Split system calculator for new house


  #1  
Old 08-25-13, 08:59 AM
F
Member
Thread Starter
Join Date: Jan 2009
Location: Kansas
Posts: 49
Received 0 Votes on 0 Posts
Split system calculator for new house

I am in the process of building a new country style house, and have read that it is important to size the AC unit correctly because it both cools and dehumidifies the interior of the house. I also have read that smaller units work more efficiently and overly large units will not run long enough to dehumidify the house.

Is there a good online calculator for determining the proper size split system to purchase for new house construction?

I know that there are many variables to consider, such as size of the house, insulation, windows, geographical area, etc. My house being built is 1300 sq. ft. main floor over a full basement. There are no vaulted ceilings. The roof is 12/12 pitch with room-in-the-attic spaces of another 400 sq. ft. The exterior walls are 2x6. For insulation, I plan a minimum of R-19 walls and R-30 ceilings. In addition, the basement walls on the interior will have 1.5" foam boards glued on, the exterior stud walls of the house will have 3/4" foam boards under the siding with blown in sticky cellulose between the 2x6 studs, the roof is standing seam light paint metal, and will have a radiant barrier installed under the roof rafters. All windows are top of the line Marvin Ultimate Low E with reflective coatings, etc. There are no skylights and no west facing windows. Windows on the south side will have awnings or overhangs to block the sun rays.

A friend of mine who is an HVAC contractor is going to install a 95% efficiency split system, but he tends to oversize units and I do not think that he understands how to size a unit for a super efficient house. For example, he told me that it is much better to have an overly large unit because they run less and last longer. The house is in south-central Kansas where everything is temperate. Given the above specs, what would be your recommendation on the AC tonnage and furnace BTU size for such a house? In the alternative, can anyone point me to a website with a detailed calculator or formulas to use to get an educated answer. I am just concerned about having to buy and live with an overly large unit sized by using the ball park method. Knowledge is always power, and this site is great!

Regards,
Chuck
 
  #2  
Old 08-25-13, 09:51 AM
C
Member
Join Date: Aug 2008
Location: Texas, California
Posts: 1,498
Received 1 Vote on 1 Post
My guess is 2.5 ton. if you really want to do some calculation (I don't trust it) , Google for
Manual J. If I were in your area, I will look for similar houses in the same area and see what they have, and go from there.
 
  #3  
Old 08-25-13, 11:17 AM
T
Member
Join Date: Jul 2008
Location: Kansas
Posts: 317
Received 0 Votes on 0 Posts
There are low cost load calc software to be downloaded. You have to understand with a new house, you are going to have to live with the decisions you make now, for the next 20 years or more. Do it right and don't count on my estimate or anyone elses. Know for sure. You want my guess? New construction, I'd guess 2 ton with 60,000 furnace. Don't trust my guess. Know for sure.
 
  #4  
Old 08-25-13, 12:29 PM
Bob14525's Avatar
Member
Join Date: Jun 2013
Location: USA
Posts: 1,630
Received 30 Votes on 25 Posts
I have a 1400 sq. ft. ranch house built in 1977. While I have added cellulose insulation on top of the 12" of fiberglass that was previously there, I wouldn't say that the house is especially "tight". I have a 21 year old 2 ton A/C unit and it didn't have any problems when we had a week of 90 degree, humid weather (western NY). So, I doubt that you want to go over 2 tons for the A/C.

As for heating, a few years ago I had a new 95% efficiency modulating furnace installed. It is 60,000 BTU and it hasn't had any trouble keeping the house warm during the coldest winters (slightly below zero temps).

While I'm not recommending these heating/cooling values for your new house, I would say that you likely won't need to exceed these values.
 
  #5  
Old 08-26-13, 04:44 AM
F
Member
Thread Starter
Join Date: Jan 2009
Location: Kansas
Posts: 49
Received 0 Votes on 0 Posts
Thanks so much for the replies. Your suggestions were about what I had thought. I have a daughter-in-law who has a degree in architectural engineering and worked for a big company sizing HVAC systems for commercial buildings. I will get her opinion and ask her to look up Manual J. After getting her recommendation, I think that I will be ready to deal with my HVAC installer. You guys are great.

Regards,
Chuck
 
  #6  
Old 08-26-13, 03:10 PM
F
Member
Thread Starter
Join Date: Jan 2009
Location: Kansas
Posts: 49
Received 0 Votes on 0 Posts
Online Load Calculator Manual J

Today, I found this load calculator based upon Manual J.

HVAC Load Calculation - Maunualj - Whole House Loadcalc

It is fairly easy to use provided that you have the correct input. It confirmed that I do not need an AC unit larger than 2 tons.

Regards,
Chuck
 
  #7  
Old 08-26-13, 10:14 PM
F
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,495
Received 36 Votes on 28 Posts
I really question that calculator. First of all, it does not have my city, which is of reasonable size and has been in existence for at least 100 years. Second, it lists Paine Air Force Base in Everett as the closest to my city. Paine AFB has not existed for at least fifty years.
 
  #8  
Old 08-27-13, 01:24 PM
HVAC RETIRED's Avatar
Member
Join Date: May 2008
Location: Wisconsin USA
Posts: 713
Received 0 Votes on 0 Posts
Online Load Calculator Manual J

Today, I found this load calculator based upon Manual J.

HVAC Load Calculation - Whole House Load-calc

It is fairly easy to use provided that you have the correct input. It confirmed that I do not need an AC unit larger than 2 tons.
Regards,
Chuck
I've checked it against using a spreadsheet; It seems to somewhat over estimate the cooling loads.

The duct system has to be engineered Right for the A/C to perform properly...

Use the calculator to figure the air infiltration CFM & put it in the correct blank or it won't figure the proper LATENT LOAD.

Redo it until you think you have it right, then print the results as you can't save it; then post results here..
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: