The First Hot Week: How to Tell If Your System Is Actually Ready
May is the audition. If the system struggles now, June will be worse. Here is what to measure and what the numbers mean.
During the first stretch of 100-degree days, measure whether your system holds setpoint, how long it runs to recover after a setback, and the temperature split between return and supply air. A system that struggles in May will fail in July, because capacity falls and load rises as outdoor temperatures climb.
Why May is the right test
The first 100-degree week is the earliest realistic stress test of the season, and it is early enough that discovering a problem is an inconvenience rather than an emergency.
It is also the last comfortable moment to get a technician quickly. By late June the Valley schedule tightens dramatically, and by July emergency no-cool calls take priority over everything else — appropriately, but that does not help you if your problem was findable in May.
Three things to measure
- Does it hold setpoint? On a 102-degree afternoon, set the thermostat to your normal summer temperature and see whether the house holds it. Not "does it cool" — does it hold.
- How long does recovery take? Let the house rise three degrees, then set it back and time the recovery. Note the number. Next year the same test tells you whether performance has changed.
- What is the temperature split? With the system running fifteen minutes, measure the return grille and a nearby supply register. Subtract. Should be 16 to 22 degrees.
Reading the results
| Observation | What it suggests |
|---|---|
| Holds setpoint easily, split 16–22° | System is in good shape for the season |
| Holds setpoint, split under 14° | Low charge or weak compressor — check now |
| Holds setpoint, split over 24° | Restricted airflow — filter, coil, or duct |
| Drifts up in the afternoon at 100° | Will fail badly at 112° — diagnose now |
| Runs continuously but holds | Marginal capacity; check charge, coil, and ducts |
| Some rooms fine, others not | Duct or balancing problem rather than equipment |
The trap of "it is cooling, so it is fine"
A system at 80 percent of its capacity cools a house perfectly well in May. The house holds 78, nothing sounds wrong, and there is no reason to suspect anything.
Then outdoor temperature climbs fifteen degrees, capacity falls further, load rises sharply, and that same 80 percent system loses ground. Nothing changed inside the equipment — the conditions simply exposed what was already true.
This is the entire argument for measuring rather than judging by feel. The temperature split reading in May tells you about July.
Room by room
While the system is running hard, walk the house and check each room. Note any room that is more than two or three degrees off the thermostat reading.
A room that is consistently four or five degrees warmer is almost always a duct or balancing issue — a crushed run, a disconnected boot, an undersized supply, or a closed damper somewhere. That is worth diagnosing separately from overall system performance, because it will not be fixed by anything done to the equipment.
The distinction matters commercially, too. A contractor who responds to "one room is hot" by quoting a larger system is not diagnosing the problem.
If something is wrong, act in May
Every year we take calls in late June and July from homeowners who noticed something in May and hoped it would resolve. It does not. Systems degrade; they do not recover.
A capacitor caught in May is a same-week appointment and a modest repair. The same capacitor failing in July is an emergency call, possibly at overtime rates, possibly after a day without cooling, and occasionally after it has damaged the compressor.
The cost difference between those two outcomes is large, and the only variable is when you acted.
Frequently asked
How do I know if my A/C is ready for summer?
Test it during the first 100-degree week. Confirm it holds setpoint rather than merely cooling, time how long recovery takes after a three-degree rise, and measure the temperature split between return and supply air — it should be 16 to 22 degrees.
My A/C cools fine now. Will it handle July?
Not necessarily. A system at 80 percent capacity cools adequately in May and loses ground at 112°F, because capacity falls as outdoor temperature rises while the load climbs. The temperature split measurement in May is what tells you about July.
What temperature split should my A/C have?
Sixteen to 22 degrees between return air and supply air. Below 14 suggests low refrigerant charge or a weak compressor. Above 24 suggests restricted airflow — a loaded filter, a dirty evaporator coil, or a duct restriction.