2 air conditioning units for new build

lniaJuly 6, 2012


We are building a new home and will be installing 2 air conditioning units. 1st floor is slightly over 1900 sq feet with 2 story great room. 2nd floor is approximately 2100 sq feet if we include bonus room. Is there a rule of thumb regarding tons for each floor of the house? Is it better that the higher tons cool 2 nd floor? I appreciate any advice or points we should consider as this is all new for us.


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The HVAC contractor needs to calculate the heating and cooling load for the home. This is referred to as a Manual J calculation. Rules of thumb lead to equipment which is either over sized or undersized.

The cooling load will usually be higher for the second floor. But you could have a large amount of south facing windows on the first floor which could make the cooling load higher than the second.

    Bookmark   July 6, 2012 at 5:38PM
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It really is west facing windows rather than south that is the issue. The manual J that I used has east and west windows as identical which is just wrong but that is what it used. I find that 2nd stories tend to have lower needs because of smaller windows which dominates over heat rising. Unless the ductwork is in the attic, which adds a lot of heat so then the 2nd story unit creeps up in size.

    Bookmark   July 7, 2012 at 5:03AM
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We will hAve 2 furnaces as well. One will be in the basement. One will be on the 2nd floor bonus room. Our house faces west. I would say our largest windows are on the 1st floor (I'd patio door, dining room window, den window,etc). Is it possible for a layman to do the manual j calculation?

    Bookmark   July 7, 2012 at 7:40AM
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Yes a lay person can do a manual J but you have a lot of inputs and need to know a lot of details. Why are you sizing this yourself or are you just checking the contractor's work?

In some jurisdictions, a manual J is required with the permit. I will say that doing a manual J and following one are not the same thing.

You will tend to piss off HVAC contractors when you do your own manual J or disagree with their's. They want to oversize in their DNA and it is very difficult to break that. Ask me how I know.

For a manual J, you need all the wall measurements, window sizes, r and u values, SHGCs for the windows, overhangs on windows. You have to speculate what the air leakage rate is although it isn't very exact. You need to know where the duct work is located.

    Bookmark   July 8, 2012 at 4:34AM
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