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Question DetailsAsked on 3/14/2016

bldg a new home,should i install HVAC ducts in attic and should the supply air and returns be in 2nd floor ceilings

My builder is putting the main trunk lines into the attic for the second floor HVAC and he is using R8 insulation with flexible ducts. He is also putting the supply and the return's in the ceiling of each second floor rooms. The supply vents are by the outside windows in the ceiling, and the return vents are in the ceiling close to inside wall.

Is this a good practice to place ducts outside of the conditioned inside space?

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2 Answers

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The duct arrangement is typical but not satisfactory. The second floor should have a seperate unit due to the fact heat rises. Using a single unit, the first floor will be comfortable but the second floor is usually warmer than the first floor and can be quite uncomfortable.

I like opposing supply and return locations: If the supplies are in the ceiling the return should be in the lower wall across from the supply to pull the supply air down for better coverage.


Source: Poppy Ross

Answered 3 years ago by PoppyRoss

0
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As Poppy says - common but not the best practice, but allowed. Running the ducting through an attic outside the thermal envelope is common, especially on retrofits, but obviously affects the thermal efficiency of the system, and can also cause condensation in the ducting in very cold weather.


Sounds like an economy bid, using flex rather than the more flow-efficient sheet metal ducting.


Ducting insulation is regulated by ASHREA 90.1 - not knowing your specific location can't say if R-6 duct insulationmeets code, but for unventilated attic or ducts on top of attic R-6 insulation R-6 would be to minimum code for Zone 5 - roughly the cornbelt, so R-8 would probably meet code for the upper midwest (but not all the way to the border) or for a broad zone of the South but might not for the deep south or Florida - depends on your specific system design temperatures. And of course how common real hot or cold conditions are mattears too - if you only get a few days a year or real hot or cold weather it is less significant than if you live in International Falls (usually has lowest Continental US temps) or the Mojave or Arizona deserts or sticky, humid deep south like LA/MS/AL/GA, for instance.


Putting supply and return ducks both in the ceiling, as Poppy says, is not considered the best practice - but is common in retrofits where the floor joist space is not reasonably accessible, and in false ceiling installations in commercial buildings. Commonly ceiling supply and return is more acceptable in A/C country than in serious heating areas, because the cold air tends to drop pretty quickly and spread across the floors and mix in the room, and the ceiling return (provided it is at the other end of the room) picks up the warmer air to recirculate. With heating, however, the warm air stays up against the ceiling and does not fill the room, resulting in a lot of system recirculation of the warm air without proper mixing - hence the outside wall floor supply and opposite wall ceiling return Poppy talks about. This would also be normal or "best practice" for areas with significant heating and cooling seasons both.


Did your architect specify the duct locations (and type) ? OR if you have an architect on the job (working for you) he/she could determine whether his installation meets local code or not.

Answered 3 years ago by LCD




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