Interactive diagrams
How your HVAC system works
Six animated, step-by-step diagrams — central AC, heat pump, gas furnace, ductless mini-split, and tank & tankless water heaters. Tap any part, walk the cycle, and understand what your technician is actually talking about.
The refrigeration cycle
Central Air Conditioning
Your air conditioner doesn't make cold air — it moves heat. Refrigerant absorbs heat from your indoor air at the evaporator coil, carries it outside, and dumps it at the condenser. Everything else — the compressor, the expansion valve, the fans — exists to keep that loop running.
Step 1 of 5: The thermostat calls for cooling
Parts legend — tap a part in the diagram or the list
- The thermostat calls for cooling — When room temperature rises above your setpoint, the thermostat closes a low-voltage circuit that wakes up the whole system.
- Refrigerant absorbs indoor heat — Cold liquid refrigerant in the evaporator coil boils into vapor as it soaks up heat from the air the blower drags across the fins — that's also where humidity condenses out.
- The compressor pressurizes the vapor — The warm vapor travels to the outdoor unit, where the compressor squeezes it — raising its temperature well above the outside air.
- The condenser rejects heat outside — Because the refrigerant is now hotter than the outdoor air, heat flows out through the condenser coil while the outdoor fan pulls air across it. That's the warm breeze off the top of the unit.
- The expansion valve restarts the cycle — The cooled liquid passes the expansion valve, its pressure and temperature plunge, and it heads back to the evaporator — while the blower keeps pushing cooled air through the ducts.
One machine, both seasons
Heat Pump
A heat pump is an air conditioner with one extra trick: a reversing valve that lets the refrigerant loop run in either direction. In summer it moves heat out of your house; in winter it pulls heat from the outdoor air — which works beautifully in Charleston's mild winters — and releases it inside. Toggle the mode below and watch the flow reverse.
Step 1 of 5: The thermostat calls for cooling
Parts legend — tap a part in the diagram or the list
Cooling mode, step by step
- The thermostat calls for cooling — Same as a central AC: when the room drifts above the setpoint, the thermostat closes the cooling circuit and the heat pump starts.
- The indoor coil absorbs heat — Cold, low-pressure refrigerant in the indoor coil soaks up heat from your home's air while the blower moves air across it.
- The compressor squeezes the vapor — The reversing valve routes the warm vapor to the compressor, which raises its pressure and temperature well above the outdoor air.
- The outdoor coil dumps the heat — Because the refrigerant is now hotter than outside, heat flows out of the outdoor coil into the yard — helped by the outdoor fan.
- The expansion valve resets the loop — Pressure drops, the refrigerant gets cold again, and the cycle repeats — in cooling mode a heat pump IS an air conditioner.
Heating mode, step by step
- The thermostat calls for heat — On a cool Charleston morning the thermostat asks for heating instead — and the same machine answers, no separate furnace needed.
- The reversing valve flips the flow — A four-port valve reverses which coil condenses and which evaporates, so the refrigerant now runs the loop in the opposite direction.
- The outdoor coil absorbs outdoor heat — After the expansion valve chills the refrigerant below the outside temperature, even 40°F winter air has heat to give up to the coil.
- The compressor concentrates that heat — Compressing the vapor multiplies its temperature, turning mild outdoor warmth into genuinely hot refrigerant headed indoors.
- The indoor coil releases heat inside — The blower pushes your home's air across the now-hot indoor coil and warm air flows out of the vents. Cycle repeats until setpoint.
Combustion, safely contained
Gas Furnace
A gas furnace makes heat the direct way: burners heat a sealed metal heat exchanger, and the blower pushes your home's air across its hot outside surface. Combustion gases never mix with your air — they stay inside the exchanger and vent out the flue, watched the whole time by safety controls.
Step 1 of 5: The thermostat calls for heat
Parts legend — tap a part in the diagram or the list
- The thermostat calls for heat — The call starts an ignition sequence — the furnace first runs a draft check before any gas flows.
- The gas valve opens and the igniter lights the burners — A hot-surface igniter glows, the gas valve opens, and a row of burner flames lights across the bottom of the furnace.
- The heat exchanger warms up — Burner flames heat the sealed metal exchanger from the inside. Combustion byproducts stay locked inside it and exit through the flue — never into your air.
- The blower pushes air across the exchanger — Once the metal is hot, the blower kicks on, moving return air across the exchanger's hot surface and sending warmed air through the ducts to your rooms.
- Safety controls watch every second — The flame sensor confirms the burners are actually lit, and limit switches cut gas if anything overheats. This is why a cracked heat exchanger is an immediate shut-it-down finding.
The same cycle, no ducts
Ductless Mini-Split
A mini-split runs the exact same refrigeration cycle as a central system — it just skips the ductwork. An outdoor inverter unit connects to a wall-mounted indoor head through a small line set that passes through a roughly 3-inch opening, and each head conditions and controls its own room independently.
Step 1 of 5: The room's own control calls
Parts legend — tap a part in the diagram or the list
- The room's own control calls — You set this room's temperature at this room's head — no fighting over one hallway thermostat.
- The inverter ramps to match the load — Instead of blasting on and off, the outdoor unit's variable-speed compressor speeds up or slows down to deliver exactly the capacity the room needs.
- Refrigerant travels the small line set — The refrigerant runs between outdoor unit and indoor head through slim insulated lines that pass through a single ~3-inch opening in the wall.
- The wall head conditions the room directly — The head's coil absorbs (or releases) heat right in the room and its fan distributes the air — no duct losses, which is a big part of mini-split efficiency.
- Every head is its own zone — Add heads for other rooms and each keeps its own schedule and setpoint — a guest room can sleep at 68°F while the office holds 74°F.
Hot water in storage
Tank Water Heater
A tank water heater is an insulated storage vessel that heats water ahead of time and holds it ready. Cold water enters through a dip tube to the bottom, the burner or element heats it, and hot water — which naturally rises — leaves from the top. The anode rod and T&P relief valve quietly protect the tank the whole time.
Step 1 of 5: Cold water enters through the dip tube
Parts legend — tap a part in the diagram or the list
- Cold water enters through the dip tube — The dip tube routes incoming cold water straight to the bottom of the tank, keeping the hot water at the top undisturbed.
- The burner or element heats the water — At the bottom — right where the cold water arrives — the gas burner or electric element adds heat.
- Hot water rises to the top — Warmer water is less dense, so it floats upward and the tank naturally stratifies: hottest at the top, coolest at the bottom.
- The hot outlet supplies your fixtures — When you open a tap, hot water leaves from the top while fresh cold water refills the bottom — and the thermostat cycles the burner to recover.
- Protection parts do their quiet work — The anode rod corrodes instead of the tank, the T&P valve stands guard against overpressure, and the sediment layer slowly builds — which is exactly why a yearly flush extends tank life.
Heat on demand
Tankless Water Heater
A tankless water heater stores nothing — it heats water the moment you ask for it. When a hot tap opens, a flow sensor detects moving water, the burner fires, and water heats instantly as it snakes through a compact heat exchanger. Endless hot water, and zero energy spent keeping a tank warm between uses.
Step 1 of 5: A hot tap opens and the flow sensor spins
Parts legend — tap a part in the diagram or the list
- A hot tap opens and the flow sensor spins — The moment water moves through the unit, the flow sensor's turbine signals the control board: someone wants hot water.
- The burner fires — The control board ignites the burner within seconds, sizing the flame to how much water is flowing and how cold it arrived.
- Water heats instantly in the exchanger — Water winds through the heat exchanger's tight passages directly above the flame, jumping to the set temperature in one pass.
- Endless hot water flows out — Because heating happens in real time, the hot water never runs out — a long shower can't drain a tank that doesn't exist.
- The tap closes and everything stops — Flow stops, the burner shuts off, and the unit sits idle using no energy — no standby losses from keeping stored water warm.
Quick answers
Questions the diagrams answer
Why does my AC have an indoor and an outdoor unit?
Because air conditioning is heat moving, not cold making. The indoor unit's evaporator coil absorbs heat from your home's air, and the refrigerant carries it to the outdoor unit, where the condenser coil and fan release it outside. Each half does one side of the exchange — that's why both have to be matched and both have to work.
How can a heat pump heat my house with cold winter air?
Even 40°F air holds usable heat. The expansion valve chills the refrigerant below the outdoor temperature, so heat flows from the winter air into the outdoor coil; the compressor then concentrates that heat to a temperature that warms your home. In Charleston's mild winters, that's exactly the workload heat pumps are best at.
Do furnace combustion gases mix with the air in my house?
No — and that separation is the whole design. Burner flames heat the inside of a sealed metal heat exchanger while your air passes only across its outside surface; the combustion byproducts vent out the flue. It's also why a cracked heat exchanger is a shut-it-down safety finding, not a wait-and-see repair.
Why doesn't a ductless mini-split need ducts?
Because the indoor wall head puts the coil and the fan directly in the room it serves — the refrigerant line set, passing through a roughly 3-inch wall opening, replaces the entire duct network. Skipping ducts also skips duct losses, which is a big part of mini-split efficiency.
Why does my tank water heater need to be flushed?
Minerals in the water settle into a sediment layer at the bottom of the tank — directly between the burner and the water it's trying to heat. That layer makes the heater work longer for the same hot water and shortens tank life. An annual flush clears it before it hardens.
How does a tankless water heater never run out of hot water?
It doesn't store hot water — it makes it in real time. A flow sensor detects the open tap, the burner fires, and water heats in one pass through the heat exchanger for as long as the water flows. The practical limit is flow rate (how many fixtures at once), not gallons in a tank.
Something's not working the way the diagram says it should? The troubleshooter walks your exact symptom, and the FAQ library goes deeper on terminology.
Now you know how it works.
When it stops working, you'll know who to call — a real person answers until 9 PM.