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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?

google found this

https://www.energystar.gov/ia/partners/ ... elines.pdf

a hvac contractor might be more precise

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.

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.