A quote is a starting number. The change order is where that number moves, and it usually moves in small steps that each sound reasonable at the door. A technician finds a worn fitting, a cracked bypass, or an old line that should be replaced while the access is open. Sometimes the find is real. Sometimes it is an upsell. Either way, you need a written record of what was added, why, and what it costs before the crew touches it.
Know what the original paper actually covers
Before anyone adds anything, pull out the signed quote or contract and read the scope. Look for the equipment model, the installation items listed, what is excluded, and whether the price is firm or an estimate that may change after inspection. Many quotes include certain fittings and leave others out, and those gaps are where change orders come from. If the quote does not say whether disposal of the old unit, permit fees, startup, or the first service visit are included, ask now and get the answer in writing.
What a real change order contains
A change order should be its own short document that refers back to the original agreement by its reference number. It should list each added item by name, its quantity, the labor charge, the material charge, and the new total. It should state in plain words what problem the item solves. A line that reads 'additional parts and labor' is not enough. Ask whether the added part carries its own warranty and for how long, because a new part installed under the original contract may have different coverage than the equipment you bought.
Phrases that should slow you down
Certain phrases tend to come right before an addition. 'While we have it open' usually means a part is being offered beyond what you agreed to. 'Code requires' is a claim you can check, so ask which code section or which inspector requires it and whether it applies to your house. 'We recommend' is an opinion, and you are allowed to decline it. 'It will fail soon' deserves a question about what the technician actually observed, not a promise about the future. None of this means the recommendation is wrong. It means the recommendation should arrive on paper with a reason attached.
Rules to set before the crew starts
Tell the lead technician that no work outside the signed scope proceeds without your written approval, and make sure the office knows the same rule. If an addition is truly needed to keep the system safe to run, ask the technician to stop, photograph the condition, and send a change order before continuing on that item. The rest of the job can keep moving only if the original scope allows it. You can also ask for time to review. A reputable company will accept a short pause on an added price without treating it as a refusal.
Keep a running total
Start a one-page sheet with the original quote total on the first line. Add each approved change order below it with its amount and a note on what it covers. Compare the running total against the final invoice before paying. Any line on the invoice that does not match an approved change order is a question for the office, not something to pay and sort out later. Keep the signed copies in the same folder as the warranty and the original contract so the paperwork tells one consistent story.