It is most important to see that the air return is air tight. Nothing else will cause more troubles than leaky returns.
1. Check if possible, by hand, to see that all rooms had airflow BEFORE replacing an inside heater or air handler.
2. All carpeting which you will walk upon should be checked BEFORE any work. Note any spots, stains, etc. to the customer. Document if necessary. Drop cloths should be used for protection of carpet.
3. Likewise, check the ceilings directly below the new equipment location, to make note of any pre-existing damage.
4. A liquid drier should be installed on any evaporator coil or condenser unit replacement. Do not leave any old driers in the system.
5. Do not tee condensate drains to other primary or secondary a/c drain lines. If piping corrections will be more than what was estimated, get the customers written approval for increased cost.
6. Dampers should be installed on all supply ducts. Tap collars for duct connections to sheet metal should be the "ductboard" type if a hole cutter is not used.
7. Slope Cooling coils at least slightly toward the drain fitting. This keeps pans much more clear of slime and water from blowing into the plenum.