On a fixed-speed system, a supply and return reading taken ten minutes into a run tells you a fair amount. The compressor runs at one speed, the air moves at one speed, and the numbers settle into a pattern you can repeat. Modulating equipment changes that picture. ACHR News reports that Bosch's new YD heat pump lineup carries "fully modulating inverter technology across every tier," so variable-speed behavior is no longer limited to the top model. In general terms, a modulating unit adjusts its output to the load instead of cycling between fixed states. That is good for comfort, but it means one reading can catch the equipment mid-ramp and tell you very little about the rest of the day.
What You Need
You need two things: a way to read the air temperature entering and leaving the indoor coil, and a notebook or a spreadsheet on your phone. A pair of probe thermometers, or a multimeter with a K-type thermocouple probe, does the job. Place one probe in the return air a few feet from the filter grille. Place the other in the supply duct past the plenum, clear of direct sunlight and clear of any backup heat. Keep both probes out of the direct stream from a register so you are measuring the air mass, not a jet of air.
Before you start, note the filter size and whether it was changed recently. A dirty filter changes the split on its own, so the log is only useful if you know the filter's condition on day one. Write down the thermostat setpoint and whether a schedule is active. If you adjust anything during the test, record that too.
Build the Log Across the Day
Log a row every 30 minutes for the first three hours after the thermostat calls for heat, then every few hours for the rest of the day. Use these columns: time, outdoor temperature, indoor setpoint, return reading, supply reading, and a short note on whether the air handler sounded like it was ramping up or holding steady. Add a column for the difference between supply and return. The difference column is the one you will scan first.
Do this on a shoulder-season day when outdoor temperatures move through a real range. A flat outdoor reading gives you a flat test, and a flat test hides the behavior you are trying to see. If the weather is mild and steady, repeat the log on a colder morning before you draw any conclusions.
Read the Trend, Not the Snapshot
A single supply-minus-return number describes one moment. Across a day, with outdoor temperature changing, the trend is what matters. If the split drifts down steadily while outdoor air holds still, something upstream of the coil deserves a look: a loaded filter, a blocked or undersized return, or a duct run leaking conditioned air into an unconditioned space. If the split moves mainly with outdoor temperature and recovers when the air warms, the unit may be doing what it was designed to do. Compare your numbers against the documentation for your specific model, since the expected behavior of a modulating unit depends on the model and its controls.
Watch the timing too. A split that is wide for the first few minutes and then narrows can be normal startup behavior on a variable-speed system. A split that starts narrow and stays narrow for the whole run is a different pattern, and it is the one worth sending to a technician with your rows attached.
Know Where the Log Stops
This is a record, not a repair. Adjusting refrigerant charge, opening the sealed circuit, or changing control settings beyond the thermostat belongs to a licensed technician. Turn off power at the disconnect before you open any access panel, and keep your hands, probe leads, and loose clothing clear of the blower wheel and the outdoor fan.
When you call for service, hand over the notebook. A week of timed rows with outdoor readings gets a diagnosis started faster than a complaint that the house feels cold in the afternoon. The technician can then check the things your log cannot show, while you already know what the equipment did, when it did it, and under what weather.