Hot Air/Cold Air Return Ductwork Layout Design Changes

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e.alleg
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Post Thu. Sep. 06, 2007 11:33 pm

My house is set up so that the hot air blows out in the center of the house in various ducts in the floor, the cold air return ducts are larger and located in the floor on the outside walls. I was thinking of either switching the flow so that hot air comes out under the windows and flows back to the center of the house, or only using one or 2 of the 4 cold air returns and pumping hot air out the (what is now) cold air returns to provide more warmth to some of the rooms. What do you guys think?

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coaledsweat
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Post Fri. Sep. 07, 2007 7:35 am

I think that would make the house more even as far as temps go room to room. I am not sure it will work but it sounds like a good idea. :)

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gambler
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Post Fri. Sep. 07, 2007 9:16 am

Usually the hot air registers are on the outside walls of the house (under cold areas such as windows) and the cold air returns are on the interior walls (center of the house).

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coaledsweat
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Post Fri. Sep. 07, 2007 10:13 am

Center heat and outside wall returns are common in very old hot air systems as the heat source was directly below the center register and they had no blowers. My first experiance with coal was with a home like this. It had a huge SEARS coal furnace. Man could that thing heat!


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e.alleg
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Post Fri. Sep. 07, 2007 3:09 pm

Interesting. Some of the ductwork was installed 10 years ago, some were here since 1887 (floor joists with tin nailed to them to make a duct). The existing furnace is undersized for the size of the house, 115k net BTU's isn't enough for 3,000 square feet with 10' ceilings, 8 doors, and too many to count six foot tall single pane windows. I'm thinking they set it up the old way to get away with an undersized furnace? Has anyone done a study or any testing to determine the difference between methods, maybe to see what uses less fuel or makes the house more comfortable? It's not hard to switch the ductwork, but switching back isn't going to be easy.

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Yanche
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Post Fri. Sep. 07, 2007 7:24 pm

e.alleg wrote:Has anyone done a study or any testing to determine the difference between methods, maybe to see what uses less fuel or makes the house more comfortable?
Hank Rutkowski, PE has a procedure for this analysis. See:
**Broken Link(s) Removed** for the Excel spreadsheet. To get the values to plug into the spreadsheet cells you must buy the book. You want "Manual J version 8", not the abridged edition. $85 for ACCA members, $130 for non-members. A good Mechanical Engineering College library should have it.
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Post Mon. Sep. 10, 2007 2:17 pm

Our current/old gas furnance (Janitrol?) is setup the same way, just a large floor register (2'x2'?)in the middle of the house with 2 cold air returns 10-15' away against the windows, just straight convection, no blowers or anything. I think the return ducts are 12-14" Diameter.

The convection flow is quite good, you can feel the heat flowing around the rooms. Not sure what the furnance is rated, but the house is 2500 sq.ft. and built in 1890. There is only 1 Vent that goes upstairs in the ceiling. I will be putting in another, if you leave the doors open upstair, the heat will flow up the front stairway and the cold air back down the rear stairway.
- Dave
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- Amsoil Authorized T1 Certified Dealer

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