Lesson 8 of 12 · Chapter 8
Air Conditioning and Heat Pumps
Air conditioning is not making cold. It is controlled heat movement: relocating indoor heat and moisture outdoors with the refrigeration cycle you already know. A heat pump is the same machine with a direction switch, and the most honest piece of equipment you will ever work on.
Step one
Read the material
Five ideas. Two on air conditioning: what it really does, and the symptoms that impersonate each other. Two on heat pumps: reversing direction, and the heat, help and ice that come with it. And one on efficiency, which the chapter insists is seasonal.
Idea one
Air conditioning moves heat and moisture
Air conditioning is controlled heat movement. An air conditioner has two simultaneous jobs at the evaporator: remove sensible heat and remove latent heat, the moisture that condenses on a coil cold enough to drop air below its dew point. The compressor makes the pressure that lets the outdoor coil throw that heat away. Nothing destroyed heat or created cold.
Airflow and time decide how well it works. Too little air and the coil heads toward ice; too much and it may not get cold enough to remove moisture. And a system that runs steadily tends to control humidity better than a system that short-cycles, because moisture removal needs sustained coil temperature.
Use checkpoints, not impressions: the temperature split as context, coil behaviour through pressures and temperatures, and condensate. If a humid house runs for hours and the drain stays dry, something is wrong. Good technicians don’t just ask whether it is cold. They ask whether it is moving heat and moisture the way the conditions demand.
Idea two
Symptoms that mimic each other
A dirty condenser coil cannot reject heat efficiently, so head pressure and compressor amperage rise, capacity drops, and a high-pressure switch may open: the customer says “it always quits on the hottest days,” and it is not superstition. Cleaning the coil is not cosmetic.
A dirty indoor coil or restricted airflow does the reverse: the coil gets colder, ice forms, capacity collapses. The customer reports weak, warm air, suction pressure looks low, and the inexperienced technician wants to add refrigerant. But low suction can be caused by low load from airflow restrictions just as easily as by low charge. Airflow is load.
So don’t just ask whether the compressor is running. Ask whether you are achieving the temperature platforms you need at the evaporator and condenser for the day’s conditions. That keeps you from blaming components that are only reporting consequences.
Idea three
The heat pump reverses direction
A heat pump does not make heat in winter any more than an air conditioner makes cold in summer. The only difference between cooling mode and heating mode is which coil is acting as the evaporator and which coil is acting as the condenser. In heating, the outdoor coil absorbs heat from outdoor air and the indoor coil rejects it into the house.
The reversing valve makes the switch, commanded by the thermostat’s O or B signal. Many brands energize O in cooling; some energize B in heating. Get that wrong and you get a system that runs smoothly, with plausible pressures and a normal-sounding compressor, but backwards: “It’s blowing cold when I set heat.”
So before condemning a reversing valve, confirm the command first: is O or B energized as it should be for the mode, is control voltage stable at the solenoid, does the coil read correctly? Only then do you move to the mechanical questions.
Idea four
COP, auxiliary heat and defrost
Because it moves heat instead of creating it, a heat pump can deliver more heat energy into the space than the electrical energy it consumes; that is COP. As the outdoor temperature drops, capacity falls and auxiliary heat, usually electric strips, helps. Heat pump supply air often feels cooler than a gas furnace’s while still delivering plenty of BTUs, and a customer who keeps raising the thermostat to feel “hot” air can trigger auxiliary heat and a much bigger bill.
In heating, the outdoor coil can frost over, so the system periodically runs in cooling mode to heat the outdoor coil and melt the ice. That is defrost. The board may slow the indoor fan and often brings on auxiliary heat so the air does not feel cold. Steam and a whoosh can be normal.
Defrost that happens too often or will not end is worth diagnosing: an outdoor coil sensor, board logic, charge, blocked outdoor coil airflow or a failing outdoor fan.
Idea five
Efficiency is seasonal
Efficiency is seasonal. The same equipment behaves differently at 75°F and at 100°F outdoors, and a heat pump that seems fine at 45°F can struggle at 20°F. So efficiency work runs through gates: airflow, then coil cleanliness, then refrigerant charge and metering, because low airflow can mimic low charge and a dirty condenser can mimic overcharge. Then electrical health, since a weak fan capacitor slows condenser airflow and raises head pressure.
Then controls. A deep overnight setback on a heat pump can make the thermostat bring on auxiliary heat to recover fast, doing exactly what it was told. Smaller setbacks and staging that favours the compressor often save more than new parts. And sealing a return leak that pulls attic air reduces the load itself: the cleanest BTU is the one you do not have to move.
Finish the way lesson five taught you: prove the result through a complete cycle, and explain it in measurements.
Step two
Work it with your hands on it
The Reversible Heat Mover. Ask for cooling or heating, put the heat pump into defrost, and set what the thermostat energizes against what the equipment expects. Watch which coil takes which job, which way the heat goes, and what the customer feels. Then mismatch O and B and see how normal everything else still looks.
The thermostat asks for
Operation (defrost applies in heating)
Thermostat configuration
The equipment
Hands on it, off the screen. On a humid day with the air conditioning running, find where your condensate line ends outdoors or at a drain, and check that water is coming out. That water is latent load leaving your house.
If you have a heat pump, check your thermostat’s installer settings (without changing them) for its O or B setting and its auxiliary heat setting, and note what they say.
Step three
Prove you understood it
One question per idea, a new situation every time. Miss one and you are sent back to the idea it was testing.
All five solid.
You can explain air conditioning as heat and moisture movement, tell look-alike symptoms apart, say which coil does which job in each heat pump mode, confirm an O or B command before blaming a valve, explain COP, auxiliary heat and defrost, and walk the efficiency gates. Next: back to the class for lesson nine.
The HVAC Apprentice’s Field Guide
Lesson eight of twelve, from chapter eight. Written by our founder, Dr. Gene A Constant, and donated to the Foundation. Published by Global Sovereign University Press.
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