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This is a little simplified, but not too much. I have architects and plans for a a two story house plus a basement. The house is about 80 feet by 20 feet with all stories extending the whole footprint. The floors between stories are supported with a 20" floor truss system.

I was told the floor truss system could pass HVAC ducts. I thought this would mean that the floor of the 1st story could provide HVAC for the basement and first floor, and the floor of the 2nd story would provide HVAC for the second story. All mechanical would be in the basement with a chase between the 1st and 2nd story.

Instead they want to serve the 2nd story from the roof (there is no attic) by adding roof trusses for ducts. And add a 2nd story mechanical room. This is wildly inelegant to me.

Since I insisted on keeping the mechanical in the basement, now they've designed a chase from the basement all the way up to the roof of the second floor. Possibly even more inelegant.

  • Will the 20" floor truss system not provide the space to service 3 stories from two floors? With a zone for each story?

  • If it doesn't provide adequate space, should I look at mini-split for the basement, hydronic floors for basement, or some other type of solution for one or more floors? It'd be nice to avoid having HVAC in the roof, right?

  • Speaking from just my personal experience, I've got a single HVAC in the basement and ducts running through the floor of the 1st and second floors. However, the return goes all the way to the attic to pull from the second floor ceiling down to the basement. Might they be recommending the chase for the return instead of the ducts? – BMitch Dec 11 '15 at 16:05
  • Did you ask them why it might be important to have ducts running that high in the home? I have a feeling if you ask them about this, their explanation will make sense. For a well balanced, efficient and comfortable system it is necessary to service through more than just the floor of a room. – BrownRedHawk Dec 11 '15 at 17:25
  • How is the roof designed? Is it a vented roof with insulation atop the 2nd floor ceiling, or is it a compact unvented roof with insulation directly attached to the underside of roof deck? – ThreePhaseEel Dec 12 '15 at 5:03
  • The roof is designed unvented with insulation attached to the underside of the deck. And I realized the difficulty in running inside the floors is because of a floor cutout in the second story floor. But mini-splits appear to be a great option to serve the two bedrooms on the other side of the cutout, along with other bedrooms (for consistency and comfort) – crazyjncsu Dec 14 '15 at 16:32
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HVAC is most efficient when you blow in the cold air from above and let it settle down. This gives a very even temperature gradient from head to toe. Just blowing it in from the floor will make your feet cold and your head hot, causing you to lower the thermostat and drive up your operating costs.

It is worth the effort to install the ducting in the ceiling or at the ceiling level, wherever possible. It may seem inelegant, but it's a good idea.

  • Similarly, many air returns in well balanced, newly designed homes are set in the ceiling. – BrownRedHawk Dec 11 '15 at 17:24
  • It seems like this is just favoring A/C efficiency/comfort over heat efficiency/comfort, right? I'm in the southeast BTW, so this isn't necessarily a bad thing. – crazyjncsu Dec 11 '15 at 17:33
  • You're right that this does favor cooling over heating. However, the level of discomfort with ceiling mounted registers in heat mode is less than floor mounted registers in cool mode. I'm not sure of the reasoning, but it's an accepted generalization. – longneck Dec 11 '15 at 19:28
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If you want to have zones, and no ducts a mini split system would be worth a look. I have not put one in that big. The smaller units controlling 3 -4 zones work great the line sets are hidden in normal walls. Their only needs to be a small drain from each of the units for condensate.

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