USSC Hotblast Question

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jeff_t

Minister of Fire
Hearth Supporter
Sep 14, 2008
4,205
SE MI
I've got two 8" pipes hooked up for hot air. Do I need 8" pipe for return air? I'm asking only because I have a bunch of 6" vent pipe already that I have no use for.

Jeff
 
I'd use at least 10" for the return. 12" would be better.
 
Some Hot Blast furnaces have furnace filter kits that have a "return" flange built into them. Ask your supplier for information on a filter kit...Cave2k
 
The filter kit has a 14x 18 opening. I had an old plenum laying around and made a collector for it. I've got enough for two 6" pipes.
 
Mr T said:
The filter kit has a 14x 18 opening. I had an old plenum laying around and made a collector for it. I've got enough for two 6" pipes.

It's all about the cross-sectional area (mostly, but friction and resistance play a part as well). For simplicity's sake, let's just look at area to start. Area is calculated as pi x radius-squared. So 2 (8") pipes x 3.14 x 4"-squared (16") is 100.48 sq. inches. You need to have at least that much in the return, or you're just handicapping the furnace. 6-inch pipes are a little over 27 square inches in cross-section, so if you can fit 4 into your system as a return, you should be in decent shape.
 
I don't have that much 6 inch, so I guess I'll start with a couple of eights and see what happens. I was thinking about this while looking at my LP furnace and how much smaller the cold air return is than the plenum coming out.
 
Could be that the LP system is not all that well designed. It wouldn't be the first time.
 
Mr T said:
I've got two 8" pipes hooked up for hot air. Do I need 8" pipe for return air? I'm asking only because I have a bunch of 6" vent pipe already that I have no use for.

Jeff
I used Dual "8"s in my return so if your doing one return it should be 16in or better
 
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