Important Tip: Don’t just pay attention to the high-end of the price range! – which is what I know you are all looking at, but also look at the low range. If a contractor is cheaper than this, ask “why?” Usually it is because they don’t have liability insurance, workman’s compensation insurance, are unlicensed, all of the above, or worst yet, they don’t use HVAC technicians. You are spending a lot of money.  Make sure that they are licensed (In California, check here at CSLB License Checker) and insured.
An alternative to packaged systems is the use of separate indoor and outdoor coils in split systems. Split systems are preferred and widely used worldwide except in North America. In North America, split systems are most often seen in residential applications, but they are gaining popularity in small commercial buildings. The split systems are a great choice for small buildings where ductwork is not feasible or where the space conditioning efficiency is of prime concern [20]. The benefits of ductless air conditioning systems include easy installation, no ductwork, greater zonal control, flexibility of control and quiet operation [21]. In space conditioning, the duct losses can account for 30% of energy consumption [22]. The use of minisplit can result in energy savings in space conditioning as there are no losses associated with ducting.
It seems all the average costs for a complete heating and cooling system replacement I've found on websites are usually at least 1/4 less, mostly half lower, then quotes I've gotten from companies. The worst quote I got was from One Hour Heating & Air Conditioning (the company with the Mike Rowe commercials), for over $15,000! I finally settled on a Carrier dealer, for $10,700 & I'll get $950 of that back in energy co rebates eventually. The company I went with's basic system would have been around $8800 (the only one that sorta matched up with web avg cost). They did all the correct planning as far as I could see, not like One Hour, who just showed me a price guide for the same size units already in place. And they do all the rebate paperwork and submit it to the energy co, One Hour never even mentioned possible rebates! Plus Carrier gets above avg reviews. I would have greatly liked a price under 9 grand, but this company got me 0% APR financing, so at least nothing out of pocket. My advice is stay away from national chains, and shop around with local area companies. Of course, One Hour says local companies will fold before you need warranty service lol. For their cost, there should be a technician living in his work van outside my house hahaha.
Maybe I took him to literally but I am pretty sure cops wont show up to ensure that my duct gets replaced. He said it would be over 1000 dollars to replace the ductwork if that is the case. Now, I spoke to a friend who did 2 ductwork lines on his own in his attic and he said it was like 15 dollars for the plastic wirewound tubing to create new ducts. Should I cancel that appointment and just do the work on my own? I feel like I’m a chump or lazy for considering paying someone to do this work.
I hired duct cleaning company out of "Service Magic" website. I did not realize the name of his company was Duck cleaning of Central Florida...that should have given me a hint.. The guy did a very poor, incomplete job, and ruined a closet full of clothes when he sprayed up to clean a vent instead of taking it down and cleaning it outside. He left me with more than 1/2 dirty vents and ducts, did not do the air handler in the garage and failed to fog the system. He spent 6 useless hours at my residence and now his insurance is denying coverage..very disappointed in Service Magic....

To qualify for a tax credit from the federal government, you must save the manufacturer's certificate from your system. The IRS won't require the certificate at the time you file your federal tax forms, but they recommend keeping it with your records in case of an audit. It proves that you purchased a qualifying product. The government offers tax credits for:
The performance of vapor compression refrigeration cycles[26] is limited by thermodynamics. These air conditioning and heat pump devices move heat rather than convert it from one form to another, so thermal efficiencies do not appropriately describe the performance of these devices. The Coefficient-of-Performance (COP) measures performance, but this dimensionless measure has not been adopted. Instead, the Energy Efficiency Ratio (EER) has traditionally been used to characterize the performance of many HVAC systems. EER is the Energy Efficiency Ratio based on a 35 °C (95 °F) outdoor temperature. To more accurately describe the performance of air conditioning equipment over a typical cooling season a modified version of the EER, the Seasonal Energy Efficiency Ratio (SEER), or in Europe the ESEER, is used. SEER ratings are based on seasonal temperature averages instead of a constant 35 °C (95 °F) outdoor temperature. The current industry minimum SEER rating is 14 SEER.[27][dead link]
PickHVAC Tips: Installing the right size central air conditioner is extremely important. Have a load calculation done to ensure proper sizing. ACs that are too big aren’t as energy-efficient as they should be, and they cycle on and off too much. This creates over-cooling and temperature imbalance. Units that are too small run constantly in very hot weather and can’t keep your home cool. When an air conditioner is properly sized, it runs at optimal efficiency and indoor comfort.
Though this installation is not always complicated, it's best to work with an air conditioning contractor to ensure it's done correctly. Permits are usually required for this type of work and can be obtained by a state-licensed contractor. You should not attempt to install central air conditioning on your own since you need a license to handle the refrigerant chemicals (Freon) involved.
Most people are now aware that indoor air pollution is an issue of growing concern and increased visibility. Many companies are marketing products and services intended to improve the quality of your indoor air. You have probably seen an advertisement, received a coupon in the mail, or been approached directly by a company offering to clean your air ducts as a means of improving your home's indoor air quality. These services typically — but not always — range in cost from $450 to $1,000 per heating and cooling system, depending on: