The Numbers We Measure
This is what we measure: every one of the HVAC measurements we take on a system, what a healthy result looks like, and what it rules in or out when it lands outside the range. These HVAC measurements are the checklist behind every report we hand over.
Airside HVAC measurements: what the air is doing
Airflow gets measured first, always. Every refrigerant reading depends on it, so charging a system before verifying airflow means charging to a moving target.
| Measurement | Healthy range | What it tells us when it is off |
|---|---|---|
| Total external static pressure | Under 0.50 in. w.c. | The single most useful reading on a residential system. High means the ducts, filter or coil are restricting the blower, and the equipment cannot deliver rated airflow no matter how new it is. |
| Return static pressure | Roughly half of total | Isolates the restriction to the return side. Undersized return ducts and filter grilles that are too small for the airflow are the usual culprits in older South Bay homes. |
| Supply static pressure | Roughly half of total | Points at the supply side instead: crushed flex, closed dampers, too few registers, or a coil that is fouled and choking flow. |
| Airflow (CFM) | About 400 CFM per ton | The volume of air actually moving. Low airflow drops capacity and can drive the coil below freezing. High airflow hurts humidity removal, which matters less here than inland but still matters. |
| Temperature split | 16 to 22 °F | Return air temperature minus supply air temperature. Low points at charge or airflow. High usually means not enough air is crossing the coil. |
| Return wet bulb | Varies with the day | Not a pass or fail number. It sets the correct charging target for the conditions, which is why the same system has a different correct charge in June than in October. |
Refrigerant side HVAC measurements
These readings say whether the charge is correct and whether the refrigerant is moving freely. They are only trustworthy once airflow has been confirmed.
| Measurement | Healthy range | What it tells us when it is off |
|---|---|---|
| Superheat | Target set by conditions | How much the refrigerant warmed past boiling before leaving the coil. The primary charge check on fixed orifice and piston systems. High superheat means the coil is being starved, low means liquid is at risk of reaching the compressor. |
| Subcooling | Typically 8 to 12 °F | How much the liquid cooled below condensing temperature. The primary charge check on TXV systems, which is most modern equipment. Low usually means undercharge or a leak, high means overcharge or a restriction. |
| Suction pressure and saturation | Read as temperature | Low side conditions. Converted to a saturation temperature because temperature is what actually moves heat. Too low and the coil ices, too high and the system is not absorbing heat properly. |
| Liquid pressure and saturation | Read as temperature | High side conditions, where heat gets rejected outdoors. High readings mean the condenser cannot shed heat: a dirty coil, a failing fan, or overcharge. |
| Condenser approach | Compared to outdoor air | How close the condensing temperature runs to outdoor ambient. A wide gap means the outdoor coil is not doing its job. Ours run dusty here, and it shows up in this number first. |
| Compression ratio | System dependent | High side against low side. A useful early warning on compressor health, and one of the readings that separates a tired compressor from a charge problem. |
What the system actually delivered
Everything above is diagnostic. These last three are the answer to the only question a homeowner actually asked.
BTU delivered
The whole output, measured rather than assumed. One ton is 12,000 BTU, so a three ton system should land near 36,000 under design conditions. Compare this to the nameplate and you know whether you got what you paid for.
The part that lowers temperature
The share of capacity actually making the air colder. When this falls below about 90 percent of expected, the system flags it, and it usually traces back to airflow.
The part that removes moisture
Moisture pulled out of the air. Matters less in our dry summers than it would in Houston, but a system spending most of its capacity here is a system that will struggle on temperature.
How the numbers fit together
This is the whole checklist on a single system. Airflow at 1,600 CFM on the left, superheat of 21.4 degrees at the evaporator, a condenser reporting Starved on the right, and the capacity totals in the middle.
Read alone, any one of those could be explained away. Read together they converge on one answer, which is the entire point of taking the full set rather than the two readings that are quickest to get.
Why all of it, and not just the easy ones. Any single reading can be normal while the system is failing. High superheat looks like undercharge, but it is also what a stuck TXV, a restricted liquid line and a fouled evaporator all look like. Taking the full set is what separates the four possibilities, and it is the difference between fixing the cause and selling a refrigerant top up every summer.
Furnaces and heat pumps get measured too
Cooling gets the attention, but the same logic applies when the system runs in reverse.
- Temperature rise across a gas furnace, compared to the range stamped on the equipment. Too high and the heat exchanger is being cooked by low airflow, which is how heat exchangers crack early.
- Static pressure in heating mode, because a filter or duct restriction does not care which season it is.
- Gas pressure and combustion, so the burner is running as designed rather than over or under fired.
- Heat pump performance against outdoor temperature, since capacity legitimately falls as it gets colder and the question is whether it is falling more than it should.
- Auxiliary heat behaviour, because strip heat quietly energising when it should not is one of the most expensive faults there is, and it is completely silent.
Where this applies
Same instruments, same checklist, whichever service brought us out.
What We Measure Before Any Part Gets Condemned
The refrigerant and airflow numbers tell us how the system is performing. Electrical readings tell us why a part failed or whether it is about to. A part does not get replaced because it looks old or because it is the usual suspect. It gets replaced because a reading says it is out of spec.
Capacitors
A run capacitor gets tested in microfarads against the rating printed on its side, with the power off and the capacitor discharged. Most are rated with a tolerance of plus or minus five or six percent. One reading ten percent low is weak. One reading zero is done. A bulged top is a clue, not a measurement.
Voltage and amp draw
Voltage gets checked at the disconnect and at the contactor under load, because a loose connection can read fine at rest and drop badly when the compressor starts. Amp draw on the compressor and both fan motors gets compared to the nameplate. A motor pulling well over its rated amps is working too hard or failing, and it usually takes something else with it.
Motor windings and contactors
Compressor and motor windings get checked for resistance and for shorts to ground. A contactor gets inspected for pitted, burned contacts and checked for voltage drop across the contacts while it is pulled in. A contactor that drops several volts across itself is cooking the compressor slowly.
Flame sensor and ignition
On a gas furnace, flame sensor current gets measured in microamps. A furnace that lights and then shuts off a few seconds later is very often a sensor reading too low to prove flame. Hot surface igniters get checked for resistance against their spec before they fail on the first cold night.
What We Measure On A Mini Split
Ductless systems get the same discipline with different access. Pressures and line temperatures still give superheat and subcooling, although many inverter systems want the readings taken in a test mode that locks the compressor at a fixed speed. Supply air temperature and humidity at each indoor head show whether every zone is pulling its weight. On a multi zone system, a single head that underperforms often points at a line set issue or a stuck expansion valve for that one zone rather than the whole system.
Indoor heads also get the blower wheel and coil inspected, because a dirty wheel cuts airflow in a way no pressure reading will flag directly. The symptom shows up as a weak temperature change at that head and a system that never quite catches up.
Humidity Is A Measurement Too
Comfort is temperature and moisture. A system can hold 72 degrees and still feel clammy if it is not pulling moisture out of the air, usually because it is oversized and short cycling or the blower speed is set too high. Measuring wet bulb and relative humidity at the return and supply tells us how much of the system capacity is going to moisture removal. It is the reading that explains why the thermostat says one thing and the people in the house say another.
Why We Write Everything Down
A measurement nobody records is just an opinion with extra steps. Everything we measure goes on the report and stays in the equipment history for that address. The next visit compares against it. That is how a slow refrigerant leak, a weakening capacitor, or a duct problem that is getting worse shows up as a trend months before it shows up as a breakdown.
What We Measure Versus What Gets Guessed
Plenty of service calls still run on feel. Hand on the suction line, a look at the pressures, a thermometer in a supply register, and a verdict. Sometimes that verdict is right. The trouble is that a system can be a pound low on refrigerant and starved for airflow at the same time, and the two problems partly hide each other on the gauges. Add refrigerant to a system that really needs a duct fix and the pressures look better for a while, right up until the compressor floods back.
That is the whole reason for measuring in order: airflow first, then charge, then delivered capacity, then electrical. Each reading either confirms the last one or tells you to stop and look again. It is slower than guessing on the first visit and much faster than guessing three visits in a row.
What You Can Measure Yourself
A few checks need no tools beyond a cheap thermometer and your eyes. With the cooling running for fifteen minutes, the air at a supply register near the equipment should be roughly fifteen to twenty degrees cooler than the air going into the return. Much less than that and something is off. Look at the filter monthly in heavy use. Check that the outdoor unit is clear and its fan is spinning, and that the copper line at the outdoor unit is cool and sweating, not frosted. None of that replaces a real diagnosis, but it tells you when to call before a small problem becomes a big one.
None of these readings require opening walls or replacing parts. They are taken through service ports, test holes in the plenum, and the electrical panel on the equipment itself. A full set takes a little longer than a quick look, and it turns a guess into an answer that you can see on paper and compare against the next time the system is checked.
Common questions
How long does a full set of measurements take?
About an hour once the system has stabilised, longer if the readings point somewhere that needs isolating. Static pressure needs two test ports drilled, which takes a few minutes and leaves holes small enough to plug.
Do you have to drill holes in my ducts?
Two small test ports, one before and one after the air handler, and they get plugged when we are done. There is no other way to read static pressure, which is precisely why so few systems ever get it measured.
Will you do this on a maintenance visit or only on repairs?
Both. On maintenance it is more valuable, because that is when you find the problem while it is still cheap. The reading that saves you money is almost always taken before anything has actually broken.
What if my system is a ductless mini split?
Static pressure does not apply the same way with no duct system, but everything else does: superheat, subcooling, delta T and delivered capacity. Ductless systems hide problems well because each head is small, so measurement matters just as much.
Where the HVAC measurements targets come from
We did not invent the numbers on this page. The targets behind these HVAC measurements checks are published by the bodies below, and you can read them yourself without going through us.
Have your system measured
San Jose and the South Bay. One owner, one truck, and the numbers in writing.