Air Conditioning Flowcharts

An AC call splits cleanly into two categories at the first branch: electrical faults (no cooling because something isn’t getting power or a control signal) and refrigerant/mechanical faults (the system runs but doesn’t cool, or cools poorly, or ices up). Most electrical faults are within scope for a general HVAC tech. Refrigerant faults quickly cross into EPA 608 territory and — for a tech working up from plumbing — are often the point where you do diagnostic value and hand off the repair to someone with more specialized training.

The flowchart discipline from Chapter 53 applies identically here: read the question, take the measurement, pick the branch, back up if the tree’s answer doesn’t match reality. What changes is the specific test points and the specific failure modes.

How the AC no-cool call typically goes

The service call sounds like one of four complaints:

1. “AC isn’t running at all.” Blower off, outdoor unit silent. Mostly electrical.
2. “Blower runs but outdoor unit doesn’t.” Classic split — thermostat is calling but Y signal isn’t closing the contactor, or 24V is blowing a fuse, or the contactor is mechanical-failed.
3. “Everything runs but it’s not cold.” System is operating but not transferring heat — refrigerant, airflow, or coil issue.
4. “Runs fine for a while then gets warm / blows ice cold then no air.” Frozen coil, intermittent compressor lockout, or thermostat placement issue.

Each maps to a different branch at the top of the tree.

The AC no-cooling decision tree — written walkthrough

Until the AC flowchart is authored into the interactive widget, here’s the structure you’d follow mentally or on paper. Future versions of this chapter will drop it into FlowchartWidget using the same pattern as Chapter 53.

Branch 1 — nothing runs at all

Same as the furnace no-call branch: start at R-C on the indoor control board. 24 VAC present means the power side is intact and the problem is in the call path. 0 V means transformer or 120V supply. Low V means a shorted load dragging the secondary down.

Walk the thermostat demand path: Y at the thermostat → Y at the indoor board → Y at the outdoor contactor. Voltage disappears where the fault is.

Branch 2 — blower runs, outdoor doesn’t (the classic AC branch)

The indoor unit has 24V and G and Y are being sent — the blower runs, which proves G is getting through. But the outdoor unit is silent: no contactor click, no condenser fan, no compressor.

Three most common causes, in order:

1. Broken thermostat wire between indoor and outdoor — squirrels, landscapers, freezing damage at a whip connection. Measure 24V at the Y terminal on the outdoor contactor during a cool call. Absent? Wire is broken somewhere between indoor and outdoor. Walk it.
2. Blown low-voltage fuse at the indoor board — a short in the outdoor control circuit (damaged wire, contactor coil shorted) blew the 3A or 5A fuse on the board. Replacing the fuse without finding the short blows it again in seconds.
3. Contactor coil failed or contactor mechanically stuck — 24V present at the coil terminals, but the contactor doesn’t pull in. Coil open (OL on ohms), or spring-welded contacts (coil energizes but contacts don’t close).

Less common but seen:

• Outdoor disconnect pulled — the pull-out disconnect at the outdoor unit may have been removed by an HVAC tech, electrician, or a homeowner who “heard the neighbor’s was the problem.” Simple visual check.
• Compressor protection locked out — modern compressors have thermal or pressure protection that can latch off; power cycling the outdoor breaker resets most.
• Capacitor failed to the point the compressor can’t start — hears a hum / click, then a thermal trip; contactor pulls in but the compressor draws locked-rotor current, the overload trips, and the contactor drops or the compressor buzzes.

Branch 3 — both run but no cold air

Now you’re out of pure electrical territory. Measure return-air and supply-air temperatures. A healthy AC gives a 15–20°F delta-T across the coil when the house has been running a while. Delta-T well below that suggests:

• Refrigerant low (needs EPA 608 work to diagnose with gauges)
• Dirty evaporator coil
• Excessive airflow (blower speed too high)
• Refrigerant overcharge (less common)

Delta-T above 20–25°F suggests:

• Insufficient airflow (dirty filter, stuck blower speed, restricted returns, blocked coil, bad blower motor)
• Iced coil starting to form (check visually or by feeling suction line for frost)

A totally iced coil gives very low delta-T (air barely moves at all, thermally very little exchange), so don’t assume low delta-T means refrigerant — check the coil first. The fix for a frozen coil is not more refrigerant; it’s airflow or a charge problem that caused the freezing.

Branch 4 — intermittent or short cycling

Watch the system through a complete cycle. Things to look for:

• Ice formation visible at suction line or indoor coil within 10–15 minutes of starting → refrigerant or airflow issue causing low evaporator temperature.
• Compressor starts, runs for a few minutes, trips off, restarts after 3–5 minutes → high-pressure cutout on high-side overheating (dirty condenser, failed condenser fan, restricted airflow outdoors), or low-pressure cutout on low charge.
• Condensate float switch tripping → clogged drain pan, drain line, or trap. Float rises, opens a switch in series with Y, shuts the system down until water drains and float falls.
• Thermostat placement causing fast satisfaction → thermostat in direct supply airflow, cycling off before the bulk of the house cools.

Frozen coil — the special case

A frozen evaporator coil is worth calling out because it’s common and it’s easy to misdiagnose. The symptoms change as the ice grows: initially the system cools fine, then cooling degrades, then airflow drops, then nothing blows from the registers at all (ice has completely blocked the coil). The homeowner calls when the last phase hits, and tech arrives to a system that’s been sitting for hours — the ice may have melted by then, and the system runs fine for the service call. An hour after the tech leaves, it freezes again.

Diagnostic sequence for suspected freeze:

The first four rows — filter, returns, blower, coil surface — are airflow issues, within scope for any tech. Fix the airflow, thaw the coil, run the system for an hour, and watch it. If it stays cold and doesn’t re-freeze, airflow was the issue. If it freezes again, charge is suspect.

The pattern

Whether you’re on a furnace or an AC, the flowchart discipline is the same: symptom at the top, branch by measurement, terminal diagnosis with next steps. What differs is the domain knowledge that informs which test to take at each branch. AC work heavily weights:

• Contactor voltage and coil behavior
• Capacitor health (run cap, dual cap, start cap if fitted)
• Thermostat wire integrity between indoor and outdoor units
• Airflow (because it drives everything refrigerant-adjacent)
• Float switches (safety device techs new to AC miss entirely)

Memorize the top-of-tree branch choices and the four most common next-measurements for each. That covers 80% of AC service calls. The remaining 20% is where refrigerant work, compressor diagnostics, and more specialized training start to matter.

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01Blower runs on a cooling call but the outdoor unit is silent. You measure 24 VAC at Y on the indoor board during the call, but 0 VAC at Y on the outdoor contactor. What's the most likely fault?

AContactor coil is openBThermostat wire between indoor and outdoor is broken somewhere along its runCThe condenser fan motor is badDLow refrigerant charge

02An AC is running but delta-T across the evaporator coil is 8°F — well below the expected 15–20°F. The air filter is clean, returns are clear, the blower is running normally. What's your next diagnostic step?

AImmediately replace the thermostatBShut off the system — likely a refrigerant or coil issue that needs EPA-608-certified work to confirmCIncrease the blower speedDTell the homeowner the system is working fine

03A homeowner reports the AC cools fine in the morning but stops blowing air entirely by afternoon. You arrive at 3 PM — no air from registers, supply lines cold to the touch, access panel shows ice on the evaporator coil. What's your first step?

ARun the system harder to blast through the iceBShut the system off and let the coil thaw before any further diagnosisCImmediately call a refrigerant techDReplace the thermostat