Nine Out of Ten HVAC Systems Have a Fault. Yours Is Probably One of Them.

There is a comfortable assumption buried in how most people think about their air conditioner: that it works correctly until the day it stops. Working, then broken. A binary.

The field research does not support that at all.

In June 2018 the U.S. Department of Energy's Building Technologies Office published Residential HVAC Installation Practices: A Review of Research Findings — a synthesis of dozens of field studies in which researchers went into real houses and measured what the equipment was actually doing. Not lab units. Installed systems, in service, in people's homes.

What the measurements found

The review's summary of fault prevalence is worth reading slowly, because each line describes a different way a system can be quietly wrong:

• Duct leakage is "the most common source of performance degradation," with most studies finding 90–100 % of systems tested needing sealing or repairs to supply or return ducts.
• Low airflow across the indoor coil is "found more than 50 % of the time in all regions studied." High airflow shows up in another 8–15 %.
• Refrigerant charge faults "range between 29–78 % undercharge and 4–50 % overcharge," depending on the study and region.

One field study inside the review measured a single population in detail: 68 % of systems were not charged properly — 47 % undercharged, 21 % overcharged — and 53 % of measured airflows were below the manufacturer's own recommendation of 350 CFM per ton.

These are not exotic failures. They are the ordinary condition of installed equipment.

Why nobody notices

Here is the part that matters more than any single percentage. A system with 30 % undercharge and restricted airflow does not stop cooling. It cools less. It runs longer. On a mild day you would never know, because the house still gets to temperature — it just takes more minutes and more kilowatt-hours to get there.

You find out in August, when the outdoor temperature climbs past the point where a degraded system can keep up, and the house simply stops getting cold. The fault didn't happen that day. It was there in April. August is only when the margin ran out.

The compounding problem

NIST studied what happens when faults occur together rather than alone, and the answer is not reassuring. In Sensitivity Analysis of Installation Faults on Heat Pump Performance, the authors found that simultaneous faults could be additive, roughly neutral — or well beyond additive. Duct leakage combined with refrigerant undercharge was specifically named in that last category.

The DOE review is candid about the limits here: "the studies do not provide sufficient data to reliably quantify the prevalence of multiple faults, or the impacts of faults occurring in combination." So the honest statement is not that combined faults are always catastrophic. It is that when a system has several small problems at once — which, given the prevalence figures above, is the normal case — nobody can currently tell you what that adds up to on your particular unit.

What follows logically

You do not need a new statistic to draw the conclusion. If faults of this kind are present in the large majority of installed systems, and if they degrade capacity rather than stopping it outright, then the only reliable way to know your system's condition is to measure it — and to keep measuring it, because the day a technician checks is not the day the fault arrives.

That is the entire premise of Vetralis. It mounts on the outdoor unit and watches refrigerant pressure and line temperature continuously, and alerts you when those readings drift out of their normal pattern. It is not a repair service and it cannot catch every possible fault. What it does is close the gap between when a system starts going wrong and when a human being finds out.