How much will is cost to install a central AC system in a 50 year old 2800 sq foot house that has a flat roof?

Question

How much will is cost to install a central AC system in a 50 year old 2800 sq foot house that has a flat roof?

Answer 1

One rule of thumb when estimating central air conditioning systems would be a cost of $2500 a ton.  (12000btu=1 ton of ac:  the measure goes back to the days when refridgeration was primarily used to make ice to be sold to individual users)  One estimate of requirement is,"1 ton AC is required per 1000 Sq Ft living space."1  Therefore your approx 3000 sq ft home should cost about $7500 to have AC installed.  A major cost is the ductwork that is required for a traditional system.  And the work is primarily done within the home with some associated damage that must be repaired.

In more recent years  ductless split systems have become more popular and sometimes cost effective in whole or partial house AC installation.  This kind of system can be very efficient with seer ratings in excess of 20 and most of the work is done on the outside of the house. (running ac, drain, and electric to each of the "heads [evaporators] located in each room.)  There is a product called "slim duct" that can be used to hide the lines or in some cases the lines can be run within the walls.  

For a more novel solution a ground (or open water) source heat pump (heat and cooling) system could be installed.  There is a much greator up front cost but runnning costs are very very cheap. (One customer was paying $200/ mo to the util on average throughout the year for heating (northeast) cooling and all other electricity requirements combined.)  As with many "alternate energy systems" combining solutions is usually more cost effective and efficient.  This kind of system might make some sense to a solar pannel (and some radiant heat) installation where a limited amount of power would be available.

Any installing would have to be a custom one for your home.  Average local temperatures/humidity, south facing windows, amount of insulation, power availability (you might have to upgrade your pannel/electrical service) are all considerations.  Get your estimates and listen to your local installers.