bosses house that had white carpet,
still dont know how he didnt get fired soot everywhere

Originally Posted by pacnw

the premiss is that the 3.5"NG setting is providing MORE gas than the 11"LP setting and causing soot production?

Keep in mind the LP orifices are considerably smaller than NG.

i guess you could take the H.E. out... sounds like a lot of work. these furnaces were working great after i got done with them, though time will :-/.

i'd think it is incomplete combustion because the velocity of the gas pulls the air into the venturi, but with out that velocity (low gas pressure) not enough air is mixed before it burns hence the yellow flame (and soot). so it is actually running rich despite less gas being output.

Originally Posted by hvacrmedic

Incomplete combustion keeps the temp down.

the premiss is that the 3.5"NG setting is providing MORE gas than the 11"LP setting and causing soot production?

I take the H.E. outside the house and flush it with water.

Originally Posted by 54regcab

I'm surprised that the furnace wouldn't immediately cycle on limit due to the extreme over firing.

Incomplete combustion keeps the temp down.

I'm surprised that the furnace wouldn't immediately cycle on limit due to the extreme over firing.

MUST READ: LP run on Nat. Gas config

I have dealt with this twice this year. One unit "we" installed, another some other jake leg installed.

If you run LP on a furnace set up for natural gas it will work, for a little while, apparently.

Soot builds up and eventually rolls out on every ignition.

Then, you can not simply convert the furnace, you must also blow out the heat exchanger.

The way I learned, after the first time, was to disconnect the igniter, call for heat, and blow nitrogen through every tube.
p.s. turn the gas off

The draft motor will blow a good portion of the soot out of the house.

Now remove the induced draft motor, clean off the squirrel cage and manifold. While the IDM is off, also blow backwards through the heat exhanger. If you put a blanket over the furnace while doing this, you'll prevent the big stuff from going EVERYWHERE.

After this, no roll outs after the conversion. I tried to get away with not doing the H.E. cleanse during the conversion and had to come back... don't do that to yourself.