Customer Says It Runs All Day Translation To Real Fault Decision Tree

Why this matters

"It runs all day and never shuts off" is a runtime complaint, and runtime is the symptom of an imbalance between delivered capacity and building load, not a fault in itself. The same words describe an undersized system on a design day (normal), a system losing capacity to a slow leak (fault), a thermostat differential set too tight (configuration), and a building that suddenly gained load from a failed return damper or an open window (external). Each branch has a different owner and a different fix. A tech who reaches for gauges first will sometimes find perfect charge on a system that simply cannot win against the load, and waste an hour proving the wrong thing. This tree separates capacity-loss faults from load-gain conditions and from normal long-runtime operation.

Symptom presentation

What the phrase guarantees and what it hides:

• "Runs all day" confirms the equipment is energized and cycling rarely or not at all. It does NOT confirm the space is failing to reach setpoint.
• The complaint often arrives on the first stretch of design-temperature weather, when long runtime is correct behavior and the customer is simply noticing it for the first time.
• A high electric bill frequently rides along, which biases the customer toward "it must be broken" when the machine may be doing exactly what it should under heavy load.

Establish the real question first: is the space holding setpoint while running continuously, or is it climbing past setpoint with the equipment unable to keep up? Continuous running that holds setpoint on a hot day is correct. Continuous running with a rising indoor temperature is a capacity deficit.

Quick checks

1. Indoor temperature versus setpoint logged over 30 minutes. Holding versus losing ground is the master fork.
2. Outdoor temperature and a rough load read (sun exposure, occupancy, open doors). A 100 F design day with a 75 F target is a 25 F-plus design split that legitimately demands long runtime.
3. Temperature split at the coil. A healthy 18 to 22 F split with continuous runtime points to load, not equipment.
4. Thermostat differential / cycle settings. A differential set to 0.5 F or a "circulate" fan mode reads as never-off to a customer.

Isolation tree

Branch A: Space holds setpoint, split healthy, hot outside

This is normal design-day behavior, not a fault. The system is correctly sized to run near-continuously at design conditions. Confirm with the load read and educate the customer. If they want shorter runtimes, that is a comfort/efficiency conversation (shading, insulation, air sealing), not a repair.

Branch B: Space loses ground, split healthy (16 to 22 F)

Capacity is being delivered but the load exceeds it. Look for load gain or undersizing:

• New load: failed economizer or fresh-air damper stuck open, a return pulling attic or crawlspace air, a failed zone damper dumping conditioned air into an unconditioned zone, added square footage, or new heat-generating equipment.
• Duct losses: supply trunk leaking into a 130 F attic loses capacity before it reaches the room.
• True undersizing: a Manual J shortfall the system was always going to have on design days.

Branch C: Space loses ground, split low (under 14 F)

The equipment is not making rated capacity. This is the gauges branch:

• Low subcooling plus high superheat: undercharge from a slow leak. Capacity fades as charge drops, and the customer notices it first as longer runtimes. Leak search before charging.
• Low superheat plus high static: low airflow starving the coil.
• High superheat plus normal subcooling plus high head: dirty condenser coil rejecting heat poorly, capacity dropping as outdoor temperature climbs.

Branch D: Equipment cycles normally but the customer perceives constant run

A fan set to ON or a circulate mode runs the blower between calls. A tight thermostat differential causes frequent short cycles the customer hears as constant operation. Confirm at the thermostat configuration, not the condenser. On a variable-speed or inverter system, low-stage operation that runs for long stretches at reduced capacity is the design intent for comfort and efficiency, and a customer used to a single-stage unit cycling on and off may report the steady low-stage run as a fault when it is correct behavior.

The master fork restated

Every branch above hinges on one measurement taken early: is the indoor temperature holding setpoint or climbing while the equipment runs continuously. Continuous run that holds setpoint is a load-and-runtime question, not a repair, and the work is education, configuration, or efficiency improvement. Continuous run with a rising indoor temperature is a capacity deficit, and the work is finding what is robbing capacity (charge, airflow, condenser, or true undersizing) or adding load (a stuck damper, an open fresh-air path, duct losses). Resolving that fork first keeps you from putting gauges on a system that is simply doing its job on a hot day.

Confirming diagnosis

• Normal long runtime: space holds setpoint, split 18 to 22 F, runtime tracks outdoor load. Documented, not repaired.
• Undercharge capacity loss: subcooling below 5 F, leak located.
• Condenser fouling: high head pressure, condenser air-side matted, head drops after cleaning.
• Load gain: a specific stuck damper, open fresh-air path, or duct leak located physically, with runtime improving after correction.
• Configuration: differential or fan mode confirmed at the stat as the cause of perceived constant run.

Remediation

• Load gain: close or repair the stuck damper, seal duct leakage, correct the return path, re-measure runtime.
• Undercharge: repair leak, evacuate to 500 microns, weigh in nameplate charge, verify subcooling.
• Condenser fouling: clean the coil with a non-acid cleaner, re-read head pressure.
• Undersizing: present a Manual J-based capacity finding; band-aiding an undersized system with overcharge is not a fix.
• Configuration: set a reasonable differential, set fan to AUTO if continuous circulation is unwanted.

References

• ACCA Manual J Residential Load Calculation, 8th Edition
• AHRI Standard 210/240-2023 Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment
• 40 CFR Part 82 Subpart F (EPA Section 608 Stationary Refrigeration and Air Conditioning Rules)
• ASHRAE Handbook of Fundamentals, 2021 Edition, Chapter 18 Nonresidential Cooling and Heating Load Calculations