You are choosing a person before you are choosing a system. The unit matters, but someone else's hands will install it, and the paperwork from the job is what you will rely on when you plan the next replacement. This walk-through covers what to check on any installer or service company, whatever system is on the table.
Check the license with the office that issues it
Many areas license trades, and the licensing office usually publishes a lookup. Search the business name and the individual's name, and confirm that the license class matches the work in your quote. A license for one trade does not cover another. Write down the license number and the date you checked it, so you can show your record if a question comes up later.
Ask whether a permit will be pulled
Where a permit is required, the installer should say who pulls it and who schedules the inspection. A job without a permit is not automatically a bad job, but it leaves you without an inspection record for the system. Ask for the permit number in writing. If the installer says none is needed, ask them to name the rule that exempts the work.
Read the written scope line by line
The scope should list what will be removed, what will be set, what will be connected, and what will be tested. The new unit should be identified by its model and serial, and the scope should say which parts are new and which are reused. If an item is described only as included, ask what it means in practice and what it looks like when it is finished.
Ask the questions that show how they work
Ask who will be on site, whether those people are employees or subcontractors, and who you call if something is wrong after the job. Ask what happens to the old unit and whether you will receive the model and serial of the removed one. Ask how they record the install date. An installer who answers plainly gives you information you can compare. One who deflects has told you something too.
Read the warranty paper, not the sales summary
Read the warranty document itself. Note who issues it, how the parts coverage and the labor coverage are written, what registration step is needed, and what the document says is not covered. The plain explainer on how service contracts and warranties work will help you read the clauses, and the published tables on this site give the typical service life of each system you are weighing. Do not accept a verbal promise about how long anything will last.