:confused:My forced air heat and air conditioning system components (1969 Coleman A/C & Gaffer Statler FAU) have exceeded their life expectancy by about double and for mostly efficiency reasons I'm looking to replace the units. One HVAC guy recommends upgrading from 4 ton to 5 ton (2300 sq. ft. 2-story house in L.A. area - we are no strangers to 100+ degree summer days). Since it is mostly the AC unit that is working overtime and still failing to keep ahead of the heat, I was thinking about doing the replacements in two steps - AC now and FAU in 6-months to a year. HVAC guys says both units should be same size for most efficient performance and therefore recommends against doing it as a 2 part job. How detrimental would running a new 5 ton AC unit with a 4 ton FAU?

2300 sf comes out to a 3 1/2 ton by my calculations. Sounds like you may have some duct work problems. I would make sure your duct work is size correctly before trying to get a bigger unit.

Seek additional opinions. If the a/c system is grossly oversized for the application, poor humidity control will result due to insufficient cycle duration. While the indoor temperature will be maintained, the space may feel "clammy".

I would make sure your duct work is size correctly before tring to get a bigger unit.
I followed tech through attic access as he checked all the ducting for upstairs and all looked OK and he mentioned that the insulation around the ducting is a good quality. Duct lines to bedrooms are 7".