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newSchoolWorks.com › VoTechWorks › Building & Construction › HVAC/R › Heat Pumps: Principles, Installation, and Service › Lesson 11
BLD-HVAC07-04 · Lesson 11 · 18 clock hours · practice

Systematic Troubleshooting: Diagnosis of Common Heat Pump Faults

Objective. The student will apply a systematic diagnostic procedure to identify the root cause of at least four instructor-seeded faults in a heat pump trainer and document findings and corrective actions on a standardized service report.

Why Systematic Troubleshooting Matters

A heat pump can fail in dozens of ways, and the symptoms of one fault often look like those of another. Guessing wastes refrigerant, parts, and time — and can expose you or the customer to unnecessary hazards. A systematic approach forces you to gather real data, compare it to specification, and isolate the root cause before touching a single component. That discipline is the core skill tested by NATE, HVAC Excellence, and NCCER credentialing bodies, and it is what separates a competent technician from a parts-changer.

Safety Before You Measure Anything

Before connecting instruments or opening any panel, follow these non-negotiable steps:

The Four-Step Diagnostic Procedure

Use this sequence on every fault, every time:

Step 1 — Gather Symptoms

Interview the "customer" (your instructor or the written fault card). Ask: What is the unit doing or not doing? When did it start? Is the problem constant or intermittent? Walk around the unit and use your senses — look for frost patterns, ice, oil stains, or tripped breakers; listen for unusual sounds; smell for burnt insulation.

Step 2 — Measure

Collect quantitative data with calibrated instruments. Record every reading with its unit and the ambient conditions at the time:

Step 3 — Compare to Specification

Every reading means nothing without a reference. Use the equipment nameplate, manufacturer's charging chart, and the NATE or HVAC Excellence reference guides in your materials packet. Ask: Is this reading high, low, or normal? Does it match what the fault symptom predicts?

Step 4 — Isolate the Root Cause

Work from the most likely, least invasive test toward the most invasive. Eliminate possibilities one at a time. For example, high discharge pressure with normal suction pressure points toward a condenser problem (dirty coil, failed fan, overcharge) rather than a compressor issue. Confirm your conclusion by performing a corrective action and verifying that the symptom disappears.

Common Heat Pump Faults and Their Signatures

Overcharged system: High discharge pressure, high suction pressure, low superheat, possible liquid slugging. Subcooling will be higher than the manufacturer's target. Corrective action: recover refrigerant to achieve target subcooling.

Failed reversing valve solenoid: The unit stays in one mode regardless of thermostat command (heating or cooling only). Test the solenoid coil with a DMM — an open coil reads infinite resistance (OL); a shorted coil reads near zero ohms. Verify 24 V is present at the coil terminals before condemning the coil. Corrective action: replace the solenoid coil or the reversing valve assembly as needed.

Open defrost sensor (thermistor): The defrost cycle never initiates, or the board faults out. A thermistor with an open circuit reads out-of-range resistance on a DMM when compared to the manufacturer's resistance-temperature (R-T) table. Corrective action: replace the sensor.

Restricted filter (air-side): Low suction pressure, low superheat, possible icing of the indoor coil in cooling mode or reduced airflow in heating mode. Measure static pressure drop across the filter and compare to design. Corrective action: replace or clean the filter.

Completing the Service Report

Each report must contain five sections: (1) observed symptoms, (2) instruments used and readings obtained, (3) comparison to specification with the specification source cited, (4) root cause conclusion, and (5) corrective action taken or recommended. Incomplete reports — missing readings or missing specifications — do not count as passing diagnoses. Think of the report as your legal record: a future technician should be able to read it and understand exactly what you found and why.

Debrief Protocol

During the group debrief, be ready to explain your diagnostic path, not just your answer. Your instructor will ask: Could another fault produce the same symptom? What would you check next if your first test had been inconclusive? This is how NATE exam questions are written — they test reasoning, not memorization.

Key terms8

SuperheatThe number of degrees a vapor is above its saturation temperature at a given pressure; measured at the suction line to evaluate proper refrigerant charge and metering device operation.
SubcoolingThe number of degrees a liquid refrigerant is below its saturation temperature at a given pressure; measured at the liquid line and used to verify adequate charge on the high side.
Reversing valveA four-way solenoid-operated valve that redirects refrigerant flow to switch a heat pump between heating and cooling modes.
ThermistorA temperature-sensitive resistor used as a sensor; resistance changes predictably with temperature and is verified against a manufacturer resistance-temperature (R-T) table.
Lockout/tagout (LOTO)An OSHA-required procedure that isolates hazardous energy sources and prevents accidental re-energization while a technician works on equipment.
Running load amperage (RLA)The normal operating current draw of a motor under rated load conditions, listed on the nameplate; compared to measured amperage to detect motor or electrical problems.
Manifold gauge setA set of two calibrated pressure gauges, valves, and hoses used to read suction and discharge pressures and to add or recover refrigerant from a system.
Defrost cycleAn automatic heat pump operating mode that temporarily reverses refrigerant flow to melt frost accumulation from the outdoor coil, controlled by time, temperature, or pressure sensors.

Worked examples4

1. You measure suction pressure of 102 psig on an R-410A system. The suction line temperature is 55 °F. The saturation temperature at 102 psig for R-410A is 40 °F. What is the suction-line superheat, and is it within a typical target range of 10–20 °F?
  1. Identify the measured suction line temperature: 55 °F.
  2. Identify the saturation temperature corresponding to the suction pressure (102 psig R-410A): 40 °F.
  3. Calculate superheat: Suction line temp − Saturation temp = 55 °F − 40 °F = 15 °F.
  4. Compare to the target range of 10–20 °F: 15 °F falls within that range.
Answer: Superheat = 15 °F, which is within the normal target range. The charge appears correct from the suction-side perspective.
2. On the same R-410A system, the liquid line pressure is 410 psig and the liquid line temperature is 95 °F. The saturation temperature at 410 psig for R-410A is 105 °F. Calculate subcooling and determine whether the system is overcharged if the manufacturer specifies 10–15 °F subcooling.
  1. Identify the saturation temperature at the measured liquid line pressure (410 psig R-410A): 105 °F.
  2. Identify the measured liquid line temperature: 95 °F.
  3. Calculate subcooling: Saturation temp − Liquid line temp = 105 °F − 95 °F = 10 °F.
  4. Compare to the manufacturer target of 10–15 °F: 10 °F is at the low end but within specification.
Answer: Subcooling = 10 °F, which is within the manufacturer's specified range. The system is not overcharged based on this reading.
3. A technician suspects the defrost termination thermistor is open. At the current outdoor coil temperature of 32 °F, the manufacturer's R-T table lists the thermistor resistance as 10,000 ohms (10 kΩ). The DMM reads 'OL' (overload/infinite resistance) across the thermistor terminals after LOTO is applied and the component is disconnected. What is the diagnosis?
  1. Apply LOTO and verify zero voltage at the thermistor circuit before disconnecting.
  2. Disconnect the thermistor leads to isolate the component.
  3. Set the DMM to resistance (Ω) and measure across the thermistor terminals.
  4. Compare the DMM reading (OL, meaning infinite resistance) to the specification (10,000 Ω at 32 °F).
  5. An infinite resistance reading indicates an open circuit inside the thermistor.
Answer: The thermistor has an open circuit and must be replaced. An open defrost sensor will prevent the defrost board from initiating or terminating the defrost cycle correctly.
4. A clamp-on ammeter reads 18 A on the compressor common leg. The nameplate lists RLA = 14 A and LRA = 84 A. The supply voltage is correct at 240 V. Is the compressor drawing normal current, and what does the elevated reading suggest?
  1. Record the measured current: 18 A.
  2. Note the nameplate RLA: 14 A.
  3. Calculate the percentage over RLA: (18 − 14) ÷ 14 × 100 = 28.6% above RLA.
  4. A compressor drawing significantly more than RLA at correct voltage suggests a mechanical problem (such as tight bearings or slugging), an electrical issue (failing winding), or a system condition causing abnormally high head pressure.
  5. Cross-reference with discharge pressure and subcooling readings to determine whether the elevated amperage is driven by refrigerant-side or mechanical causes.
Answer: The compressor is drawing 28.6% above its nameplate RLA, which is abnormal. Further investigation of discharge pressure, refrigerant charge, and compressor windings is required before condemning or operating the unit.

The math in this lesson2

Learners compute superheat and subcooling from line temperatures and R-410A saturation temperatures.
7.NS.17.RP.3
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor seeds four faults (for example: overcharged system, failed reversing valve solenoid, open defrost sensor, restricted filter) in the heat pump trainer; students use a systematic approach — gather symptoms, measure, compare to specification, isolate fault — without being told what the faults are. · Students complete a service report for each fault: list observed symptoms, instruments and readings used, comparison to specification, conclusion (root cause), and corrective action taken or recommended. · Debrief as a group: each student presents one fault diagnosis; class discusses alternative diagnostic paths and any safety steps that must be taken before and during each measurement.
Materials: Heat pump trainer unit with seeded faults, manifold gauge set, digital multimeter, clamp-on ammeter, digital thermometer, service report forms, NATE Air-to-Air Heat Pump and HVAC Excellence troubleshooting references
Check: Four service reports reviewed: student correctly identifies root cause and corrective action for at least three of four faults; all reports include instrument readings compared to specification; pass mark: 3 of 4 faults correctly diagnosed with supporting data.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NATE — Heat Pump System Diagnosis
1. A technician notices that a heat pump runs in cooling mode only, regardless of the thermostat setting. The FIRST electrical check on the reversing valve solenoid coil should be:
EPA 608 / HVAC Excellence — Refrigerant System Fundamentals
2. Measured subcooling is significantly higher than the manufacturer's specification. This reading most directly indicates:
EPA 608 — Regulatory Requirements
3. Before attaching manifold gauge hoses to an operating heat pump, the technician must:
NCCER HVAC — Troubleshooting Air Distribution
4. A heat pump's indoor coil is icing over during a cooling cycle. Suction pressure is low and superheat is below the normal target. The MOST likely cause is:
HVAC Excellence — Electrical Diagnosis
5. A DMM reads 'OL' when measuring resistance across a disconnected defrost thermistor at a temperature where the manufacturer specifies 10 kΩ. This result means the thermistor is:
NCCER HVAC — Safety
6. Which safety step is required by OSHA before a technician removes a panel and works on an electrical component inside a heat pump cabinet?
NATE — Diagnostic Reasoning
7. During the debrief, a classmate argues that high discharge pressure alone is enough to condemn the reversing valve. Which statement BEST explains why this reasoning is flawed?
HVAC Excellence — Professional Documentation
8. A service report is considered incomplete and does not count as a passing diagnosis if it:
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