New DIY central AC system


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Old 12-20-11, 07:53 PM
R
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New DIY central AC system

I have a detatched game room that is 25' x 30' and it has a 12' tall ceiling that slopes down to 8'. It's a metal building with 3.5" of blown insulation in the walls and the ceiling has R19. The game room is one large open room.

I'm locate in South Texas and it gets 104 to 108 Deg F here in the summer. I've been using a 25,000 BTU Window unit for 10 years now and it barely keeps up and I replace it on an average of every 4-5 years. During the summer it rarely cycles and is very loud.

I just bought a new 3 ton split system heat pump which may be just a little large. (I figured a 2.5 ton would be fine but wanted extra in case I expand some).

Here are my plans.
On the 25' wall with the 8' ceiling, I'm going to build my air handler closet in the corner with the return vent on the bottom (25" x17"). I'm planned to use duct board to make a discharge duct 11"x 20" to go up 2 feet to the main duct.

I planned to make the main duct from duct board as well and run it in a single 25" line at the ceiling.

What size rectangle should I be shooting for for this main (only) duct and how many registers. I do want a decent velocity from the registers to "feel" it across the room.

BTW, I'm doing all the carpentry work, wiring, and ducts and I will have a local guy come by and solder the line kit and commission the system. They don't have anyone available to come look and size the ducts until after the first of the year.


Thanks
Richard
 
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Old 12-20-11, 08:54 PM
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You really don't "shoot for" duct work!
You need to determine the btu/h needed for each room then size the ducts to satisfy each area.
You might get some numbers thrown around but without calculating the load it would be a crap shoot.

Hvac-calc is a program that would help if you wanted to do the sizing yourself.
 
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Old 12-20-11, 09:40 PM
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Greg is correct, and that load calc can give you the correct size system you need for your home. A undersized system will run constantly and eat up electricity. An oversized system will run too short of cycles and not dehumidify and not cool properly. Proper sizing is ESSENTIAL!
 
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Old 12-21-11, 09:32 AM
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According to the program, I only require a 2 ton system.
Sensible Gain is: 16,450
Latent: 9483
Total Heat Gain 26,000
Heat Loss is 19,600

However I know that 25,000 BTU WIndow unit will not keep this place real cool in the heat of the summer. It's a metal building with galvanized roof. 4" Sprayed in Rock Wool insulation on all walls, R19 in the ceiling. No air gaps in insulation since it's sprayed in. There are steel purlins that carry heat to the room as well and you can see it via infrared.


Adding 25% to the total heat gain comes to 32,500 so the 3 ton shouldn't be too awefully oversized.

I ordered a 3 ton system, 1200 CFM at 1" is the requirement.
1200 CFM says I need the following main duct size 16 round, 22x10, or 18x12. It never asked for the length of the run which is odd.

Another question reminding I have a sloped ceiling 8' to 12'. The Return is located at the 8' wall. Do I want the ducts on the opposite side of the room from the return or will it even matter that much? I'd like them on the same wall as the airhandler at ceiling level.


Richard
 
 

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