Frequency
Impressions divided by reach: how many times the average person in your audience saw the ad.
A frequency of 3.0 means the average person saw it three times. It is an average, so a meaningful share of the audience saw it considerably more.
Why it matters for HVAC
Small service areas make this the constraint that binds first. A national advertiser at a $10,000 monthly budget may never see frequency climb; a contractor spending the same into one metro will watch it climb within weeks, because the audience cannot refresh.
Some repetition is the point. Nobody books a system replacement from one impression, and demand-generation advertising works partly by being seen more than once. The question is where useful repetition ends and waste begins, and that boundary is visible rather than theoretical.
How it is measured
Reported directly, and worth reading weekly rather than as a monthly average, which smooths over exactly the trend you are looking for. Read it against link click-through rate: frequency rising while click-through holds is fine, and the two diverging is the signal.
A worked example
An account holds frequency at 2.2 with link click-through at 1.7% through the first month. By week seven frequency is 4.6 and click-through is 0.9%. Cost per lead has risen 70%. The audience is saturated, and the fix is creative rather than budget.
These figures are invented to show the arithmetic. They describe no real business.
Where this is demonstrated
The Creative production pillar scores every listed agency on what they document here, and gives you the questions that tell you which one you are being sold.