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Heating & Air Conditioning Building a Home

Air conditioner/Heat sizing new build

2 years ago

I live in houston and my plot recieves an average amount of sun. we have spray foam insulation. the whole home is 4600 sq ft with 2600 on the first and 2000 on the second. my builder says their sub contractor sizes our home with a 2 ton and 2.5 ton seer 16 system. im not an hvac expert i understand there are many variables that go into this calc but my builder will not share with me the calc. i have other homes that are smaller and use bigger systems. how should i verify this is the right size? appreciate the help.

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Comments (7)

millworkman
2 years ago
Is this a custom home that you own the land or is this a tract or developer type home where you complete the land/home purchase upon completion?
David Cary
2 years ago
The overwhelming criteria is window orientation. If you don't have large west facing windows, you are likely fine. The vast majority of systems are oversized.

Most of the calcs are done so poorly, that is probably why they don't want to share them. I find this unacceptable but good luck with that. I would imagine this means it is a tract house.

I have always been able to get my Manual J's and they have always had significant mistakes. The mistakes are usually to justify oversizing.
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I found this note on page 11 of the installation manual under upflow units: NOTE: Where the maximum airflow is 1800 CFM or more, both sides or the bottom must be used for return air. This is saying the bottom return by itself will be enough. There was note under the upflow unit section, but I assume it also applies to a horizontal installation. You want the filter and grill cover to be at least the size of the opening to the furnace. Making the grille bigger is not going to help if return itself is smaller. I don't know how this is going to work with the filter size you quoted. I would think you would need two 14X24 filters. It would be nice if you get get a 4 inch filter instead of the 1 inch. Otherwise you will be changing the filter every month. The 6x6 stud obstruction should be removed and you want a tightly sealed return duct. The return duct should not have a right angle. It should be a curved elbow to make the 90 degree transition. Sharp angles create more air resistance. How big was the original house and AC, and much area are you adding? Most houses don't have the properly sized duct work to handle a 5 ton AC. Do you have confidence in this HVAC contractor?
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Neil Gupta
Original Author
2 years ago
Thisnis a custom home
mike_home
2 years ago
Based on the size of the house and the Houston location, the sizes seem small. However a custom built home could have above average insulation and the sizes may be correct.

If you are up for it, you can do your own Manual J calculation. Take a look at HVAC Calc. You can purchase a single use version of the software for $49. You will have to get all the dimensions of the house and the insulation values from the builder.
sktn77a
2 years ago
last modified: 2 years ago
"im not an hvac expert i understand there are many variables that go into this calc but my builder will not share with me the calc."

If he can't or won't share the calculations with you, you can assume none were done. Follow the golden rule.... He with the gold makes the rules!

(ie YOU!)
David Cary
2 years ago
Does the builder have an explanation for why he won't give a manual J? Of course, the HVAC sub usually does the calculation. A better way to do it is have an energy rater do the calc and check over the HVAC sub's work.
PRO
Charles Ross Homes
2 years ago
Hi, Neil,

I believe Texas has adopted the International Energy Conservation Code (IECC) which requires calculations to be performed as the basis for sizing/selecting HVAC equipment and duct work. In Houston, it appears there is a formal acknowledgement to be signed off by the HVAC Contractor. You should be able to get additional information from the Houston Permitting Center 832.394.8850

Since the calculations are required, I don't know why you wouldn't be provided with a copy.

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