For a whole house system (hvac) or just a window unit? What are you looking for?>
I would definitely (based on past experience) purchase from a company that not only installs but also maintains the unit. HD doesn't do that nor do most repairmen do that. This is only my opinion based on owing 1 home & 2 rental homes.
I actually purchased mine new system from HD when they had a display and salesman in-store. They usually do. It was a good experience.
Got two units two year ago to use in our tack stalls when we travel with our horses. We got one at HD the other at Lowe's, Both were reasonable in price and both different brands work well. We purchased the largest 110 volt unit they had. After each show I take off the front cover and use my garden hose to clean it out. Lots of dust at those places. You can purchase one from a repair company that can install it for you, but you will be paying the price for that service.
Window and wall units are pretty much the same as far as parts and components. All sealed systems, so other then keeping them clean there is not much to worry about on where to buy it.
Just be sure you keep the filter clean and purchase one that is easy to do. Several units require you to remove the entire front of the unit to access the filter, while others simply have a little door that you open and pull the filter out.
In a reply on another post, you can check out heatandcool.com for great prices and a variety of HVAC units. If you find a unit and like the price, you can then contact a HVAC contractor for pricing to install (and warranty the work) or as I have done, check places like craigslist for competent installers. When looking for an installer be sure to ask for references (and verify them) and above all - get a written bid and warranty for the installation.
Woodbridge knows what to do, when to do it and how to do it properly. If I knew just some of what he knows I wouldn't have any maintenence contacts.
Competent + Craigslist = Oxymoron!
Also, not a good idea to purchase equiptment and have someone install it. Most in the HVAC buisness will only warranty equiptment they sell. THEY purchased and sold the equiptment, THEY honor the warranty..... YOU purchase the equiptment, YOU are responsible w/ dealing w/ warranty issues w/ the manufacturer.
Picture this.... you buy a piece of garbage online that is an off-brand and is known in the HVAC industry for "break-downs". Why would a company install this for you and offer to cover this equiptment during it's warranty period knowing they will have countless service calls to run on it for no money?
Aside from that, the existing unit you have may be undersized and unless you know how to size the unit to the square footage of your house, you may purchase something too small or too big. You really need a pro to offer you the right solution for your situation so that you can get maximum comfort and efficiency out of your investment.
And adding even to that, you can buy the "Cadillac" of the heating and cooling industry and it may have every bell and whistle known to man on it but, if your ductwork is inadequate, undersized, improperly sealed, or not enough returns you are throwing all of the equiptment's efficiency away.
This is only my opinion though!
One of the most expensive purchases you will ever make for your home is an air-conditioner. Since you will be living with it for a long time, you have to choose wisely before purchasing. Pick a trusted contractor. Do pick a contractor based on recommendations from friends or neighbors or a reputable referral network. Check on how long the contractor has been in business.