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Old 04-22-2010, 11:47 AM   #1
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AC sizing


Recently purchaced a small cabin that I will be remodelling/finishing. It currently doesn't have any AC or heat. Looks like the cheapest & best option is to do a thur the wall unit. While AC is a must, also wanted a unit with heat as well. I have checked a couple of sites on the web, and sizing seems to be all over the place - anywhere from 12 to 18 btu's. So... what would be an appropriate sized unit? Any suggestions on brand (more importantly which ones to avoid).

Cabin is 32x24. Will have r13 for walls and r50+ for ceiling. 2 bdr, 1 bath, with open area for kitchen/living room. No shade trees to speak of, with average number of windows.

thanks.

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Old 04-22-2010, 12:10 PM   #2
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Are you planning on installing yourself, or hiring an HVAC pro? If you are going to take bids just make sure the quote includes a "Manual J" or heat load calculation.

If you plan to do the install yourself you can use free software to do the load calc.

http://hvac-loads.com/main.asp

There are a lot of things to take into account other than just the R-value of the wall and ceilings.

In your post you stated that "12 to 18 btu's" was what you came up with by researching on the web...I don't know how you came up with these figures, but just as a rough estimate based on the size of the cabin I'd say you are probably going to need closer to 75,000 btu heating and 1.5 tons of cooling, depending on your location.

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Old 04-22-2010, 01:18 PM   #3
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Originally Posted by fabrk8r View Post
depending on your location.
http://www.degreedays.net/
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Old 04-22-2010, 01:23 PM   #4
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I am located in Nebraska, 69101 zip, and plan on installing myself. What I meant to say was 12-18k BTUs so 1 to 1.5 tons of cooling.

I tried the software tool below, but not sure what it is telling me. It is giving me a heat gain of 531 (ceiling), 2120 (people) and 708 (windows and Heat Loss of 1257 (ceiling) and 1603 (windows). Nothing else has values, and not seeing anything that says "you need a xxxx sized AC to cool your area"

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Old 04-22-2010, 07:14 PM   #5
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Pay the 49 dollar fee, and it will give you the full results
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Old 04-24-2010, 04:41 AM   #6
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Pay the 49 dollar fee, and it will give you the full results
lol he got sucked in by the teaser.
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Old 04-24-2010, 05:05 AM   #7
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OK. Let's be practical for a moment......

Load calcs hae their place but on a well insulated small structure an abbreviated calculation will suffice: sq ft X insulation factor
.furnace efficiency
Out put 34357 btu
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Old 04-24-2010, 05:48 AM   #8
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So what size A/C do you come up with.
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Old 04-24-2010, 11:36 AM   #9
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If he is going to use a PTAC, and with his room size, all this talk about sizing is kinda mute, as his only option is the largest of the PTAC or PTHP units being about 15,000 btu of cool and slightly more of heat.

I agree with Hvaclover in that if going with a conventional split, again, minimal requirements are sort of dictated by the base size of most furnaces being in the 40-45K range and with the amount of ceiling insulation, i would head toward a 1 ton AC. Or, a 1.5 ton HP system with 7.5 KW of back up.

Having said that, I am a firm believer in the load calc in almost all cases, but here it seems that building size creates the limitations.
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Old 04-24-2010, 09:30 PM   #10
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The GE line has a heat pump that can run both the heat pump and electric heat at the same time.
So there is no real reason to over size the cooling. Like with many PTACs.

So a load calc will help him size for humidity control.

Why put in a 15,000 BTU, if a 9,00 is more then enough.
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Old 04-24-2010, 09:58 PM   #11
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Why put in a 15,000 BTU, if a 9,000 is more then enough.
Cause the 24k unit wasn't much more then the 18k unit after a rebate
My last house I had a 12k 10v in wall in the main room that didn't do much (600 sq ft)
I looked at the 18k & the 24k wasn't that much more
So I had an AC that was big enough to do the whole house
I put the 12k in the master bedroom

Main room had a flat ceiling, direct Southern sun & poor insulation
To me it makes sense to have seperate heat & AC, not sure about cost wise
This house I put a 14k in the ~250 sq ft sunroom & another in the master bedroom
Sunroom ceiling slopes from 9' up to flat ceiling at 11'
Its open along one wall ~12' to the kitchen
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Old 04-25-2010, 04:13 AM   #12
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Hey Scuba !

Red Sox suck!
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Old 04-25-2010, 10:31 AM   #13
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Hey how them Tiggers doing ?
Sox won last nite

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