Peak humidity weeks make rooftop units work every occupied hour, and a floor that felt fine in June starts collecting warm complaints by mid afternoon. Before a facility team opens a ticket that only says the building feels off, one repeatable reading tells you far more than a thermostat number. Measure the temperature difference between the return air a unit pulls in and the supply air it pushes back out. That split, taken with a simple probe thermometer at a return grille and a nearby supply diffuser, separates a unit that is still cooling from one that has quietly stopped keeping up. This guide is for property and engineering staff across the Southeast who want a number to attach to a work order. Pair the reading with Garrett Mechanical commercial HVAC service and preventive maintenance when the split points past a filter swap.
If symptoms could be mechanical, plumbing, or electrical, start with the facility symptom priority quiz so the reading lands on the correct owner. If people are in danger now, pause and follow your emergency plan first.
Measure the split at the grille, not the thermostat
Hold a probe thermometer at the return grille that feeds a running unit and let the reading settle. Then take a supply diffuser fed by the same unit and let that reading settle too. Subtract the supply number from the return number. The gap is your cooling split for that zone. A thermostat on the wall reports room air, which drifts for many reasons, so it cannot tell you whether the equipment above the ceiling is actually pulling heat out of the air.
Take the reading on a full cooling afternoon, not first thing in the morning. Note the outdoor temperature, the unit tag, and which grille and diffuser you used so the next person can repeat the same two points. One number without those details is hard to trust a week later.
What a low split usually means
A small gap between return and supply air on a hot afternoon often means the unit is moving air without removing much heat. Loaded filters, a dirty coil, or low airflow show up this way before anything trips. Confirm filter condition and coil faces first, since those are the checks a facility team can do without opening a refrigerant circuit. Read how to check filters and coils on packaged units and clear what you can reach safely.
If the split stays low after clean filters and a clear coil face, the cause is usually past visual work. Refrigerant charge, a weak compressor, or a control problem all read as a tired split. Schedule commercial HVAC diagnostics with the number in hand so the technician starts where the reading points instead of guessing.
What a wide split can hide
A large gap looks encouraging, but it can mean the unit runs cold while very little air actually moves. A blower fighting a clogged filter or a slipping belt can drop airflow enough that the coil chills the trickle that reaches it, which reads as a wide split at a single diffuser. Put a hand near the outlet and feel whether real volume comes through, not only cold air.
Weak airflow with a cold coil also risks the coil icing during long run hours, which cuts cooling further once the frost builds. If the split reads wide but the room stays warm and the diffuser feels faint, note that combination on the ticket. Airflow problems and cooling problems get fixed differently, and the number plus the hand check tells the technician which one to chase.
Read the split beside setpoints and schedules
A healthy split with a warm room usually points at the calendar, not the coil. Setpoints that felt fine during mild weeks can fall behind long humid afternoons, and a unit that cools well still loses ground when the schedule assumes lighter load. Compare your readings against spring setpoints meeting long hot stretches before you lower every thermostat in the building.
Change one setpoint or schedule per problem zone and wait a day before stacking more changes. If the complaint might be plumbing or electrical rather than the rooftop, run the peak cooling quiz so the loudest symptom lands with the right trade first.
Log the number so it means something next week
Write the split, the outdoor temperature, the unit tag, and the two points you measured. A single sheet per rooftop turns scattered readings into a trend. A unit that read fine in June and thin in July tells a clearer story than a fresh ticket that starts from zero every visit. Keep the log where the next facilities lead can find it, not only on one phone.
When the same unit reads low two weeks running, treat that as a signal, not a coincidence. Steady readings before peak weeks cost less than stacked comfort tickets once every floor runs cooling together, and they help preventive maintenance land on the units that need it.
What to send before the visit
Attach the split readings, dated photos of filters and coil faces, the unit tags, and a short note on which zones feel weak. If occupied space is already uncomfortable and a unit will not cool at all, schedule through emergency service rather than waiting for the next routine window.
Contact Garrett Mechanical with that packet when the numbers point past filter and coil work. Browse more facility notes in the knowledge base for filter passes, setpoint drift, and storm week prep on a steady calendar.