HVAC/R Systems Installation: Understanding Self-Contained AC, Central AC, and Air-to-Air Heat Pumps

Self-contained air conditioning units: basic components and what each one does (5.2.1)

A self-contained air conditioning unit is an air conditioner where the major refrigeration and air-moving parts are packaged together in one cabinet. Common examples include window units, many portable AC units, and packaged terminal air conditioners (PTACs) often seen in hotels. You can think of it as a complete “mini HVAC system in a box”—it both moves air and performs refrigeration without needing separate indoor and outdoor equipment connected by field-installed refrigerant piping.

Understanding the components matters because most troubleshooting and safe installation practices come down to recognizing (1) how heat is picked up indoors, (2) how it’s rejected outdoors, and (3) how air is delivered and drained safely. Even if you later work mostly on split systems (central AC and heat pumps), the same refrigeration cycle components show up again and again.

The vapor-compression refrigeration cycle (the idea that ties all components together)

Self-contained AC units almost always use the vapor-compression cycle. The goal is not to “make cold” but to move heat from the indoor air to the outdoor air.

At a high level:

• The system absorbs heat from indoor air at the evaporator coil.
• The system rejects heat to outdoor air at the condenser coil.
• A compressor and a metering device (expansion device) keep refrigerant moving and create the pressure/temperature conditions that make heat transfer possible.

A useful memory aid is COME:

• Compressor
• Outdoor coil (condenser)
• Metering device
• Evaporator

In a self-contained unit, “outdoor” and “indoor” sides are separated by partitions and directed airflow, even though they’re inside one cabinet.

Basic components you should be able to identify

Below are the core parts you’re typically expected to recognize in a self-contained AC unit, along with what they do and why they matter.

Compressor

The compressor is the pump of the refrigeration system. It takes low-pressure refrigerant vapor leaving the evaporator and compresses it into a higher-pressure, higher-temperature vapor.

• Why it matters: Without compression, the refrigerant wouldn’t reach temperatures/pressures that allow it to reject heat at the condenser.
• What goes wrong: A failed compressor can trip overloads, cause no cooling, or create unusual noises. A common misconception is that “a compressor makes cold.” It doesn’t—it enables heat movement.

Condenser coil (heat rejection coil)

The condenser is where hot refrigerant gives up heat to the “outdoor” airstream. As the refrigerant rejects heat, it condenses from vapor toward liquid.

• Why it matters: If the condenser can’t reject heat (dirty coil, blocked airflow), system pressures and temperatures rise and cooling capacity drops.
• What goes wrong: Dirty condenser fins, failed condenser fan, or blocked discharge air can lead to overheating and shutdown.

Condenser fan (outdoor fan)

The condenser fan moves outdoor air across the condenser coil.

• Why it matters: Heat transfer depends heavily on airflow. Even a good coil can’t reject heat if air isn’t moving.
• What goes wrong: Incorrect replacement fan direction or blade mismatch can reduce airflow significantly.

Metering device / expansion device

The metering device (often a capillary tube in small self-contained units) creates a pressure drop that allows refrigerant to expand and cool before entering the evaporator.

• Why it matters: This is what separates the high-pressure side from the low-pressure side and sets up the conditions for heat absorption.
• What goes wrong: Restrictions or incorrect device sizing can starve the evaporator, reducing capacity and potentially causing icing.

Evaporator coil (indoor cooling coil)

The evaporator is where refrigerant absorbs heat from indoor air. Moisture from indoor air often condenses on this cold coil—this is also how the unit dehumidifies.

• Why it matters: The evaporator provides the cooling you feel and removes moisture for comfort.
• What goes wrong: Dirty filters, blocked return air, or low airflow can cause coil icing. A common mistake is assuming ice always means “low refrigerant”—low airflow can do it too.

Evaporator fan / blower (indoor fan)

The evaporator fan circulates room air across the evaporator coil and back into the space.

• Why it matters: Cooling capacity depends on steady airflow across the evaporator.
• What goes wrong: A weak blower or clogged filter reduces airflow, increasing icing risk and reducing comfort.

Air filter

A filter captures dust and debris before air reaches the evaporator coil.

• Why it matters: A clean evaporator coil transfers heat efficiently; a dirty one insulates itself with dust.
• What goes wrong: A neglected filter is a common root cause for poor cooling and freeze-ups.

Condensate collection and drainage

Because the evaporator dehumidifies, water must go somewhere. Self-contained units typically use:

• A drain pan under the evaporator coil

• Some method of draining or slinging condensate (some window units sling water onto the condenser to improve efficiency)

• Why it matters: Poor drainage can cause leaks, microbial growth, odors, and corrosion.

• What goes wrong: Improper unit tilt (for window units) can cause water to drain indoors instead of outdoors.

Controls: thermostat and safety devices

Self-contained units include user controls and internal protections such as temperature sensors, overloads, and sometimes pressure-related safeties.

• Why it matters: Controls maintain comfort and protect equipment.
• What goes wrong: Misdiagnosis—people often blame “low refrigerant” when the real issue is a failed control sensor or a tripped safety.

“Two air paths” inside one box (a concept that prevents many installation errors)

A key idea: self-contained units manage two separate airstreams.

• Indoor airstream: room air passes through the filter and over the evaporator, then returns to the room cooled and dehumidified.
• Outdoor airstream: outside air passes over the condenser to remove heat.

If these airstreams mix due to missing panels, bad gaskets, or poor installation, performance drops quickly. For example, if hot condenser discharge air recirculates back into the condenser intake, the unit “breathes its own heat” and capacity falls.

Example: identifying components in a window unit by symptoms

Suppose a window unit runs but cools poorly, and the front grille feels warm rather than cool.

• Warm front discharge suggests the evaporator side isn’t absorbing heat effectively.
• Common component-related causes to check first (before assuming refrigerant loss):
  • Dirty filter reducing airflow across the evaporator
  • Evaporator fan not running at full speed
  • Evaporator coil clogged with dust/lint

The lesson: component identification isn’t just naming parts—it guides your diagnostic sequence.

Exam Focus

• Typical question patterns:
  • Identify parts on a diagram of a window/portable unit and state each part’s function (e.g., “What does the metering device do?”).
  • Explain why a unit ices up using airflow vs refrigeration-cycle reasoning.
  • Describe the two airstreams (indoor vs outdoor) and what happens if they mix.
• Common mistakes:
  • Confusing the condenser and evaporator (remember: evaporator absorbs heat indoors; condenser rejects heat outdoors).
  • Assuming “ice on coil = low refrigerant” without considering low airflow.
  • Forgetting condensate handling—installing without proper tilt/drain considerations.

Central air conditioning systems: identifying major assemblies and installing the system (5.2.2)

A central air conditioning system (most commonly a split system) cools an entire building or a large zone by separating equipment into an outdoor unit and an indoor unit, connected by refrigerant lines and controlled by a thermostat. It’s “central” because cooling is delivered through a building distribution system—typically ductwork and supply/return grilles.

This matters for installation because, unlike a self-contained unit, you create the system connections in the field: refrigerant piping, condensate drains, electrical power and controls, and airflow setup. Many performance problems in central AC come from installation quality rather than defective equipment.

What you should be able to identify in a typical split central AC system

A common split central AC system has these identifiable sections.

Outdoor condensing unit

This cabinet generally contains:

• The compressor
• The condenser coil
• The condenser fan

Why it matters: The outdoor unit’s job is heat rejection. Its location and clearances affect airflow and therefore capacity and reliability.

Indoor evaporator coil and air handler/furnace

Indoors, the system includes:

• The evaporator coil (often installed in a case/plenum)
• A blower that pushes air through the coil and into ductwork
• In many homes, the blower is part of a furnace or an air handler

Why it matters: The indoor unit determines airflow across the evaporator and controls condensation management. Incorrect airflow can cause humidity issues, icing, noise, and comfort complaints.

Refrigerant line set

The indoor and outdoor coils are connected by two refrigerant lines:

• Suction line (larger, insulated; carries cool, low-pressure vapor back to the compressor)
• Liquid line (smaller; carries high-pressure liquid from the condenser toward the metering device)

Why it matters: Line routing, insulation, leak-free joints, and keeping tubing clean/dry are essential. Moisture or debris can damage compressors and restrict metering devices.

Metering device

Depending on the design, the metering device may be at the indoor coil. Common types in split systems include thermostatic expansion valves (TXVs) or fixed orifices/pistons.

Why it matters: The metering device strongly affects evaporator performance and system charging procedure.

Condensate drain system

Cooling removes moisture; that water must drain safely via:

• A primary drain pan at the evaporator coil
• A drain line routed to an approved disposal point
• Sometimes a secondary (overflow) drain and/or float switch to prevent water damage

Why it matters: Drain problems cause ceiling damage, mold risk, and callbacks.

Electrical and controls

Central systems typically include:

• A thermostat
• A low-voltage control circuit (often powering contactors/relays)
• A disconnect near the outdoor unit for service safety

Why it matters: Safe lockout practices and correct control wiring prevent equipment damage and safety hazards.

How central AC works (step-by-step, from a building comfort perspective)

1. The thermostat calls for cooling.
2. The indoor blower moves warm return air across the evaporator.
3. The refrigerant in the evaporator absorbs heat; moisture condenses and drains away.
4. The compressor pumps refrigerant to the condenser.
5. The outdoor coil rejects heat to outdoor air.
6. Cooled, dehumidified air is supplied to the conditioned space through ducts.

The big installation takeaway: comfort depends on both the refrigeration circuit and the air distribution system. A perfectly charged AC unit cannot fix undersized returns, leaky ducts, or blocked airflow.

Installing a central air conditioning system (typical workflow and key quality checks)

Installation details vary by manufacturer, local codes, and system type—always follow the equipment instructions and applicable regulations. The steps below describe the typical logic of installation and the “why” behind each step.

1) Plan equipment location and service clearances

You choose locations for the outdoor unit and indoor coil/air handler to ensure:

• Adequate airflow through coils (avoid tight enclosures)
• Reasonable refrigerant line routing (avoid unnecessary length and sharp bends)
• Service access (panels must be removable for maintenance)
• Noise and vibration considerations (keep away from sensitive spaces when possible)

A common error is placing the outdoor unit where hot discharge air recirculates or where landscaping later blocks airflow.

2) Set and secure the outdoor unit

Typically the outdoor unit sits on a stable pad or mounting base. The goals are:

• Keep the unit level and secure
• Reduce vibration transmission
• Keep it above conditions where standing water or debris accumulate

3) Install the indoor evaporator coil and tie into ductwork

If installing an evaporator coil with an existing furnace/air handler:

• Ensure correct airflow direction through the coil (coils are designed for a specific airflow path)
• Seal duct connections to prevent air leakage
• Confirm filter location and accessibility

Air leakage and poor return design are frequent causes of low comfort even when the refrigeration side is fine.

4) Route and connect refrigerant lines (line set)

Good line-set work is both mechanical and cleanliness-focused:

• Route tubing with support, gentle bends, and protection from rubbing
• Insulate the suction line continuously to prevent energy loss and condensation on the line
• Keep tubing ends capped until connection to limit moisture/dirt entry

Connections are commonly made by brazing or flared fittings, depending on the system. Poor joints are a major leak source.

5) Pressure test and leak check (without introducing moisture)

A standard practice is to pressure-test with an inert, dry gas (often nitrogen) and then check joints for leaks using appropriate methods. The point is to verify tightness before evacuation and releasing refrigerant.

6) Evacuate (remove air and moisture) before opening the system to refrigerant

Evacuation uses a vacuum pump to remove non-condensable gases and moisture from the refrigerant circuit.

• Why it matters: Air and moisture reduce capacity, raise operating pressures, and can create acids that damage components over time.

A common misconception is that “purging refrigerant through the lines” is an acceptable substitute for proper evacuation—it is not an equivalent practice and may violate regulations.

7) Release or weigh in refrigerant charge per manufacturer procedure

Many outdoor units come with a factory charge intended for a specified line length; additional charge may be required depending on the actual installation. Proper charging methods are manufacturer- and metering-device-dependent.

• Why it matters: Incorrect charge can cause poor cooling, compressor damage, and efficiency loss.

8) Wire power and controls safely and correctly

Electrical work must follow applicable electrical codes and manufacturer instructions. From a systems standpoint, you should be able to identify:

• Line-voltage power feeding the outdoor unit
• Low-voltage control wiring between thermostat, indoor unit, and outdoor unit

Always confirm proper disconnecting means and safe service access.

9) Commissioning: verify airflow, temperature change, and drain operation

Commissioning is where you prove the system works as intended:

• Verify indoor airflow is not restricted (filter, coil, blower settings)
• Confirm condensate drains properly (no leaks, proper slope, safety switch function if present)
• Confirm the outdoor fan and compressor operate and that air leaving supply ducts is cooler than return air under steady operation

Example: diagnosing an installation issue vs an equipment failure

Scenario: A newly installed central AC cools poorly, and the outdoor unit is extremely hot with frequent shutdowns.

• If the outdoor unit lacks clearance or is in a confined area, airflow across the condenser may be restricted.
• That installation problem raises condensing temperature/pressure, which can trigger protective shutdowns.

The important reasoning skill: don’t jump straight to “bad compressor.” First evaluate installation factors that affect heat rejection.

Exam Focus

• Typical question patterns:
  • Label a split-system diagram (outdoor condenser, indoor evaporator, suction/liquid lines, metering device, condensate drain) and describe refrigerant flow direction.
  • Explain why evacuation and leak testing are required before startup.
  • Describe common installation checks that ensure performance (airflow, drainage, clearances).
• Common mistakes:
  • Treating central AC like a self-contained unit—forgetting that line-set workmanship (leaks, insulation, contamination) is critical.
  • Ignoring the condensate drain during installation and commissioning.
  • Misidentifying suction vs liquid line (suction is typically larger and insulated).

Air-to-air heat pumps: identifying components and installing for heating and cooling (5.2.3)

An air-to-air heat pump is a refrigeration system that can provide cooling in summer and heating in winter by reversing the direction of refrigerant flow. It’s called “air-to-air” because it transfers heat between indoor air and outdoor air (as opposed to geothermal systems that exchange with the ground).

This matters because installing a heat pump is similar to installing central AC—but with extra components, extra control wiring, and extra operational modes (including defrost). Many comfort complaints happen when the heat pump is installed correctly mechanically but set up incorrectly in controls (thermostat type, auxiliary heat staging, or defrost-related wiring).

The key concept: a heat pump doesn’t create heat—it moves it

In heating mode, the outdoor coil becomes the place where heat is absorbed from outdoor air, even when it feels cold outside. The refrigerant cycle is still moving heat; it’s just moving it in the opposite direction compared with cooling mode.

A practical analogy: a heat pump is like a reversible conveyor belt for heat. The compressor is the motor; the reversing valve is the switch that changes direction.

Components that make a heat pump different from straight AC

A heat pump includes the same core refrigeration components (compressor, indoor and outdoor coils, metering device), plus several additional parts you should be able to identify.

Reversing valve

The reversing valve changes the direction of refrigerant flow so the system can alternate between heating and cooling.

• Why it matters: This is the defining component of a reversible heat pump. If it fails or is mis-controlled, the system may be stuck in heating or cooling.
• What goes wrong: Miswiring the thermostat/control circuit can cause incorrect mode operation.

Defrost system

In heating mode, the outdoor coil can become cold enough that moisture freezes on it. Heat pumps use a defrost control strategy to periodically remove frost/ice.

• Why it matters: A frosted outdoor coil can’t absorb heat effectively, reducing heating capacity and efficiency.
• What goes wrong: If defrost controls fail, the outdoor unit may ice over heavily, leading to poor heating.

Auxiliary (backup) heat

Many air-to-air heat pump systems include auxiliary heat (often electric resistance heat or a furnace) to help during very cold weather or during defrost.

• Why it matters: The heat pump’s available heating capacity generally decreases as outdoor temperature drops. Auxiliary heat maintains comfort when the heat pump alone can’t keep up.
• What goes wrong: Incorrect thermostat configuration can prevent auxiliary heat from energizing when needed—or energize it too often, increasing energy use.

Indoor air handler and blower (often with heat strips)

If the system uses electric auxiliary heat, heat strips are typically installed in the air handler.

• Why it matters: The air handler must be correctly sized and wired for safe operation. Airflow is critical—low airflow can cause overheating of electric elements.

How an air-to-air heat pump operates in both modes

Understanding mode switching helps you correctly identify components and verify proper operation after installation.

Cooling mode (like central AC)

• Indoor coil functions as the evaporator (absorbs heat from indoor air).
• Outdoor coil functions as the condenser (rejects heat outdoors).

Heating mode (reversed)

• Outdoor coil functions as the evaporator (absorbs heat from outdoor air).
• Indoor coil functions as the condenser (releases heat to indoor air).

A frequent student mistake is thinking the “evaporator” is always indoors. In a heat pump, which coil is evaporator vs condenser depends on the mode.

Installing an air-to-air heat pump (what’s the same as central AC, and what’s added)

Mechanically, installing a heat pump resembles installing a central split AC: you still set equipment, run refrigerant lines, evacuate, charge, and commission. The differences show up in controls and in ensuring the system can operate safely in heating mode.

1) Equipment placement and refrigerant piping: same fundamentals, higher stakes

You still need:

• Proper outdoor clearances for airflow
• A solid mounting base
• Well-supported, insulated suction line
• Clean, leak-free refrigerant connections

But for heat pumps, outdoor placement deserves extra attention in cold climates because snow drifting, roof runoff, or poor drainage can create ice buildup around the unit. Installation planning should keep the unit where meltwater won’t refreeze into a block that restricts airflow.

2) Install the indoor unit with auxiliary heat considerations

If auxiliary electric heat is present, confirm the indoor unit installation supports:

• Correct airflow (to avoid overheating)
• Proper electrical supply capacity and safe wiring practices (per codes/manufacturer)
• Correct control staging so auxiliary heat isn’t running unnecessarily

3) Controls and thermostat setup: the most common installation pitfall

Heat pumps often require a thermostat and control wiring that supports:

• Reversing valve control (energized in heating or cooling depending on system design)
• Compressor contactor control
• Indoor fan control
• Auxiliary heat call
• Defrost and associated signals (depending on system)

Because control conventions vary by manufacturer, the safest installation mindset is: use the wiring diagram and setup instructions for that exact equipment, and verify operation mode-by-mode during commissioning.

4) Commissioning a heat pump: verify both heating and cooling performance

A complete startup verifies:

• Cooling mode: stable operation, good airflow, condensate draining
• Heating mode: warm supply air, correct reversing valve action
• Defrost behavior (as applicable): system can defrost without causing unsafe conditions indoors
• Auxiliary heat: energizes when commanded and shuts off appropriately

A common real-world error is only testing cooling on a mild day and leaving without confirming heating operation—then the first cold snap reveals a control or reversing valve issue.

Example: identifying which component is likely involved from a symptom

Scenario: The system is blowing cool air indoors when the thermostat is set to heat.

• This points to a mode problem—often involving the reversing valve or its control signal.
• Before concluding the valve is “bad,” you would verify:
  • Thermostat configuration for heat pump operation
  • Correct control wiring per diagram
  • Whether the reversing valve is being energized when it should be

This example highlights a key heat pump skill: symptoms often involve controls as much as refrigeration.

Central AC vs air-to-air heat pump: quick conceptual comparison (to prevent mix-ups)

Exam Focus

• Typical question patterns:
  • Explain how a heat pump provides heating (describe the reversed heat flow and role of the reversing valve).
  • Identify additional heat pump components (reversing valve, defrost controls, auxiliary heat) and state their purpose.
  • Given a scenario (icing outdoor coil, wrong mode operation), identify the likely system or control area involved.
• Common mistakes:
  • Claiming the evaporator is “always inside” (it changes with mode).
  • Forgetting auxiliary heat’s role during defrost or cold weather capacity limits.
  • Treating heat pump commissioning like AC commissioning—failing to test heating mode and control staging.