Refrigerators, Air Conditioners, and Heat Pumps: COP, Load, and Common Failure Modes

What These Machines Actually Do (and What to Measure)

Refrigerators, air conditioners, and heat pumps are the same core machine used in different ways. They all move heat “uphill” from a colder place to a warmer place by consuming input work (usually electrical power to a compressor and fans). The practical difference is which side you care about:

• Refrigerator: useful effect is removing heat from the insulated box (cold side).
• Air conditioner: useful effect is removing heat from indoor air and rejecting it outdoors.
• Heat pump: useful effect is delivering heat to the indoor space (warm side), typically for heating.

For makers and troubleshooters, the most useful mindset is: there are always two heat flows and one power input. If you can estimate any two of them, you can sanity-check the third. In the field, you rarely measure heat flow directly, so you infer it from temperatures, pressures, airflow, and electrical power.

Key measurable signals

• Electrical input: watts to compressor + fans (plug-in power meter for small units; clamp meter + voltage for larger).
• Air-side temperatures: return and supply air temperatures across the indoor coil; outdoor air across the outdoor coil.
• Airflow: approximate CFM (fan curve, anemometer, or “good enough” checks like filter condition and duct restrictions).
• Refrigerant-side pressures and line temperatures: suction and discharge pressure, suction line temperature, liquid line temperature (service gauges + pipe clamps).
• Coil condition: frost/ice patterns, dirt, bent fins, blocked airflow.

COP: Coefficient of Performance You Can Use

For these machines, “efficiency” is usually expressed as COP rather than a percentage. COP is a ratio of useful heat moved to input work. There are two common COPs:

• COP_cooling (refrigerator/AC): COP = Q_cold / W
• COP_heating (heat pump): COP = Q_hot / W

Because the machine moves heat rather than creating it, COP can be greater than 1. A heat pump with COP 3 delivers about three units of heat indoors for each unit of electrical energy consumed.

How COP relates to what you feel

When the temperature difference between indoors and outdoors is small, the machine can move heat with less “effort,” so COP is higher. As the lift (the gap between the cold-side and hot-side conditions) increases, COP drops. Practically:

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• AC on a mild day: tends to have higher COP than on a very hot day.
• Heat pump on a mild winter day: tends to have higher COP than on a very cold day.
• Dirty coils, low airflow, wrong charge: all reduce COP because they force larger temperature differences and/or reduce heat transfer.

Field-friendly COP estimation (air-side method)

You can estimate cooling capacity from air temperature drop and airflow. This won’t be lab-accurate, but it is excellent for diagnosing “why is it weak?”

Step-by-step (ducted AC or heat pump in cooling mode):

• 1) Measure return and supply dry-bulb temperatures at the indoor unit (avoid measuring right at the coil face where stratification is strong). Record ΔT = T_return − T_supply.
• 2) Estimate airflow (CFM). Use manufacturer data, fan speed setting, static pressure measurement if available, or a reasonable estimate based on system size and blower settings.
• 3) Estimate sensible cooling: Q_sensible ≈ 1.08 × CFM × ΔT (BTU/hr). This ignores moisture removal; in humid climates total cooling is higher than sensible.
• 4) Measure electrical power input to the whole outdoor unit (and indoor blower if you want system COP). Convert to BTU/hr: W × 3.412 = BTU/hr.
• 5) Compute an approximate COP: COP ≈ Q_cooling / (W × 3.412) if Q is in BTU/hr and W is in watts.

Notes: If humidity is high, the coil condenses water and latent cooling can be significant. A low ΔT does not always mean low capacity; it might mean high latent load. If you can measure humidity (return and supply), you can estimate total capacity more accurately, but even the sensible-only method is a strong diagnostic when compared to past readings on the same system.

Field-friendly COP estimation (heating mode)

In heating mode, you can do a similar air-side estimate using ΔT = T_supply − T_return. For ductless mini-splits, airflow is harder to estimate; you can still use temperature rise and fan setting as a relative indicator and compare to nameplate input power.

Load: The Heat You Must Move (and Why It Changes)

Load is the rate at which heat must be removed (cooling load) or delivered (heating load) to hold a target temperature. Load is not a property of the machine; it is a property of the space or box and its environment. The machine’s job is to match the load on average.

Common sources of load

• Infiltration and ventilation: outdoor air leaking in (hot/humid in summer; cold/dry in winter).
• Internal gains: people, cooking, electronics, motors, lighting.
• Solar gains: sun through windows and heating of building surfaces.
• Conduction through insulation: walls, roof, refrigerator cabinet, door seals.
• Moisture load: humidity removal in cooling mode; defrost cycles and door openings in refrigerators.

Why load matters for COP and reliability

Load determines runtime and operating conditions. A system that is undersized runs continuously, often at high temperature lift, reducing COP and increasing wear. A system that is oversized may short-cycle (frequent starts), which can reduce efficiency and stress compressors and contactors. Variable-speed systems mitigate this by modulating capacity, but they still suffer when airflow is restricted or coils are fouled.

Practical load checks you can do

• Refrigerator: check door gasket sealing (paper-strip test), frequency of door openings, and whether warm food is being loaded frequently.
• AC: check for open windows/doors, attic access leaks, return duct leaks, and clogged filters.
• Heat pump: check if auxiliary electric heat is running unexpectedly (big COP killer), and whether outdoor unit is icing excessively.

Performance Indicators: Superheat, Subcooling, and Approach Temperatures

When diagnosing vapor-compression systems, technicians often use superheat and subcooling as “health indicators” of refrigerant charge and metering behavior. You don’t need to re-derive the cycle to use them; you need to measure pressures and line temperatures correctly.

Superheat (evaporator outlet protection)

• Meaning: how many degrees the suction vapor is above its saturation temperature at suction pressure.
• Why it matters: too low can risk liquid refrigerant returning to the compressor (liquid slugging); too high often indicates starved evaporator (low charge, restriction, or low airflow).

Step-by-step:

• 1) Measure suction pressure at the service port.
• 2) Convert suction pressure to saturation temperature for the refrigerant (use a PT chart/app).
• 3) Measure suction line temperature near the evaporator outlet (insulate the probe from ambient air).
• 4) Superheat = T_{suction line} − T_saturation.

Subcooling (liquid line quality)

• Meaning: how many degrees the liquid line is below saturation temperature at the measured high-side pressure.
• Why it matters: too low can indicate low charge or flash gas in the liquid line; too high can indicate overcharge or a restriction causing liquid backing up in the condenser.

Step-by-step:

• 1) Measure discharge/high-side pressure.
• 2) Convert to saturation temperature for the refrigerant.
• 3) Measure liquid line temperature leaving the condenser.
• 4) Subcooling = T_saturation − T_{liquid line}.

Approach temperatures (coil effectiveness)

Another maker-friendly metric is approach: how close the coil gets the air to the refrigerant’s effective temperature. Large approaches can indicate dirty coils, low airflow, or refrigerant-side issues. Examples:

• Evaporator approach: return air temperature minus evaporating saturation temperature.
• Condenser approach: condensing saturation temperature minus outdoor air temperature.

These are powerful because they connect what you can measure (air temps, pressures) to what the machine is “trying” to do.

Common Failure Modes and What They Look Like

Most real-world problems show up as one of a few patterns. The goal is to recognize the pattern, confirm with a small set of measurements, and avoid “parts cannon” repairs.

1) Airflow problems (indoor or outdoor)

Causes: clogged filter, dirty evaporator coil, blocked return, closed registers, failing blower motor/capacitor, dirty condenser coil, blocked outdoor fan, bent fins, debris around outdoor unit.

Symptoms:

• Reduced capacity, longer runtime, higher power draw.
• In cooling: indoor coil may freeze; suction pressure often low; superheat can be high or low depending on metering and icing progression.
• In heating (heat pump): outdoor coil may frost quickly; indoor supply air may feel lukewarm; system may rely on backup heat.

Practical checks:

• Inspect and replace/clean filters; verify blower wheel cleanliness.
• Check coil face for dirt or ice; verify condensate drain is clear (standing water can re-evaporate and worsen humidity).
• Measure temperature split across coil; compare to typical values for that system and conditions.
• Outdoor unit: ensure clearances, clean coil, verify fan speed and direction.

2) Low refrigerant charge (leak)

Causes: slow leaks at flare fittings, Schrader cores, brazed joints, evaporator corrosion, vibration-induced cracks, service valve caps missing or loose.

Symptoms:

• Reduced capacity; longer runtime; poor dehumidification in AC.
• Low suction pressure; often higher superheat; subcooling often low.
• Evaporator may show partial frosting near the inlet, with the rest of the coil warm (starved coil pattern).

Practical step-by-step diagnostic:

• 1) Confirm airflow is acceptable first (a low-airflow system can mimic low charge).
• 2) Measure superheat and subcooling; compare to manufacturer targets if available.
• 3) Inspect for oil stains at joints and coils (oil often travels with refrigerant).
• 4) Leak-check with soap solution on accessible joints; for deeper checks use an electronic leak detector.
• 5) If a leak is confirmed, repair before recharging; otherwise the “fix” won’t last.

3) Overcharge or non-condensables

Causes: adding refrigerant without proper measurement, charging by pressure alone, air left in system after improper evacuation.

Symptoms:

• High head pressure; higher compressor power; reduced COP.
• Subcooling may be high (overcharge) while superheat may be low to normal.
• Condenser runs very hot; outdoor fan may run constantly; safety trips possible.

Practical checks: verify condenser coil cleanliness and outdoor airflow first; then evaluate subcooling and head pressure relative to outdoor temperature. Non-condensables often show as persistently high head pressure even with good airflow and correct charge procedures.

4) Metering device problems (capillary tube, fixed orifice, TXV/EEV)

Causes: debris, moisture/ice at the restriction, waxy oil deposits, TXV bulb issues, EEV control faults.

Symptoms depend on failure direction:

• Restriction/underfeeding: low suction pressure, high superheat, low capacity; may see temperature drop across the restriction and a cold spot where flashing begins.
• Overfeeding (TXV stuck open, bulb detached): high suction pressure, low superheat, risk of floodback; compressor may sound different, and oil return can be affected.

Practical checks: look for a sharp temperature change across filter-drier or cap tube inlet; measure superheat stability (TXV hunting shows oscillations); verify sensing bulb placement/insulation on TXV systems.

5) Compressor and electrical failures

Causes: start capacitor failure, run capacitor drift, relay/contactor issues, winding insulation breakdown, overheating from high head pressure or poor cooling, liquid slugging damage, inverter drive faults (variable-speed units).

Symptoms:

• Hard starting, humming then tripping breaker, intermittent operation.
• High current draw, overheating shell, thermal overload trips.
• Low or no pressure differential (worn valves) leading to poor capacity even though it “runs.”

Practical step-by-step checks (maker-safe level):

• 1) Measure supply voltage under load; low voltage can mimic bad compressor starts.
• 2) Measure running current and compare to nameplate.
• 3) Inspect capacitors (bulging/leaking) and test capacitance if you have a meter.
• 4) Listen for short-cycling or repeated start attempts; check for overheated wiring or burnt contactor points.

Note: Refrigerant systems involve high pressure and mains voltage. If you are not trained and equipped, limit yourself to non-invasive measurements (air temps, power draw, visual inspection) and hand off sealed-system work appropriately.

6) Defrost and icing problems (heat pumps and refrigerators)

Causes: failed defrost heater, defrost thermostat/sensor failure, control board faults, blocked drain causing ice buildup, door gasket leaks adding moisture, low airflow across coil.

Symptoms:

• Refrigerator/freezer: frost blanket on evaporator coil, weak airflow from vents, warmer cabinet temps, fan noise changes.
• Heat pump outdoor unit: heavy ice accumulation, reduced heating capacity, frequent defrost cycles or no defrost at all.

Practical checks:

• Inspect coil frost pattern: light, even frost can be normal; a solid ice block indicates a defrost/airflow issue.
• Verify evaporator fan operation (refrigerators) and that air passages are not blocked by food packaging.
• Check drain pan and drain tube for ice blockage; a clogged drain often causes recurring ice sheets.

7) Sensor and control problems (thermistors, pressure switches, boards)

Causes: drifted thermistors, loose connectors, moisture ingress, miscalibrated sensors, pressure switch trips due to airflow issues, firmware/control logic faults.

Symptoms:

• System runs too long or not long enough; erratic cycling.
• Defrost timing wrong; fan behavior odd; error codes on modern units.
• Heat pump stuck in one mode due to reversing valve control issues.

Practical checks:

• Read fault codes if available; document conditions when they occur.
• Measure sensor resistance vs temperature if service data is available.
• Wiggle-test connectors (power off) and inspect for corrosion.

Step-by-Step Troubleshooting Workflows

Workflow A: “Not cooling enough” (AC or refrigerator symptom)

• 1) Confirm the complaint with numbers: measure indoor return/supply temps (AC) or cabinet/freezer temps (refrigerator). Note ambient conditions.
• 2) Check airflow and cleanliness first: filters, coils, fans, blocked vents, dirty condenser, door gaskets (refrigerator).
• 3) Check electrical input: is power draw unusually high (restricted condenser, overcharge) or unusually low (underfeeding, weak compressor)?
• 4) Look for icing patterns: iced evaporator suggests airflow/defrost problems; partial frost suggests low charge or restriction.
• 5) If you have gauges: measure suction/discharge pressures, superheat, subcooling; compare to expected targets and ambient conditions.
• 6) Decide the branch: airflow/coil issue, charge/leak issue, metering issue, compressor issue, or controls.

Workflow B: “High power bill / low COP”

• 1) Measure real power (watts) over a representative period; note duty cycle.
• 2) Check condenser-side heat rejection: dirty outdoor coil, blocked airflow, failing fan, recirculating hot air.
• 3) Check indoor-side heat pickup: low airflow, dirty evaporator, duct leaks, poor return path.
• 4) Check operating lift indicators: high head pressure and high condensing temperature relative to outdoor air are classic COP killers.
• 5) For heat pumps: verify auxiliary heat isn’t running unnecessarily; check defrost behavior and outdoor coil condition.

Workflow C: “Short cycling”

• 1) Identify what is stopping the cycle: thermostat satisfied, safety switch trip, control board reset, thermal overload.
• 2) Check airflow and coil cleanliness: many safety trips are secondary to overheating (high head) or freezing (low suction).
• 3) Check electrical components: weak capacitors, contactor chatter, low voltage under load.
• 4) Check refrigerant-side indicators: extreme pressures, very low superheat (floodback) or very high superheat (starvation) can cause unstable operation.

Practical Examples Makers Can Replicate

Example 1: Quick COP sanity check on a window AC

Goal: determine whether a “weak” unit is actually underperforming or simply facing a high load.

• Measure input power with a plug-in watt meter.
• Measure return and supply air temperatures at the grille (avoid direct line-of-sight to the coil).
• If you can estimate airflow (even roughly), compute sensible cooling with Q ≈ 1.08 × CFM × ΔT.
• Compare Q to electrical input converted to BTU/hr (W × 3.412). If COP seems far lower than expected for that unit class, inspect coils and airflow before suspecting refrigerant issues.

Example 2: Diagnosing a refrigerator with warm fresh-food section but cold freezer

Likely pattern: evaporator coil is cold, but airflow to fresh-food is blocked or the evaporator fan is failing.

• Check that the evaporator fan runs when the door switch is pressed (if accessible).
• Inspect vents between freezer and fresh-food for ice blockage.
• Look for heavy frost on the evaporator cover panel (suggesting defrost failure).
• Check door gaskets for leaks that add moisture and accelerate frosting.

Example 3: Heat pump struggling in heating mode with frequent defrost

Likely pattern: outdoor coil is not rejecting/absorbing heat effectively due to airflow restriction or low refrigerant mass flow.

• Confirm outdoor fan operation and coil cleanliness; remove leaves and ensure clearances.
• Observe frost: a thin uniform frost that clears during defrost is normal; thick ice that persists suggests a defrost control issue or poor heat transfer.
• Measure power draw and indoor temperature rise; if power is high but heat output is low, suspect high lift (dirty coil, restricted airflow) or refrigerant-side problems.

Common Mistakes That Create “Phantom” Problems

• Charging by pressure alone: pressures depend strongly on conditions; use superheat/subcooling targets and proper procedures.
• Ignoring airflow: many refrigerant-side symptoms are caused by dirty filters/coils or failing fans.
• Measuring temperatures poorly: uninsulated clamp probes read ambient air; always insulate the sensor from surrounding air and ensure good contact.
• Not accounting for latent load: in humid weather, a system can be doing lots of moisture removal with a modest temperature split.
• Skipping the “simple” checks: blocked condenser coils, crushed ducts, stuck dampers, and bad door seals are common and cheap to fix.

Quick Reference: Symptom-to-Cause Map

• Low cooling + partial evaporator frost + low suction + high superheat: likely low charge or restriction.
• Low cooling + full evaporator icing + low airflow signs: filter/coil/blower problem or defrost issue.
• High power + very hot condenser + high head pressure: dirty condenser, poor outdoor airflow, overcharge, or non-condensables.
• Short cycling + safety trips: overheating (high head), freezing (low suction), electrical start components, or control faults.
• Heat pump low heat + aux heat running: outdoor coil icing/airflow issues, defrost faults, or refrigerant-side capacity loss.