Getting Forced Air Around the entire house

Problems related to residential installations.

Getting Forced Air Around the entire house

Postby fgordon1 » Mon Nov 05, 2012 4:39 pm

I'll try to keep this brief, but apologize for the length of this post.
My century home (only about 1800 sqft over 2 stories) has no basement. We have had a new downdraft (heat comes out of the bottom of the furnace) F.A.G furnace/AC installed. To get upstairs the heat/ac runs through ducts in the crawl space under the house, up through the 1st floor with a trunk line, then splits in two to feed 2 bedrooms upstairs. We can not seem to get heat/ac into those rooms without the aid of an in-duct "booster" fan. This "boster fan" is large and very loud (both in the room in which it is located and the 2 bedrooms that are the recipients of the heat). I'm about to renovate the living room (where the trunk and said fan are located) and box the trunk/fan in. Even when I do this, the box is going to be huge and I'm still afraid of noise (right now you can barely have a conversation in the room if that fan is running). My question - is there no way that I can get better flow of air through the ducts without the use of this booster fan? I see MASSIVE houses being built, sometimes more than 3 stories high, and they don't seem to have trouble getting heat from a basement all the way up to a 3rd story without the use of these fans. The reality is I need even more of them in order to bring the flow of air upstairs (and even into my living room) to other rooms more to a normal level. Suggestions? HELP!
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- Getting Forced Air Around the entire house

Postby Freon » Wed Nov 07, 2012 4:24 pm

The first thing you need to tell us is the size of the ducts in the 2nd floor you are feeding air too. In your case I am certain there are too many 90 degree bends from the furnace through the trunk line up to the 2nd floor. You simple answer is make sure you use 2@45 degrees if you must have a 90 degree bend. And second, you will definitely need a bigger diameter duct going upstairs. The exact size depends on the size of the ducts to each room.
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- Getting Forced Air Around the entire house

Postby fgordon1 » Thu Nov 08, 2012 9:18 am

Hi there - thanks for writing. It is a 6" duct that then splits into two 6" ducts running to each of the bedrooms.
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- Getting Forced Air Around the entire house

Postby Freon » Thu Nov 08, 2012 5:00 pm

A 6" duct can handle ~110 cubic feet per minute of air flow (cfm). With 2@6" you need a minimum of 220 cfm... and the 6" duct feeding the 2@6" will never give you that amount of air flow. Usially the wall cavity is too small and the HVAC guys just fit what will fit.

You need a minimum of an 8" duct feeding the 2@6" ducts. However I would go at least 10" because you can always decrease the air flow if it's necessary. And we haven't even talked about 90 degree bends in the ducts or duct lengths.
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- Getting Forced Air Around the entire house

Postby fgordon1 » Fri Nov 09, 2012 10:26 am

Thanks - now we're really getting somewhere.
The duct that the company used appears to be a flexible duct that is then wrapped in an insulated sleeve. It doesn't really have any 90 degree bends per se... it runs from the main trunk under the house, directly to a corner of the livingroom, where it does bend (more or less at 90 degrees) and runs up the height of the living room, where it joins one (a pre existing) 6" duct in a "T" intersection... each part of the top of the "T" goes to a different bedroom, the left to the North and the right to the south. I hope this makes sense and you can give me a bit more feed back about a more appropriate solution. I can take photos if required.
With thanks,
Fred Gordon
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- Getting Forced Air Around the entire house

Postby Freon » Fri Nov 09, 2012 2:50 pm

Flex duct is the worst when bent at 90 degrees. Usually the duct gets pinched reducing the diameter even more.

If you want to use flex that's fine... I hope you don't have mice because they love to nest in it. A crawl space may have mice but you'd know best.

Use 10" flex or 8" hard ducting. I will describe what I would do with flex but it's the same with 8" hard duct, only different reductions

Re-run the line from the furnace to where it bends 90 degrees to go upstairs. At the point where the duct attaches to the furnace plenum there should be a metal take-off from the plenum. Make sure it's 10"... if not, replace the take-off with a 10" one. Install a damper in that takeoff (it's easy to do).

If you use flex, at the point where there is the 90 degree bend going upstairs, use 2@45 degree 10" hard elbows. If you pull back the flex insulation 12 " and then cut the inner flex duct, you can then slide the insulation over the hard duct elbows. Make sure you use good duct tape on all connections with at least 3 screws. Follow the flex manufacture's installation directions

Continue the 10" ducting up to the 2nd floor where you want to have 2@45 degree 10" elbow to the 2@ 6" lines. You will need to buy reducers to transition from 10" to 6". But do the reducing AFTER the 90 degree bend.
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- Getting Forced Air Around the entire house

Postby Katherin » Sat Dec 08, 2012 5:48 am

If you need Forced Air around the Entire Home means first you choose the good Air conditioning device which one is suitable to your two bedrooms. Nested on the fixing degree also it’s to be differ.
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