Incremental lift
The additional business caused by advertising, above what would have occurred anyway.
Every account has a baseline. Customers who would have found you, repeat buyers, people acting on a recommendation. Reported conversions include a share of that baseline. Incremental lift is what is left after you subtract it, and it is the only figure that answers whether the spend was worth it.
Why it matters for HVAC
Home services carries a large baseline. Referrals, repeat customers, a van someone has seen for years. An ad shown to a metro where you already have brand presence will collect credit for a great deal of work it did not cause.
This is why the heaviest criterion on this site asks whether an agency measures this at all. Across all nine agencies reviewed, not one publishes evidence of having tried.
How it is measured
Only by experiment. A holdout where a randomly chosen group is not shown the ads, a geo test where comparable areas run and do not run, or a deliberate blackout compared against a matched period. Anything else is inference dressed as measurement.
A worked example
An account reports 60 conversions a month. A geo holdout across matched postcodes shows the untreated areas producing 41 comparable enquiries at the same rate. The lift is roughly 19, not 60, and the real cost per incremental job is about three times the reported figure. The campaign may still be worth running; it is now being judged on the right number.
These figures are invented to show the arithmetic. They describe no real business.
Where this is demonstrated
The Tracking and the Conversions API pillar scores every listed agency on what they document here, and gives you the questions that tell you which one you are being sold.