Original message
| cwtdallas
 |  "Correct AC System Sizing"
, posted Thu 1 Feb 23:14  
I'm looking for help in determining the correct HVAC system size for a small house I am rennovating. I've used the HVAC-Calc program to perform the Manual J calcluations. Based on my design conditions Sensible Gain is 20,943 BTU/H, Latent Gain is 2,690 BTU/H, Total Heat Gain is 23,633 BTY/H and Total Heat Loss is 36,130 BTU/H. I've provided this information to multiple contractors and none seem interested in the heat gain/loss figures. They all want to know what size the house is and how old it is. They each have given different recommendations on sizing ranging from a 1.5 ton to 3 ton AC condensor and a 40K to 72K furnace.
I need an AC condensor that will provide at least 20,943 BTU/H Sensible capacity. Most condensor spec sheets that I can find only provide total capacity. A Rheem model I'm considering does provide Sensible capacity and a 2 ton unit has a capacity of about 16,500 BTU/H depending on the coil, a 2.5 ton unit has the capacity of about 20,500 BTU/H and a 3 ton unit has a capacity of about 24,500 BTU/H. Is a 2.5 ton the right match for my requirements?
I plan on using an 80% AFUE furance. What is the proper size furnace for my requirements? I've read the output capacity of the furnace should be about 1.25 times of the Total Heat Loss BTU/H. Is this correct? If so I would need a furnace with an output of about 45K BTU/H. At 80% AFUE a 60K BTU/H would seem to be the closest match. How much flexibility in sizing do I have. Would a 80% AFUE furnance with 66K BTU/H and 54K BTU/H output be too large?
I've found a complete Carrier package consisting of a 2.5 ton AC condensor with coil and 66K BTUH furnace that is a great price but I want to make sure it's a good match before I purchase it.
cwtdallas
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Replies:
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| yuri
| "Re(1):Correct AC System Sizing" , posted Fri 2 Feb 11:14
Been a few years since I studied that stuff for my Journeyman ticket. Your calculations sound good. One suggestion I have is you may try contact your utility co. and see if they have anyone who can help you for free. Or try Rheem/Carrier/Lennox etc. They all want your business. Our hydro co. owns the gas co. and elec. and I talked to a professional engineer for free when building my new house and they gave me some free info. The direction your house faces, exposure to sun at peak times of the day can influence your A/C calculations. Low E argon windows and shades help a lot.
Hope this helps
Yuri
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| acefurnacefxr
| "Re(1):Correct AC System Sizing" , posted Fri 2 Feb 11:13
Why 80% vs a 90 + ?DO NOT go with the rheem product, you will regret it. Get another quote from a lennox dealer. go with 21/2 ton (a little more in cooling that small will be a reward on the hot summer days) in Texas.Also you should be looking at Variable speed motors (DC DRIVE)in stead of ac motors...with energy cost sure to escilate as long as dumbya keeps playing war in the middle east, the variable speed motor will save you big $$$ in short times.
On the heating side, look into a Two stage unit, 40,000 in lows (most of the time in texas that should do it) and 68.000 in high for those cold dallas night (s).When its wired to a two stage stat, and set up properly the system will be a gem in the rough!
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| | cwtdallas
| "Re(2):Correct AC System Sizing" , posted Fri 2 Feb 15:52
Thanks for your response.
This is for a rental property and I've decided on an 80% unit because a 90% effecient unit won't increase my rental income or resale value. For the same reason I plan to go with a single speed furnace.
What would you recommend for size of a single speed furnace? A 50K 80% furnace would output 40K. While it might have to work hard on those really cold days I'm thinking it may be the best fit. My Total Heat Loss is 36K BTUH at a design temperature of 28 degrees outside and 72 degrees inside.
cwtdallas
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