A visual learner’s companion, from foundation to deeper reasoning.
Generic single-zone packaged rooftop DX air conditioner with electric resistance heat, a centrifugal supply blower, a PSC condenser fan, fixed minimum outdoor air, and barometric relief. Six rooms share one thermostat. Educational cutaway; not a replica or installation design.
Editable Blender geometry. Upper walls and cabinet panels are removed for teaching.
How to work through the lab
Start at Foundation and choose Journeys → Follow the room air. Explain each stop aloud before moving on.
Select a component you recognize from field experience. Compare its job with a nearby part whose name sounds similar.
Switch to Working knowledge without changing your selection. Look for what the component depends on.
At Advanced, identify the measurements and assumptions you would need. The example arithmetic is not a field target.
Open Field cases, predict what should change, and compare with the stated evidence. Finish with Test yourself at each level.
Three levels, one model. Level 1 establishes a serious first-month foundation. Level 2 connects components and operating conditions. Level 3 adds measurement reasoning and ambiguity. No level is locked.
The coil walls separate air from refrigerant. The blower moves air; the compressor acts on refrigerant. Heating here uses electric elements. It does not reverse the refrigeration cycle.
The supply blower pushes air into this branching network. Each diffuser spreads the air into a room so it can mix with room air. In cooling, supply air is cooler than the room; in heating, it is warmer. Follow the blue markers down from the rooftop unit, along the main duct, and into all six rooms.
Level 2 · Working knowledge
Duct resistance and balancing determine how much air reaches each branch. Closing one outlet changes the network; it does not create an independent temperature zone. This building has one thermostat and no zone dampers. Air distribution and the building load must be considered together.
Level 3 · Advanced
Airflow is set by the fan and system operating point. External static pressure must be measured at the manufacturer-defined locations and compared with the correct fan table. Sensible airside heat transfer is approximately 1.08 × CFM × temperature difference in Btu/h for standard-density air; density, altitude and moisture affect the approximation.
Look for this: Trace two branches and find the outlet at each end. Keep the distinction clear: A cold supply temperature alone does not prove adequate cooling capacity.
The blower creates a pressure difference across the air path: air leaves through the supply and returns through these grilles. Returns usually have no fan of their own. Returning air still contains useful conditioned air; much of it will be filtered and circulated again.
Level 2 · Working knowledge
Each room has a ducted return in this teaching building. Other buildings use transfer grilles, door undercuts or ceiling plenums. Blocking a return adds resistance and can upset room pressure. A return grille is distinct from an exhaust outlet that discharges air outdoors.
Level 3 · Advanced
Room-to-corridor pressure and return-path resistance can explain comfort complaints that resemble equipment problems. Evaluate supply and return flow together. This model draws all branches explicitly; it does not calculate leakage, duct friction, room pressurization, or actual mixing patterns.
Look for this: Find the orange path from a room back up to the rooftop. Keep the distinction clear: A return is not automatically fresh air or exhaust air.
The unit mixes some outdoor air with recirculated room air before filtering and conditioning it. This is ventilation. The outdoor air entering this intake is separate from the outdoor air that passes across the condenser.
Level 2 · Working knowledge
The damper controls an opening; its angle is not itself a measured outdoor-air percentage. Our animation illustrates minimum ventilation. An economizer is a different control strategy that may increase outdoor air for cooling when conditions are suitable; that strategy is explained here but not simulated.
Level 3 · Advanced
A ventilation design needs occupancy, use, airflow and applicable local requirements. Economizer suitability can depend on temperature and humidity/enthalpy logic. Sensor faults, stuck dampers and missing relief paths can alter the entering coil load. No damper position or ventilation rate in this model is a compliance setting.
Look for this: Compare the green intake markers with the separate outdoor condenser stream. Keep the distinction clear: Outdoor air does not flow through the refrigerant tubing.
Adding outdoor air requires a way for some indoor air to leave. This relief opening is connected to the return-air side of the cabinet. It helps prevent the building from becoming excessively pressurized.
Level 2 · Working knowledge
This conceptual barometric relief path is upstream of the mixed-air section. Real buildings may use powered exhaust or other engineered paths. Return, relief and exhaust have different destinations even if all carry air from indoors.
Level 3 · Advanced
Mass balance connects outdoor air, relief/exhaust, infiltration and exfiltration. Wind, stack effect and exhaust systems also affect building pressure. The animation shows the relationship, not a pressure solution or exact ventilation balance.
Look for this: Watch the short orange path leave the cabinet before the filter. Keep the distinction clear: All return air does not have to pass through the cooling coil.
Air passes through the filter media before the cooling coil. The filter captures particles and helps keep the coil clean. A loaded filter can make it harder for the blower to move air. It does not make the air cold.
Level 2 · Working knowledge
Pressure drop across the filter rises with resistance at a given airflow. A constant-speed blower generally delivers less air as system resistance rises; a controlled blower may increase effort until reaching a limit. The restricted-filter scenario assumes fixed blower speed.
Level 3 · Advanced
Compare filter pressure drop with the approved media and operating airflow, and distinguish it from total external static. Filter efficiency and pressure drop are different properties. A low-flow symptom can also come from a dirty coil, blower problem or duct restriction; this scenario deliberately supplies additional evidence.
Look for this: Select the restricted-filter scenario and compare airflow with the baseline. Keep the distinction clear: A denser filter is not automatically suitable for every fan system.
Room air passes over metal fins while refrigerant flows inside the tubes. Heat crosses the tube and fin surfaces into the colder refrigerant. The refrigerant boils inside the coil as it absorbs heat. The two fluids stay separate.
Level 2 · Working knowledge
The refrigerant entering the evaporator is a low-pressure liquid/vapor mixture. Most useful evaporation happens at saturation; the vapor then gains superheat. If coil surfaces are below the entering air dew point, moisture condenses on the outside and drains away.
Level 3 · Advanced
Superheat = measured vapor-line temperature − saturation dew temperature at that location’s pressure. An illustrative 52°F line and 40°F dew saturation give 12°F superheat. This is arithmetic, not a target. Coil leaving air depends on airflow, entering moisture, refrigerant conditions and bypass; air temperature drop is not refrigerant superheat.
Look for this: Look for the blue air path crossing the fin pack and the separate refrigerant line. Keep the distinction clear: The evaporator does not spray refrigerant into the air.
This fan moves building air through the filter, coil, heater and ducts. Its motor turns the wheel. The same blower operates during both cooling and electric heating; the compressor has a different job in the sealed refrigerant circuit.
Level 2 · Working knowledge
This cutaway uses a direct-drive centrifugal wheel. Air enters the wheel near its center and leaves outward into the housing. Different real rooftop units use other fan and motor designs. The selected fault model uses constant speed, so added air resistance reduces flow.
Level 3 · Advanced
Fan speed, wheel condition and system resistance affect the operating point. For the same fan and similar density, affinity relationships connect speed with flow, pressure and power under their assumptions. A commanded fan signal does not prove rotation, airflow or correct direction. This lab does not calculate a fan curve.
Look for this: Compare the indoor blower with the outdoor condenser fan. Keep the distinction clear: The compressor is not the fan that pushes air into the rooms.
In heating mode, air passes over electrically heated elements downstream of the blower. Heat moves into the air and the supply carries it to the rooms. In this configuration, the compressor and condenser fan remain off during heating.
Level 2 · Working knowledge
The controller enables heat through switching devices and protective controls with the required blower operation. Loss of airflow can overheat the heater. Real controls may stage the heat and continue the fan after heat switches off. Timing is equipment-specific.
Level 3 · Advanced
For a resistive heater, input power and air mass flow govern temperature rise, subject to losses and actual voltage. Limit and airflow protections must follow the equipment design. This is electric resistance heat, not a reversing heat pump and not a gas furnace; there is no reversing valve, burner or flue in this model.
Look for this: Switch to Heating and watch the supply color change while refrigerant flow stops. Keep the distinction clear: Heating does not reverse the refrigeration cycle in this unit.
The compressor takes in low-pressure vapor and discharges higher-pressure, hotter vapor. That pressure difference helps refrigerant circulate through the circuit. It works with the condenser, expansion valve and evaporator to move heat outdoors.
Level 2 · Working knowledge
Compression adds energy. The condenser therefore rejects the evaporator load plus compressor work, with a simplified energy boundary. The compressor is intended to receive vapor; returning liquid can damage it. It sits in the outdoor equipment compartment, separate from the building airflow.
Level 3 · Advanced
Use suction and discharge pressures together with temperatures, load, airflow and manufacturer data. Neither current nor one pressure reading establishes charge or compressor health alone. Suction superheat at the compressor includes line heat gain and can differ from evaporator outlet superheat.
Look for this: Trace the hot discharge line from the compressor to the condenser. Keep the distinction clear: The compressor changes refrigerant pressure; it does not compress the room air.
Hot refrigerant enters the condenser. Outdoor air passing over its fins carries heat away. As heat leaves, the vapor condenses into liquid inside the tubing. This outdoor air then leaves the unit; it is not the supply air for the rooms.
Level 2 · Working knowledge
The condenser handles heat absorbed indoors plus compressor work. The refrigerant changes from hot vapor through condensation to liquid, often with subcooling before it reaches the metering device. Dirt or insufficient outdoor airflow can impair heat rejection.
Level 3 · Advanced
Subcooling = saturation bubble temperature at liquid-side pressure − measured liquid-line temperature at that location. An illustrative 110°F bubble saturation and 100°F liquid line give 10°F subcooling, not a charging target. Interpret measurements with the correct refrigerant chart and manufacturer procedure.
Look for this: Follow the outdoor stream through the coil and up out of the fan. Keep the distinction clear: The condenser does not make cold air for the rooms.
This propeller fan serves the outdoor coil. It runs with mechanical cooling in our simple sequence. The supply blower serves the rooms, so a rooftop unit can have two different fans doing separate jobs.
Level 2 · Working knowledge
Air is drawn through the condenser coil and discharged upward. Recirculating that hot discharge or losing fan airflow can increase condensing temperature. Our impaired-condenser scenario slows only this illustrated outdoor stream, not the indoor supply.
Level 3 · Advanced
A fan fault, obstructed coil and high outdoor temperature may produce overlapping symptoms. Compare observed rotation, coil condition, airflow direction and operating data before forming a conclusion. Motor design determines whether a separate run capacitor applies; this teaching fan uses a PSC motor.
Look for this: Try condenser restriction and compare the outdoor and indoor flow markers. Keep the distinction clear: A spinning indoor blower does not establish that the condenser fan works.
Liquid refrigerant reaches this valve from the condenser. Passing through the restriction drops its pressure, and part flashes into vapor. The resulting colder liquid/vapor mixture feeds the evaporator.
Level 2 · Working knowledge
A thermostatic expansion valve uses its sensing bulb and pressure forces to regulate evaporator outlet superheat within its design range. It does not command the room temperature directly. A small sensing connection is not another building-air duct.
Level 3 · Advanced
Throttling is approximated as constant enthalpy, with a pressure decrease and flash gas formation. A TXV requires suitable inlet liquid conditions and pressure difference to feed correctly. Superheat behavior alone cannot distinguish a valve issue from load, airflow, bulb mounting or liquid supply problems.
Look for this: Trace the liquid line into the valve and the mixed-phase line out. Keep the distinction clear: The expansion valve does not remove heat the way the condenser does.
Water on a cooling coil comes from the air, like water on the outside of a cold glass. The pan collects it and the drain carries it away. This drain is separate from refrigerant piping.
Level 2 · Working knowledge
The trap must suit the pressure at the pan so air pressure does not prevent drainage. The model shows a draw-through cooling coil and a conceptual trapped drain, not a trap dimension. A blockage can fill the pan and activate overflow protection.
Level 3 · Advanced
Condensation requires a surface below the local air dew point. Drainage, slope, trap seal, venting and discharge arrangements depend on the unit and installation requirements. The blocked-drain scenario shows one optional float interlock that stops cooling; actual interlock wiring varies.
Look for this: Use the blocked-drain scenario and inspect the float-switch explanation. Keep the distinction clear: Water at the drain is not leaking refrigerant and does not measure cooling capacity.
This wall control compares sensed temperature with its setpoint. It requests cooling or heat, and the equipment controller decides how to operate the unit. All six rooms share this request in our single-zone system.
Level 2 · Working knowledge
Conceptual conventional signals include G for fan, Y for cooling and W for heat. R and C provide the control-power circuit. Communicating systems work differently. Actual timing, staging and protective logic belong to the equipment, not just the thermostat.
Level 3 · Advanced
Sensor location, deadband, schedules and building load influence cycling. An unmet request can reflect a protective lockout, control problem or insufficient delivered capacity. These symbolic signal lines show logic relationships, not terminal-by-terminal wiring instructions.
Look for this: Open the Controls path and step through a call for cooling. Keep the distinction clear: Turning the setpoint lower does not necessarily make a single-stage system blow colder air.
The disconnect is an electrical switching device near the equipment. It is distinct from the thermostat and the protective devices inside the unit. The viewer identifies its role without offering a live-service procedure.
Level 2 · Working knowledge
Control power may be derived from line power through a transformer. Separate feeds, stored capacitor energy and site-specific wiring matter when establishing an electrically safe work condition. An on-screen Off mode means the lesson is idle, not that real equipment is safe to touch.
Level 3 · Advanced
Read equipment-specific diagrams and use the required isolation, lockout and verification practices under qualified supervision. The illustrated electrical connections are functional blocks only. Ratings, conductors, overcurrent protection and clearances are intentionally not designed here.
Look for this: Compare this device with the contactor inside the control compartment. Keep the distinction clear: Thermostat Off is not electrical isolation.
A contactor is an electrically operated switch. Its coil responds to the control circuit, and its contacts switch power to a larger load. The thermostat does not carry the compressor motor current.
Level 2 · Working knowledge
An energized coil and closed contacts are separate conditions. A request can exist while a protective control prevents operation. A control signal can also be present even if contacts or a downstream load are faulty. The displayed signal path is a logical sequence.
Level 3 · Advanced
Diagnosis distinguishes control voltage at the coil, contact state, supply availability and load behavior. Contactors and relays share an electromagnetic switching idea but differ in application and ratings. The model never treats a clicked on-screen contactor as proof of real voltage or current.
Look for this: Follow thermostat → controller → contactor → compressor in the cooling sequence. Keep the distinction clear: A contactor is a switching device, not an energy-storage capacitor.
This metal can is a run capacitor for the illustrated permanent-split-capacitor condenser fan motor. It helps establish the phase relationship between motor windings. It is not the component that makes refrigerant cold.
Level 2 · Working knowledge
A run capacitor stays in its motor circuit while running; a start capacitor, where fitted, is used differently. ECMs and three-phase motor arrangements can differ substantially. This model does not imply that every motor or compressor has the same capacitor arrangement.
Level 3 · Advanced
Capacitance, voltage rating, wiring and replacement requirements come from the actual equipment. A capacitor may retain hazardous energy after power is removed. The lesson covers recognition and circuit purpose, not discharge or live testing. A motor symptom alone does not establish capacitor failure.
Look for this: Locate the capacitor beside the control compartment and compare its shape with the contactor. Keep the distinction clear: Capacitor, contactor, compressor and condenser are four different components.
The controller coordinates the unit. The transformer supplies the appropriate control voltage, and relays switch control or load circuits according to their ratings. Think of the controller as the coordinator rather than the source of heating or cooling itself.
Level 2 · Working knowledge
The lesson sequence is request → permission checks → fan and cooling/heat outputs → operation → satisfied request. Real products may include delays, stages, fan proving and additional sensor logic. Our stepper is deliberately untimed and does not reproduce firmware.
Level 3 · Advanced
A complete diagnosis follows input, logic, output and physical response separately. Requested operation, commanded operation and proven operation are distinct. Our generic model omits refrigerant-specific leak mitigation systems and other model-specific controls; consult the selected equipment manual before applying a sequence in the field.
Look for this: Pause at permission checks and ask what could stop the next output. Keep the distinction clear: A command from the board is not proof that the commanded component operated.
Pressure switches monitor refrigerant circuit pressure; the heater limit responds to excessive heater temperature. They protect equipment and operation. They are different from the thermostat’s comfort request.
Level 2 · Working knowledge
A high-pressure trip can accompany poor condenser heat rejection; low pressure can have several causes. A heater limit can respond to inadequate airflow. The observed trip is a clue about a condition, not a complete diagnosis of the failed part.
Level 3 · Advanced
Trip and reset values, manual versus automatic reset, and lockout logic are equipment-specific. Repeated resets or bypassing protections can hide the cause and create hazards. The lesson offers no bypass controls or generic switch setpoints.
Look for this: Compare the condenser restriction scenario with the restricted-filter scenario. Keep the distinction clear: A tripped protective switch does not automatically mean the switch is bad.
This float rises with water in the pan. In the selected scenario its interlock stops mechanical cooling so continued condensation does not make the blockage worse. The thermostat can still be requesting cooling.
Level 2 · Working knowledge
A cooling call with no compressor operation may therefore have an airside water-management cause. This model assumes the fan continues while the cooling output is inhibited; some installations interrupt different circuits.
Level 3 · Advanced
Trace the actual interlock logic and evaluate the drainage cause. Fan-only behavior, alarm reporting and reset requirements vary. Water shown here is illustrative; the model does not calculate pan volume, drain flow or time to overflow.
Look for this: Choose Blocked drain and compare request, command and actual operation. Keep the distinction clear: No compressor operation does not always indicate a compressor defect.
Level 1. Follow the complete air path. In cooling, the coil takes heat out of the indoor air and the outdoor coil rejects it.
Level 2. The baseline assumes clean air paths, a working blower and available control permissions. It does not calculate load or room temperatures.
Level 3. Use this baseline to compare qualitative relationships. The lab supplies no refrigerant pressure targets or charging diagnosis.
No fault imposed.
Illustrative airflow markers show direction, not measured velocity.
Explain: Which two fluid streams exchange heat at the evaporator without mixing?
Restricted filter
Level 1. Air meets more resistance at the filter. With our fixed-speed blower assumption, less air reaches every supply outlet.
Level 2. Reduced airflow may lower evaporator temperature and increase icing risk. A colder supply reading would not prove greater delivered capacity. No ice-growth calculation is performed.
Level 3. The case gives elevated filter pressure drop and a fixed blower speed. This makes filter resistance the intended explanation. Without that evidence, blower, coil and duct problems remain alternatives.
Filter media visibly loaded in the lesson.
Filter pressure drop elevated relative to its clean baseline.
Blower speed held constant; indoor flow reduced.
Explain: Why can colder supply air coexist with less useful cooling?
Condenser airflow reduced
Level 1. The outdoor stream has slowed. The condenser has a harder time releasing heat even though the room blower is still moving air.
Level 2. Reduced outdoor heat rejection tends to raise condensing temperature and pressure. A high-pressure protection may eventually interrupt operation; no trip threshold is simulated.
Level 3. A dirty outdoor coil, fan problem and hot-air recirculation can produce similar conditions. The case isolates outdoor airflow, but a real diagnosis needs observations and equipment-specific data.
Outdoor stream reduced; indoor stream unchanged.
Heat-rejection difficulty is imposed.
No charge conclusion or pressure setpoint is supplied.
Explain: Why would cleaning a room return grille fail to correct an isolated outdoor fan fault?
Blocked drain / float open
Level 1. The drain cannot carry water away. The pan level reaches the float, which interrupts cooling in this example. The fan can still move air.
Level 2. The room thermostat is calling, but the interlock prevents compressor and condenser-fan operation. With refrigeration stopped, this air stream is no longer being cooled by the evaporator.
Level 3. Separate a demand from a permitted output. Our chosen float logic inhibits Y while leaving fan operation available; actual wiring can interrupt a different circuit. Verify the real diagram.
Contactor switches a power circuit; run capacitor supports the selected motor circuit
Both are names for the condenser coil
Answer B. A contactor is an operated switch. A capacitor is a separate electrical component; applicability depends on motor design.
Level 2 · Where did water in a cooling-coil pan originate?
Water vapor in the entering air
Refrigerant released from the tubes
Water made by the compressor
Answer A. Air moisture condenses on a coil surface below the air dew point. It is separate from the sealed refrigerant circuit.
Level 2 · The thermostat calls, the blower runs, and the overflow interlock inhibits cooling. What is established?
The compressor is necessarily defective
Demand exists but cooling permission is absent
The condenser must be overcharged
Answer B. This evidence establishes a blocked control permission, not a failed compressor or refrigerant-charge diagnosis.
Level 3 · Vapor-line temperature is 52°F and local dew saturation is 40°F. What is superheat?
12°F
92°F
−12°F
Answer A. Superheat is vapor temperature minus saturation dew temperature: 52 − 40 = 12°F. This example is not a target.
Level 3 · Local bubble saturation is 110°F and the liquid line is 100°F. What is subcooling?
−10°F
10°F
210°F
Answer B. Subcooling is saturation bubble temperature minus liquid temperature: 110 − 100 = 10°F. Use the correct refrigerant chart.
Level 3 · Condensing pressure is elevated. Which conclusion is justified by that measurement alone?
The charge is too high
The condenser fan capacitor has failed
More evidence is required to distinguish causes
Answer C. Airflow, outdoor conditions, coil condition, recirculation, charge and other factors can overlap. One reading does not isolate a fault.
Level 3 · Air is sensibly heated without adding or removing moisture. What stays approximately constant?
Humidity ratio
Relative humidity
Dry-bulb temperature
Answer A. Moisture per mass of dry air stays approximately constant; temperature rises and relative humidity falls. This is distinct from cooling with condensation.
Model boundaries
The six rooms share one thermostat. They are not six independently controlled zones.
The rooftop assembly is enlarged and opened for visibility. It represents a generic configuration, not a product replica or a load-sized design.
Ducts and pipes show connectivity. Diameters, openings, trap geometry and service clearances are not construction dimensions.
Flow markers are symbolic and not proportional across circuits. No CFD, load, pressure, fan-curve, refrigerant-charge or electrical-network solution is performed.
The native Blender animation is a cooling overview. The browser provides mode-dependent operation and fault behavior.
Indoor temperature, relative humidity, saturation temperatures and pressures are not predicted. Calculator values are freely editable arithmetic examples.
The unit uses electric resistance heat and a generic PSC condenser fan. It is not a heat pump, gas furnace or a specific manufacturer’s complete refrigerant-safety design.
Control lines show functional relationships, not actual terminals or wiring. This guide does not authorize work on energized circuits, refrigerant systems or protective devices.
Copeland — Refrigeration troubleshooting fundamentals Refrigerant state and compressor protection concepts. The article contains reversed subtraction wording; use the explicit formulas in this lab, corroborated by the pressure-temperature reference.
