Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 4 — Vapour Compression Cycle
4.5 — Heat Pumps
A heat pump is a vapour compression system that can move heat in either direction.
By reversing the refrigerant flow, the indoor and outdoor coils swap roles —
the same equipment that cools a building in summer can heat it in winter.
Jump to
4.5.1 — How a Heat Pump Works — Reverse-Cycle Operation
A heat pump uses the same vapour compression cycle as a standard air conditioner.
The critical difference is that a 4-way reversing valve can switch
the direction of refrigerant flow so that the indoor coil acts as a condenser
(heating mode) rather than an evaporator (cooling mode).
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Cooling Mode
Identical to a standard A/C: the indoor coil is the evaporator
(absorbs heat from the room), and the outdoor coil is the condenser
(rejects heat to outside).
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Heating Mode
Refrigerant flow is reversed: the outdoor coil becomes the evaporator
(absorbs heat from cold outdoor air), and the indoor coil becomes the
condenser (releases heat into the building).
♻️
Key Insight
In heating mode, a heat pump moves heat rather than generating it.
Even at outdoor temperatures as low as −15°C (5°F), there is still
usable heat energy in the air that the refrigerant can absorb.
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Heat pumps move heat — they do not create it
This is why a heat pump can deliver 3–4 kW of heat for every 1 kW of
electrical energy consumed. The rest comes from the outdoor environment, effectively
giving a COP (Coefficient of Performance) of 3–4 under mild conditions —
far more efficient than electric resistance heating (COP = 1).
4.5.2 — The 4-Way Reversing Valve
The 4-way reversing valve is the component that enables a heat pump to switch between
heating and cooling. It has four refrigerant connections and a slide mechanism that
redirects discharge gas from the compressor to either the indoor or outdoor coil.
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Four Connections
1. Discharge — from compressor (always high-pressure hot gas)
2. Suction — to compressor (always low-pressure vapour)
3. Outdoor coil — bi-directional
4. Indoor coil — bi-directional
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Solenoid Pilot Control
The valve is shifted by a small solenoid-operated pilot valve that directs
discharge pressure to one end of the valve slide. Most systems are
de-energised in heating mode and energised in cooling mode
— but this varies by manufacturer.
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Common Failures
A partially stuck or leaking reversing valve allows high-pressure discharge gas
to bleed to the low side. Symptoms include: high suction pressure, low discharge
pressure, low capacity in one or both modes, and warm suction line.
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Testing the Reversing Valve
To confirm the reversing valve is shifting properly:
With the system running, switch between heating and cooling modes and listen for an audible
click as the valve shifts.
Use a thermocouple to confirm the indoor coil temperature changes from cold (cooling) to warm (heating)
within 30–60 seconds of switching.
Check solenoid coil for continuity if the valve does not shift.
A valve that is stuck mechanically can sometimes be freed by tapping lightly with a rubber
mallet while switching modes — but replacement is the correct fix.
4.5.3 — Check Valves in Heat Pump Systems
Because the outdoor and indoor coils swap roles between heating and cooling, a heat pump
typically has two metering devices (one for each coil) and
check valves to direct refrigerant through the correct metering device
depending on the mode.
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How Check Valves Work
A check valve allows refrigerant flow in only one direction. In
heating mode, the check valve bypasses the indoor metering device (liquid flows freely
into the indoor coil, which is now a condenser) and forces the refrigerant through the
outdoor metering device. In cooling mode, the opposite path is used.
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Dual Metering Devices
A biflow TXV (bi-directional) or two separate metering devices — one in
each coil circuit — may be used. Each is sized for the heat load in the mode
where it is active. A check valve in parallel with each metering device allows
liquid to bypass it when it is not metering.
⚠️
Check Valve Failures
A stuck-open check valve in the wrong position bypasses the active metering device,
flooding the evaporator. A stuck-closed valve prevents refrigerant from reaching the
active coil. Both cause poor capacity in one mode and are diagnosed by comparing
heating-mode and cooling-mode pressures.
4.5.4 — Cooling Mode — Refrigerant Flow
In cooling mode, a heat pump operates identically to a standard split-system air
conditioner. Refrigerant flow follows the standard vapour compression cycle.
High Side
① Compressor
Compresses low-pressure vapour from the indoor coil. Discharge gas goes to the
reversing valve.
→
High Side
② Outdoor Coil (Condenser)
Hot discharge gas condenses. Heat is rejected to the outdoor air. Subcooled liquid
exits the coil.
→
Transition
③ Outdoor Metering Device
Check valve bypasses this device in cooling mode. Liquid flows freely to the
indoor metering device.
→
Low Side
④ Indoor Coil (Evaporator)
Refrigerant boils, absorbing heat from the indoor air. Superheated vapour returns
to the compressor via the reversing valve.
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Reversing valve position in cooling mode
On most residential heat pumps, the reversing valve solenoid is
energised in cooling mode, directing discharge gas to the outdoor
coil. In heating mode the solenoid de-energises. Always verify with the
manufacturer wiring diagram before diagnosing mode-switching issues.
4.5.5 — Heating Mode — Refrigerant Flow
In heating mode, the reversing valve shifts to redirect discharge gas to the indoor
coil. The indoor coil becomes the condenser and releases heat into the building.
The outdoor coil becomes the evaporator and absorbs heat from the cold outdoor air.
High Side
① Compressor
Compresses low-pressure vapour from the outdoor coil. Discharge gas goes to the
reversing valve — now directed to the indoor coil.
→
High Side
② Indoor Coil (Condenser)
Hot discharge gas condenses inside the building, releasing heat to the indoor air.
Subcooled liquid exits the coil.
→
Transition
③ Outdoor Metering Device
Liquid is throttled through the outdoor expansion device, dropping in pressure
and temperature to a cold mixture well below outdoor air temperature.
→
Low Side
④ Outdoor Coil (Evaporator)
Cold refrigerant absorbs heat from the outdoor air (even in sub-zero conditions).
Vapour returns to the compressor via the reversing valve.
🌡️
Heating Capacity at Low Outdoor Temperatures
As outdoor temperature drops, the evaporating pressure and temperature in the
outdoor coil also drop. This increases the compression ratio and reduces the
mass of refrigerant the compressor can move per revolution, lowering heating
capacity. At very low outdoor temperatures:
Heating capacity may fall below the building’s heat loss
A supplementary electric resistance heater (auxiliary heat) activates automatically
Many systems include a balance point temperature below which auxiliary heat is
always needed (typically −10 to 0°C / 14 to 32°F)
Cold-climate heat pumps use variable-speed compressors and larger coils to
maintain useful capacity down to −25°C (−13°F) or lower
4.5.6 — The Defrost Cycle
During heating mode, the outdoor coil operates below 0°C. Moisture in the outdoor
air freezes on the coil surface, building up a layer of frost that restricts airflow
and dramatically reduces the coil’s ability to absorb heat. The defrost cycle
melts this frost.
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When Defrost Initiates
Most systems use a time-temperature or
demand defrost control. Time-temperature initiates defrost at
set intervals (e.g., every 90 minutes) if the outdoor coil temperature is below
a set point. Demand defrost uses sensors or algorithms to initiate only when
frost accumulation is detected, saving energy.
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How Defrost Works
The reversing valve switches to cooling mode, directing hot
discharge gas to the outdoor coil. This melts the frost from the coil. The
outdoor fan is turned off during defrost to prevent cold air from counteracting
the hot gas. Indoor auxiliary heat activates to compensate for the temporary
loss of heating.
✅
Defrost Termination
Defrost ends when a coil temperature sensor (typically a thermistor) reads a set
point (e.g., 20°C / 68°F), confirming the frost is melted. A time-out
(typically 10 minutes) terminates defrost if the coil does not reach the set
point, preventing indefinite defrost operation.
⚠️
Steam from the outdoor unit during defrost is normal
Water melting off the coil during defrost and then evaporating when hot gas
arrives creates a visible steam cloud from the outdoor unit. Homeowners sometimes
mistake this for a fire or refrigerant leak. Explain defrost operation as part of
the commissioning handover.
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Diagnosing Defrost Problems
A heat pump that runs in heating mode but accumulates heavy ice on the outdoor
coil is not defrosting correctly. Common causes:
Defrost control board failure — no defrost initiation
Outdoor coil temperature sensor (thermistor) open or shorted —
control reads incorrect temperature
Reversing valve not shifting during defrost — hot gas not directed
to outdoor coil
Outdoor fan not stopping during defrost — cooling the coil faster
than the hot gas can melt ice
Low refrigerant charge — insufficient hot gas temperature to melt ice
4.5.7 — COP, SEER, and HSPF — Measuring Efficiency
Heat pump efficiency is expressed differently depending on whether the system is
heating or cooling, and whether the rating is instantaneous (COP) or seasonal (SEER/HSPF).
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COP (Coefficient of Performance)
The ratio of useful heat output (or cooling effect) to the electrical energy input,
at a single set of operating conditions. A heat pump with COP 3.5 delivers
3.5 kW of heat per 1 kW of electricity consumed. Higher is better.
❄️
SEER / SEER2 (Seasonal Energy Efficiency Ratio)
Measures cooling efficiency over an entire cooling season. Units are BTU of cooling
per Wh of electricity. SEER2 uses updated test conditions (2023+) with higher
external static pressure. Minimum residential SEER2 requirements vary by region.
Measures heating efficiency over an entire heating season. Units are BTU of heat
per Wh of electricity, including defrost cycles and auxiliary heat operation.
HSPF2 uses the same updated test conditions as SEER2.
Rating
What It Measures
Typical Range
Higher = ?
COP (heating)
Instantaneous heating efficiency
2.5 – 4.5 (at 8°C / 47°F outdoor)
More heat per kWh
COP (cooling)
Instantaneous cooling efficiency
2.5 – 4.5 (at standard conditions)
More cooling per kWh
SEER2
Seasonal cooling efficiency
14 – 30+ BTU/Wh
Lower annual cooling cost
HSPF2
Seasonal heating efficiency
7 – 12+ BTU/Wh
Lower annual heating cost
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COP drops as outdoor temperature drops
As outdoor temperature falls in heating mode, the compression ratio increases
and the mass flow rate through the compressor decreases. Both effects reduce COP.
A heat pump rated COP 4.0 at 8°C may have a COP of only 2.0 at −10°C.
Variable-speed (inverter-driven) compressors maintain higher COP over a wider range
of conditions than single-speed equipment.