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I couldn't figure how this was working!?


randynavarro
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Today I found an old oil furnace. It appears to be original to the house (circa 1951).

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It was mounted on a slab and there was no ductwork attached except at the bottom.

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It appeared to be the supply plenum to the crawl space.

Each room had a return and a supply register and I could feel air returning and being supplied in the registers.

Where's the return plenum?

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Wait a minute; it's on a slab but has a crawlspace? No ductwork except at the bottom? What's that mean - does that mean that there were ducts in the crawlspace or in a slab or what? I must be missing something there.

OT - OF!!!

M.

Yeah, you're missing your brain! D'oh, did I say that?

Seriously, Jim's got it right. The furnace sits in what used to be the attached garage. The garage is now enclosed and finished. That plenum you see unde the furnace is a big elbow that just shoots right in to the crawl space.

But I'm still totally stumped. How is the air getting back to the furnace?

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Maybe it's just sucking in what's in the plenum, reheating it and shoving it out into the plenum again and the warm air is pushed out of the registers into the home. Either that, or you're blind and missed something. D'oh, did I say that?

ONE TEAM - ONE FIGHT!!!

Mike

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This is the configuration of the supply and return air Randy.

Shouldn't all oil furnaces have barometric dampers too? Oil is pretty rare in this neck of the woods.

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tis early on a Sunday morn and I have only had two cups so far. Please splain to me how you can have a supply and return in the same chamber? The supply on that furnace should be at the top rear. The view port is in the combustion chamber, which is the inside of the heat exchanger.

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I agree that you would typically see the discharge at the top or top rear of the unit but I assumed that Randy didn't see any supply duct work coming from those areas.

Based on that assumption I tried to picture how it might work. The duct at the bottom of the unit, that is visible, is the return air duct for sure. It looks like a duct in the front area (although a better shot of the blower compartment would be helpful).

When Randy checks in perhaps he'll have some more information.

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The furnace sits in what used to be the attached garage. The garage is now enclosed and finished. That plenum you see under the furnace is a big elbow that just shoots right into the crawl space.

How is the air getting back to the furnace?

I think Terance is right, but the pic is confusing. The arrow is pointing to the blower housing which blows up into the heat exchanger.

There must be ductwork in the wall behind the furnace for the heated air.

The elbow under the unit is return air. The blower usually draws air in thru its sides from that portion of the unit, then forces the air thru the heat exchanger above.

If the heat blows out the bottom, the blower will normally be above the burner.

Good question about the Barometric damper. I wonder about all the times that unit has been serviced and no service technician has felt it was needed?

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There is absolutely nothing else attached to that furnace. Nothing behind, on top, or on the sides. I could easily see all sides of the cabinet.

I don't have good pics of this but no matter -- nothing else is attached--I'm positive.

I also don't have a better pic of the inside of the cabinet. That green portion of metal Terence arrowed in his diagram is the blower motor housing.

Assuming the air is returning from the elbow at the bottom --that makes sense--to where is the warm air being sent?

BTW, the tank was out of oil so no heat. Only blower motor function.

And yes, the flue should have a barometric damper. (It should also be sloped up.)

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The red arrow isn't pointing to the blower housing, it is pointing to what appears to be duct work in front of the blower.

It looks like a counter-flow furnace with some convoluted type of airflow arrangement. The return air enters as shown, the blower shoots it up, over the heat exchanger, and then the air makes a 180 to head back down to the supply air distribution. The return air and supply air enter through the slab - much as the way a rooftop unit is configured when sitting on its curb.

The older I get, and the more I see, never say never holds a lot of truth.

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