Sizing for 3rd floor with barn-style roofline


  #1  
Old 06-20-05, 09:09 PM
BeagleBoy
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Question Sizing for 3rd floor with barn-style roofline

Have a fairly complex sizing problem that I hope a more experienced person could help answer.

Finishing a 500sq ft. 3rd floor "bonus room" that is long, has a lineset run by the builder for future HVAC, and a barn-style roofline. I'll try to explain as best as I can.

Imagine a stop sign, chopped at the bottom. Walls are 5' tall, angle at about 45 degrees for another 5'5" then level off to a 5'7" horizontal ceiling to make the room 13'5" wide, running north to south. The room is 37' long, running west to east. On the east side, there is one standard size double pane vinyl window. In the southeast corner, there is one standard size interior door that exits to a stairway.

The slanted part of the wall/ceiling is comprised of the 2"x6" beams used for the roof, so you can imagine there is no airflow between the beams (only from 2nd floor roof soffits to 3rd floor apex vented roof). This also causes uneven temperatures in the room as the sun travels from one end to another, heating the air between the beams. Currently open 2"x4" walls and 2"x6" roof beams. Have R-19 batts that are to be installed after running HVAC ductwork.

Above the ceiling, there is a truss structure which has one vertical piece descending from the apex to a 2"x4" used to form a ceiling line. This permits up to a 14" round duct on each side of the vertical support brace in the ceiling above the room and below the apex of the roof. Keep in mind that this is also where all of the hot air collects before escaping out of the vented roofline.

Located in Richmond VA, so I'm more concerned about cooling than heating. Summers are in the low 90s, winters tend to be mild as we get ice but rarely snow.

Room will be used as a game room with more need for cooling than heating. Biggest issue is how to size with the barn-style roofline comprised of the roof beams. Thought about a 1.5 ton, but fear that it may short cycle. Mini-split ductless systems may not be practical with this roofline or the long room dimensions. Would prefer 4 registers to evenly distribute.

ANY help would be greatly appreciated. Thanks!
 
  #2  
Old 06-21-05, 05:14 AM
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Here is a very trusted sizing software that should work great for your needs http://www.hvaccomputer.com/main.asp . Where were you going to put the air handler?
 
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Old 06-21-05, 05:37 AM
BeagleBoy
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At the end of the west part of the long room, there is the horizontal 2 ton Goodman heat pump used for the second floor. Next to it is a large space with lineset and electrical connections for pre-wired area for the future heat pump. Trunk lines for the 2nd floor run along the north side of the 3rd floor room in the crawlspace to flex ducts in the crawlspaces and under the 3rd floor floor.
 
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Old 06-21-05, 06:59 AM
T
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BeagleBoy,
Both supply and return duct to be stubbed in? You will need returns for good air distribution and movement. 2 supplies, 2 returns minimum.

Make certain room is adequately insulated(don't take any shortcut) and attic area is ventilated properly-possibly look at a power vents in roof.

Size the HVAC equipment correctly for good heating and cooling. Depending on how much room will be utilized, look at min 12 SEER rated heat pump. If HP unit can be sized right, I would prefer this over a mini split system.

Good Luck!
 
  #5  
Old 06-21-05, 07:15 AM
BeagleBoy
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2 returns would probably be needed, I agree, with such a long room. Will try to insulate it as best as I can. There is blown fiber in the crawlspace 6" deep that is on top of the 2nd floor ceiling. Have R-19 batts for the walls and ceiling of the 3rd floor room. Installing temperature activated powered fans at each end above the 3rd floor ceiling would be a darn good idea.

Three major problems with ductless mini-splits are: required height (wall is 5' tall before 45 degree slope), lineset length (placed in 3rd floor), and length of the room (37' long).

Most of the software and calculators based on measurements say go with a 1.0 ton heat pump unit, but I am only finding 1.5 minimum! Considering 12-13 seer unit, if I could find one.
 
  #6  
Old 06-21-05, 11:48 AM
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I don't think a 1/2 ton over will be a big deal. Just get a big screen tv to use up the extra cooling.
 
  #7  
Old 06-21-05, 01:08 PM
BeagleBoy
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Would a variable speed air handler (available on the 13 seer) be beneficial to reduce the oversizing for the 3rd floor? Currently using two independent 2.0 ton 10 seer Janitrol units with gas heat for the first two floors (original equip).
 
 

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