Sequence of Operation — Heat Pump

A heat pump is an air conditioner that can run backwards. Everything you learned in the AC sequence of operation (Chapter 26) still applies — compressor contactor, condenser fan, evaporator coil, refrigerant loop, 24V control voltage from an outdoor transformer or the indoor air handler. What’s added is a reversing valve that flips the refrigerant flow direction on command, plus a defrost control board that periodically runs a cooling cycle in the middle of winter to melt ice off the outdoor coil, plus an auxiliary heat path (electric strip heat, or a dual-fuel gas stage) that fills in when the heat pump can’t keep up.

The thermostat wiring is where the conceptual shift starts. On an AC, Y calls for cool and G runs the blower. On a heat pump, Y calls for the compressor — which can mean cooling or heating, depending on the state of the reversing valve. What determines which way refrigerant flows is the O terminal (or sometimes B — they’re opposite conventions). The thermostat sends 24V to O continuously any time the system is in cool mode; if O loses power, the reversing valve de-energizes and the system flips to heat. Same Y call, different outcome depending on O.

Cool mode — the familiar path

On a cooling call, the thermostat simultaneously energizes G (blower on), Y (compressor on), and O (reversing valve to cooling position). At the outdoor unit, the O signal energizes the reversing valve solenoid, shifting refrigerant flow so that the outdoor coil is the condenser (rejecting heat) and the indoor coil is the evaporator (absorbing heat from the house). Y energizes the compressor contactor. The condenser fan runs. The system looks and behaves identically to a straight-cool AC.

This is important to internalize before you move on: cooling is the default state with O energized. It’s the same path you already know.

Heat mode — the reversing valve drops out

On a heating call, the thermostat de-energizes O (or energizes B on Rheem/Ruud systems, which is the inverted convention) and energizes Y and G. The reversing valve solenoid de-energizes, the valve shifts by spring pressure, and refrigerant flow reverses. Now the outdoor coil is the evaporator and the indoor coil is the condenser. The outdoor fan still runs, the compressor still runs, but the air coming out of the supply registers is warm instead of cool.

Defrost mode — looks like a failure, isn’t

In heat mode, the outdoor coil is running below ambient temperature (it has to, to pull heat out of the air). When ambient is below about 45°F and humidity is present, moisture condenses on that cold coil and freezes. If allowed to continue, ice blocks airflow through the coil and efficiency collapses. The answer is to periodically run a cooling cycle outdoors — flip the reversing valve, send hot refrigerant out to the outdoor coil, melt the ice off.

This is managed by the defrost control board, which tracks time and outdoor coil temperature. Two common logics:

• Time/temperature defrost (older, simpler): every 30, 60, or 90 minutes of compressor runtime, the board checks the defrost thermistor. If the outdoor coil is below about 30°F, defrost initiates. It terminates when the coil warms to about 50°F, or on a maximum time limit of 10 minutes.
• Demand defrost (newer, smarter): the board tracks the difference between outdoor ambient temperature and outdoor coil temperature. When that delta widens beyond a threshold (the coil is getting colder relative to ambient, which means ice is accumulating), defrost initiates. Uses fewer and shorter cycles.

During defrost, three things change:

1. The reversing valve energizes — outdoor coil becomes the condenser, refrigerant flow flips to cooling direction.
2. The outdoor fan shuts off — running it would blow cold air over a hot coil and work against the cycle.
3. Auxiliary heat energizes — the indoor coil is now the evaporator, which means cold air would come out of the supply registers; strip heat masks this.

From the homeowner’s perspective, defrost looks like steam coming off the outdoor unit (that’s the ice melting and flashing to vapor) and warm air continuing from the registers. To a tech arriving on a “heat pump isn’t heating” call in the middle of a defrost cycle, the system looks broken — the compressor is running in cooling direction, the outdoor fan is off, strip heat is running. Don’t diagnose during defrost. Wait 5–10 minutes for it to terminate and go back to normal heating, then evaluate.

Auxiliary heat — fills the gap below balance point

A heat pump’s heating capacity falls as outdoor temperature drops. There’s a point — called the balance point — below which the heat pump alone can’t keep up with the house’s heat loss. Typically around 25–35°F for residential units, depending on equipment and house. Below balance point, auxiliary heat (resistance strip heat, or a gas furnace stage on a dual-fuel setup) supplements the heat pump.

Auxiliary heat is energized by the thermostat on the W terminal (sometimes W2, or auxiliary-specific terminals). Most thermostats use one of two logics to decide when to bring aux heat on:

1. Staged activation — if the room temperature falls more than about 2°F below setpoint, or stays below setpoint for more than a configured time (e.g., 10 minutes), the thermostat stages in W.
2. Outdoor sensor / balance point — if an outdoor sensor reads below a configured balance point, the thermostat permits aux heat to run whenever Y is calling.

Emergency heat (E terminal) is a homeowner-selectable mode that disables the compressor entirely and runs aux heat only. Used when the heat pump has failed or is known to be compromised.

The full heat-mode SOO

Putting it all together, a cold winter heat call on a heat pump looks like this:

Verifying heat pump SOO on a no-heat call

procedure

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Check your understanding

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01A heat pump is running in heat mode. The outdoor fan stops, the compressor continues, the reversing valve energizes, and strip heat comes on. What's happening?

AThe reversing valve has failedBA defrost cycle has initiated — the system is melting ice off the outdoor coilCThe thermostat has switched to cool mode by mistakeDThe compressor is about to overheat

02On a standard heat pump, when does the O terminal carry 24 VAC?

AIn heat mode onlyBIn cool mode only (and during defrost)CContinuously, whenever the thermostat is poweredDOnly during auxiliary heat

03A homeowner complains of high electric bills on a heat pump in winter. The heat pump is running, strip heat is also running frequently. What's the most likely cause?

AThe thermostat is brokenBThe compressor has failedCBalance point or staging logic is set aggressively, OR heat pump capacity is degraded so aux heat fills in more than it shouldDThe reversing valve is stuck