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Old 03-16-2009, 12:29 PM   #1
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Need help determining the size of an upstairs HVAC unit


I am finishing a second level bonus room, about 780 sq feet. It will have a family room, bathroom and a bedroom. How do I know what size unit I need. It will be all electric, heat pump, split unit. I worry about undersizing and getting hot in the summer due to the cielings being next to the roof. But, I dont want to oversize and use too much energy. Thanks, Eric


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Old 03-16-2009, 12:44 PM   #2
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Need help determining the size of an upstairs HVAC unit


you will need a manual J load caculation performed.

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Old 03-16-2009, 12:46 PM   #3
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Need help determining the size of an upstairs HVAC unit


Pay the 50 dollar fee and do your own Load Calc
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Old 03-16-2009, 12:49 PM   #4
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Need help determining the size of an upstairs HVAC unit


Ok thanks, I will look into that.
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Old 03-16-2009, 02:21 PM   #5
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Need help determining the size of an upstairs HVAC unit


Too many unknowns. Geography, insulation, ceiling height, glass, doors, direction. Is the only load going to be that space or will it be servicing transitional spaces?

On 780 sq ft, most of us could "Guess" 1.5 or 2 ton and probably be correct, but you are going to be spending thousands of $$, why risk it? Either go the $50 for the HVAC Load Calc, and or go with a company who will furnish with your quotation (not just a guess).

Good Luck

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Old 03-16-2009, 03:26 PM   #6
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Need help determining the size of an upstairs HVAC unit


For more info on the importance of heat loss see the FAQ here
http://www.comfort-calc.net/faq.html
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Old 03-16-2009, 03:56 PM   #7
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Need help determining the size of an upstairs HVAC unit


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Originally Posted by beenthere View Post
Pay the 50 dollar fee and do your own Load Calc
You don't think this one is accurate?
http://www.hvaccomputer.com/hvac/sizer.asp
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Old 03-16-2009, 04:04 PM   #8
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Originally Posted by jerryh3 View Post
You don't think this one is accurate?
http://www.hvaccomputer.com/hvac/sizer.asp
LOL... I used that one when my laptops battery goes dead.
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Old 03-16-2009, 05:12 PM   #9
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Need help determining the size of an upstairs HVAC unit


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LOL... I used that one when my laptops battery goes dead.

I did a stint as a V.P of Sales for a wholesale distribution company a few years ago. I gave a training class to our sales staff on sizing. I think we were using Rezcalc or Elite, one of the Manual J programs. Anyway, I handed out that house sizing sheet and before I could explain that one of our missions was to make sure that our contractor base was correctly sizing systems, one of the salesmen said "Wow! This is cool". He actually thought it was legit (note: He was there when I got there, I did not hire him....owners nephew or something).

It is frustrating to know that a contractor will spend a butt load of money on a newspaper ad or the Yellow Pages, or buy into some rip-off supplier's Bahama trip program and yet will not spend a few hundred dollars on one of the great sizing programs. The reports alone, and their value as a closing tool are well worth the minuscule price.
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Old 03-17-2009, 10:08 AM   #10
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Need help determining the size of an upstairs HVAC unit


I checked with my local Electric coops G&T and they have guy that will come out and let you know what size they recommend. Its free and he is not a contractor. It couldnt hurt to see what he has to say. The bid I got from a contractor was for a 2 ton 19 seer. It was not cheap, so I was just wondering if he was trying to sell me his most expensive unit instead of selling me what I needed.
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Old 03-17-2009, 11:42 AM   #11
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The bid I got from a contractor was for a 2 ton 19 seer. It was not cheap, so I was just wondering if he was trying to sell me his most expensive unit instead of selling me what I needed.

The Size decision is separate from the SEER decision in my opinion. The correct size is critical. The SEER becomes a choice that falls in the "pay now or pay later" category.

The reason he gave the 2 ton bid was probably not based completely on the load requirement, but also on the fact that at 19 SEER, probably his smallest unit was 2 ton.

Let us know what the power company guy has to say.

Jay
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Old 03-17-2009, 03:25 PM   #12
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Need help determining the size of an upstairs HVAC unit


You need more estimates.

That guy is just giving you a price on a unit figuring that if 2 tons is too big. Then it will just run in first stage.

Get someone out that knows how to size.

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