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Old 07-04-2006, 03:04 PM   #1
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New Suburban furnace intake question

We purchased an ST-20 Suburban (the space saving model) furnace to replace our old unit. They both vent to the outside with the 2 small holes. One is the exhaust and one is the air intake - isn't that odd.. to suck air right beside the gas exit? Shouldn't the furnace be sucking in air from inside the coach?

I must be missing something. Any input is greatly appreciated!!
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Old 07-04-2006, 03:23 PM   #2
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The air from the two holes to the outside is for the combustion chamber which needs to be isolated from the trailer air.

The return air for heating inside the trailer comes in through the sides of the furnace from under the cabinet openings.
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Old 07-04-2006, 03:26 PM   #3
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Combustion air not the hot air that it blows around. Two separate sides , trouble is when combustion burner can rust out and then they mix.
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Old 07-04-2006, 07:25 PM   #4
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Quote:
Originally Posted by sequoiacoast
We purchased an ST-20 Suburban (the space saving model) furnace to replace our old unit. They both vent to the outside with the 2 small holes. One is the exhaust and one is the air intake - isn't that odd.. to suck air right beside the gas exit? Shouldn't the furnace be sucking in air from inside the coach?

I must be missing something. Any input is greatly appreciated!!
The two holes on the Suburban furnace are the intake and exhaust for the combustion chamber of the furnace only. They have nothing to do with the hot air that the blower motor circulates inside the trailer. The burner heats a plenum where the circulation air flows thru, getting hot and circulating thru the trailer. They are totally separate systems and air pathways so there is never any combustion exhaust flowing into the trailer.
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Old 07-05-2006, 12:01 PM   #5
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Thank you all!!! Now I feel much better and know a little more about something else I never knew I'd want to know !!
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Old 07-06-2006, 08:47 PM   #6
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The old furnace had a contoured back so that it followed the curve of the airstream. The new furnace has a straight back with pipes that need to make a positive seal with the exterior skin.
But the skin has a contour and won't allow a flat and straight connection with the intake, specifically. We have built a plywood stand to elevate the furnace to match up with the existing holes... Can we put our stand at an incline, having the furnace off level, so that the intake can attatch square with the ext. skin? The base that the furnace sits on would be flat but inverted, and everything screwed down? PROBLEMS???????

Also, Do most have grates for their air return. It seems the only mention in the instructions lets the air come from inside the cabinet - without a grate through the cabinet. Does Suburban need special venting kits to retrofit with the old vents? (I have a 1968)

Thank you!!!
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Old 07-06-2006, 09:46 PM   #7
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The furnace box should not touch the exterior skin. Vibration will cause marks to show through to the outside and may even eventually wear through the aluminum. The intake/exhaust pipes have gaskets that allow them to be adjusted to fit to the outside of the skin and still slide over the furnace pipes to make a seal. The back edge of the furnace box is straight and will not conform to the exterior skin shape. There should be sufficient room in the cabinet to allow the furnace to sit flat. Some early '70 models had a sheet metal base for the furnace that was tapered a little to allow the back of the box to fit more closely to the wall, but that was mostly because of the limited space in the cabinet. If you have floor ducting, you will have to modify the base ( or make a new one ) to fit the floor plenum. If you have above-the-floor ducts of metal, you will need to make some adapters to go from the round outlets on the furnace to the rectangular metal ducts. The other method would be to remove the metal ducts and use the expandable fiber/wire ducts. Those are easier to route and reduce the amount of transmitted furnace noise. If a plywood base box is made, it should be lined with light aluminum sheet. It's also a good idea to avoid having the air blow into square corners by placing curved pieces of aluminum in the corners. Think of how a river flows - forced air works the same way. Darol
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Old 07-07-2006, 01:10 AM   #8
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Another thing

Something that is also VERY important is the # square inches of ducting output that you have available. Every furnace has a minimum amount required for proper operation to promote air flow and prevent overheating of the plenum and short cycling.

Your new unit should have this number either in the installation manual or located right on the unit. Be certain that you meet the minimum requirements for this!
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Old 07-07-2006, 10:04 AM   #9
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Thank you for this help, for clarity....

Okay, so I know we're confronting the same possible issues with installation, I'm going to try to post a pic showing my concern. This is a drawing, so don't laugh.

What I think will happen when attatching the intake square seal and the exhaust pipe with trim is that it will dip down at a slight angle in order for the trim to rest flat on the exterior skin. To allow the trim to stay flat, the pipe with have to flex to meet the angle of the skin. If the pipes are left straight, it would seem to me that the exhaust and bottom of the trim would touch the exterior but as the top goes it would stand off. Hopefully the drawing helps.

Bottom line is it okay to flex the pipes to meet the angle of the trim? Or should the whole unit be angled slightly to create the same angle of the trim as it mounts on a contoured skin?
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Old 07-07-2006, 10:41 AM   #10
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Sequoia,

IIRC, the exterior plate has 2 tubes in it for intake and exhaust and they slide into (or around) the same tubes on the furnace. Once the tubes are mated, I don't think that there will be a leakage concern. I have removed a few for service where they were not completely inserted due to design constraints from the OEM.

You should get the best seal possible where the mounting plate mates to the outside of the skin. How big is the gap at the top that you have to deal with? Perhaps you can fabricate a matching plate that will be flat on the outside and curved on the inside so that all surfaces will be flush relative to each other. Then be sure to Sikaflex all of the surfaces to assure that your install is totally sealed. That should do the trick.
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