Placement of air handler

Bernie508November 7, 2011

I am planning to build a 950sq ft guest house. A basic layout one bed room, living room with open kitchen area and a bath area. Hurrican rated windows Dade County. I live in south Florida and the unit being considered is not a heat pump but a traditional A/C unit about 2 to 2.5 tons with a variable speed fan.The condensor will be on a slab outside the house. I am trying to save space by placing the airhandler in the attic. The roof and attic area will be insulated with Icenyne (Zip Foam). I was informed that moving the airhandler into the attic where heat will be a greater problem, a higher seer rated unit is needed and the cost of placing the whole HVAC unit would increase by about 1/3. Does this seem reasonable?

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If this is a typical peaked roof with attic, seal the attic and spray foam the roof line with low-density foam. My opinion for the AC is a couple of mini-split heat pumps or two indoor units run from one outdoor unit.

    Bookmark   November 8, 2011 at 2:23PM
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If you are foaming the rafters then it isn't particularly reasonable. But, generally you would be better off to sacrifice the 15 sqft for the air handler (and not pay to have the rafters foamed (IMO). But it you are building on slab, I'm not sure who is putting ductwork there anymore so your best option maybe to put the ductwork and air handler in the attic. While you maybe increasing the size of the unit by 1/2 ton, the install costs should be about $500 more - but the run costs will be about 20% higher (this would be for a vented attic). But if you are going to foam the rafters anyway, then go ahead and put the airhandler up there which shouldn't add any costs.

I wouldn't be shocked by a code requirement to go to Seer 15 if the ductwork and air handler was not in conditioned space and then that could increase the costs by 1/3. But none of this should apply if you are foaming the rafters.

    Bookmark   November 8, 2011 at 3:31PM
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You have to place the insulation somewhere. The OP did not make it clear if it will be on the top or bottom of the attic. Given that the building is in humid FL, I'd go with the top all other things being equal. If the ducts and AH are up there, the gains are larger than if the HVAC is under the attic, but there are still advantages.

"Venting attics in the South was dreamed up by some disgruntled Yankee pissed about the Civil War and wanting to get even."

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    Bookmark   November 10, 2011 at 7:44PM
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