Properly sizing central A/C

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Posts: 17
Joined: Sat Jul 07, 2012 8:19 pm

Mon Jul 14, 2014 7:06 pm

I was wondering if there's a guide to properly sizing a central A/C system.

I have a 14x66 with a 14x12 addition (so around 1100 sq ft.) My A/C furnace supports up to 3 ton capacity as far as I remember. Built in 1980, so 2x4 walls. R14 in the walls, R22-24 in floor/ceiling. I discovered the addition was pretty piss-poor done construction-wise, and reinsulated all the walls and ceiling, then found out there was none in the floor, so I did that too and put belly tarp there. There are heat ducts that I ran to the addition although the airflow isn't less than I thought it'd be, I probably should have put a baffle in the main duct to direct some air into the 6" pipe that runs to three vents in the addition. Hindsight is always 20-20. ;)

I live in a climate where four months of the year it can get pretty warm (between 30 and 35 C or 85 to 95 F) with decent humidity, usually between 45 and 80%, but it can get higher.

I just want to do some calculations myself so when I get some quotes for a new compressor and evaporator I can tell if they're out to lunch. The reason being is I used to have a 1972 12x66 and the local guys sold me a one ton cooler and if it got over 30 C it couldn't keep up, a few times on the 35 C days it could only cool to 28/29C running nonstop.

Anyone got tips?

Norm Frechette
Posts: 189
Joined: Fri Aug 05, 2011 7:34 am
Location: Norwich, CT

Tue Jul 15, 2014 6:23 am

google found this ... elines.pdf

a hvac contractor might be more precise

Posts: 17
Joined: Sat Jul 07, 2012 8:19 pm

Tue Jul 15, 2014 10:15 pm

Thanks! I see that they do recommend 1-1.5 for my area for newer models. Mine most certainly isn't insulated near as well as new units. I only have 2x4 walls (2x2 inside walls) so that would most likely require more cooling. I'm pretty sure new units need to have at least 2x6 walls.

Posts: 385
Joined: Wed Apr 07, 2010 10:07 am

Tue Jul 15, 2014 11:21 pm

With the humidity level you have I would consider either a 2.5 or 2 ton unit. 3 ton would cool the home well but won't run long enough to remove humidity. You will reach your target temperature too quickly with the 3 ton and not reduce the humidity very much. I would probably consider a 2 ton condenser with a 2.5 ton coil. The larger coil inside would remove more humidity and that might make your home more comfortable. That high humidity puts you in a pretty unique cooling situation. You don't want to be in a cold clammy environment.

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