There are several things that a contractor can leave out, or not do, that will either make their price lower or their cost less, but not necessarily the price you pay.
1) If they are not replacing the line-set (the copper connecting the outside condenser to the coil that is inside), they might not flush out the line-set. Flushing removes refrigerant oils from the old refrigerant which may not be compatible with the new refrigerant, or the oil may be holding contaminants.
Result: Not doing it can shorten the life of your system, up to 50% or more.
2) Using nitrogen while brazing the line-set to the condenser and coil will prevent trash from building up inside the system during the process. This trash can cause clogging and poor operation of the system, and possible bread downs in the future.
Result: Poor operation resulting in shorter system life.
3) A good vacuum of the system. Not like the vacuum in your house. A vacuum reduces the atmospheric pressure inside the copper line and removes any moister from the system. This is important because moisture left in the system can cause acid to form, eating away at your new system from the inside out.
Result: Not having it done can shorten the life of the system, up to 50% or more.
4) Correctly charging the system after installation. Most systems come with a pre-charge in the Condenser for 15 feet of line-set. Very few systems have exactly that number of feet. So after installation the contractor should adjust the charge to make sure it is correct. In 1995 PSO of Oklahoma shared that having the refrigerant charge off by as little as 2 ounces could cost as much as $40.00 per month in operating expense.
Result: Higher utility bills than necessary, possible comfort and system issues.
5) Giving you the AHRI Certificate*. If a contractor quotes you a SEER**, that the system he is selling will provide. Make them prove it with an AHRI Certificate. AHRI is a third party that test the equipment and provides documentation (proof) of what a system will actually do for you, “If installed correctly”.
Result: Not getting what you paid for.
Make no mistake the quality of an installation is paramount to how long a system will last and how efficiently it will serve you. Price is important but knowing who will put it in and having everything in writing is more important. Check out a company. Ask if you can see their work. Check with the BBB and Google Reviews. Check out their Facebook page. Find out what people think of them.
*AHRI: Air Conditioning Heating and Refrigeration Institute
**Seasonal Energy Efficiency Rating (SEER): SEER is to A/C system what MPG (Miles Per Gallon) is to a car.