Open the plan and go to the definitions first. Before the coverage list and before the price sheet, find the page or section that explains what the words mean. Most disagreements that come up later start with one word that meant one thing when it was sold and another thing when it was written into the terms.
Start with four terms. Read how the plan defines normal wear and tear, because that line decides what stays with you as the owner. Read how it defines a pre-existing condition, and check which date it uses: the sign-up date, the install date, or the date of the first inspection. Read how it defines an authorized provider, because the person who arrives for a repair may or may not be the person the plan has in mind. Read how it defines a covered system. A plan may list units by type, by brand, or by model, and a system that is installed in your home but does not match the listing may fall outside the terms.
Next, look for the conditions attached to the coverage. Some plans ask you to keep service records, to perform specific upkeep, or to report a problem within a set window. Find each of those requirements and write down what you would have to keep and how long you would have to keep it. Then ask what happens if a record is missing. The answer is in the terms, not in the sales conversation.
Ask whether the definitions can change. Many plans reserve the right to update their terms by notice. Find out how notice is given, where the updated terms are posted, and whether you have to accept them to keep the plan in force.
When you compare two plans, put the definitions pages side by side and match them term by term. If the same system is described two different ways, the headline comparison is not a true comparison. Do not compare on the name of the company or on how the plan is advertised. Compare on what each definition includes and leaves out.
Waiting periods
A waiting period is a stretch of time after sign-up or install during which certain repairs are not yet eligible. Find its length in the terms. Then find what it is measured from, whether it applies to every covered system or only some, and whether it restarts after a change to the plan.
Compare the waiting period to the age of the system. If the system is already well along its service life, check whether the waiting window would end before or after the stretch when you are most likely to need a repair. The system guides on this site turn an install year into a planning date, and that date is the one to compare against the waiting window.
Cancellation and transfer
Read how cancellation is requested. Find out whether it must be in writing, whether it takes effect on the day you ask or at the end of the current term, and whether the terms describe any refund method. Do not assume a refund exists unless the terms say how it is calculated.
For transfer, find whether the plan follows the house or the owner. Check whether the new owner must act within a set window, what paperwork is required, and whether the transfer changes any definitions or waiting periods. Transfer is not automatic in every plan, so confirm it in the terms before you rely on it.
What to ask before you decide
Ask the seller to point to the definition of each term you have marked. Ask what records you must keep and for how long. Ask whether any maintenance step is a condition of coverage. Ask who may open a claim and through which channel it must be opened. Ask whether the terms can change and how you would be told. Write the answers next to the matching clause so you can check them later.
The published figures for each system are in the guides on this site, and the planning dates in those guides are the place to start. The Gas Furnace page turns an install year into a planning date you can carry into the terms. Use the planning date to decide whether a plan is worth reading in full, then use the definitions to decide whether it is worth buying.