Heat pump expansion valve: role, types and advice

Understanding the role of the expansion valve in a heat pump

Regulating refrigerant pressure and flow to stabilise the heat pump

In a heat pump, the expansion valve is the circuit's "narrow gate". It lowers the refrigerant's pressure at the condenser outlet and doses the evaporator's supply. By adjusting the opening, it keeps pressure and flow consistent despite variations in load, outdoor temperature or heating demand.

Impact on efficiency, comfort and component lifespan

A correct setting improves the COP and limits swings. Too open, the risk is liquid returning to the compressor. Too closed, the evaporator "lacks" refrigerant, power drops and the heat pump runs longer. The result: a drop in seasonal efficiency, discomfort and premature wear on the refrigerant components.

Common signs of a poorly set or faulty heat pump expansion valve

A few typical signs come up often.

• Unstable indoor temperature, short cycles, more frequent defrosting.
• Abnormal frost on the heat exchanger or on the suction line.
• Hissing noises, clicking, rising electrical consumption.
• Refrigerant pressures out of range and recurring faults. A check by a qualified professional is essential.

The main types of expansion valves used in heat pumps

Thermostatic expansion valve: how it works, strengths and limits

On a heat pump, the thermostatic expansion valve regulates the refrigerant flow using a bulb and a diaphragm. It targets stable superheat at the evaporator outlet, to avoid liquid returning to the compressor. Strengths: a robust solution, fairly tolerant, well known for repairs. Limits: less responsive when conditions change quickly, fine-tuning can be tricky, risk of hunting if the installation is poorly balanced.

Electronic expansion valve: precision, value in modulation and installation conditions

The electronic expansion valve (EEV) controls the opening step by step using sensors. It handles modulation on inverter heat pumps better and improves efficiency at partial load. For installation, you need compatible controls, careful wiring, and rigorous commissioning (sensors, parameterisation, filtration) to avoid superheat faults.

Capillary tube and specific solutions: where you still find them on certain heat pumps

The capillary tube is a fixed orifice. It's mainly found on simple or older heat pumps, where cost comes first. It doesn't adapt the flow, so it's sensitive to the charge, the length of the lines and temperature differences. On some models, you also see hybrid solutions (calibrated orifice, solenoid valve) depending on the heating/cooling mode.

Properly sizing and choosing the expansion valve based on the heat pump and installation

Refrigerant compatibility, power ratings and operating ranges

Start by checking the right refrigerant (R32, R410A, R290) and the pressure class. The expansion valve is also chosen based on a nominal power and a modulation range. On an inverter heat pump, look at the minimum and maximum capacity at the expected evaporation and condensation temperatures, not just the catalogue kW figure. An electronic expansion valve helps keep the evaporator's supply stable over a wide range.

Air-to-water, air-to-air, hybrid heat pumps: criteria that change on site

On air-to-air heat pumps (direct expansion), the installation matters as much as the part itself. Bulb position, insulation, line length and mounting direction affect the regulation. On air-to-water heat pumps (plate heat exchanger), watch for pressure losses and frost risk at low temperature. On hybrid systems, synchronise the expansion with the boiler switchover logic to avoid swings. Goal: a steady flow.

Avoiding common mistakes: undersizing, superheat and instability

To avoid: an expansion valve that's too small (lack of flow, high superheat, dropping COP) or too large (hunting, liquid return). On site, check the charge, the filter drier, the cleanliness of the heat exchangers, then measure superheat and subcooling under stabilised conditions. To go further on overall sizing, see the 5 most common sizing mistakes. Repeated instability is often fixed through settings and diagnosis, not by adding refrigerant.

Settings, checks and troubleshooting: your on-site benchmarks

Useful measurements: superheat, subcooling and stability under load

On a heat pump, start with simple measurements: suction and liquid temperatures, pressures, and air or water ΔT. Useful superheat and subcooling are best read as a trend. Under stabilised load, look for steady values, with no drift, and compare against the manufacturer's table rather than a "universal" figure.

Typical failures: frost, lack of power, noise, short cycles

Persistent frost often points to insufficient airflow, a fouled evaporator or defrosting that isn't doing its job. Lack of power: check filters, flow rates and setpoint first, then refrigerant charge. Noise: track down vibrations, mountings, the fan and cavitation on the hydraulic side. Short cycles. Common causes: oversizing, a poorly positioned sensor, flow rate too low, or too aggressive a heating curve.

When to replace the component: filters, circuit moisture and network cleanliness

Fouled filters: replace as soon as pressure loss rises or flow rate drops. If the refrigerant circuit has been opened, replace the filter drier and carry out a thorough vacuum pull to limit residual moisture. On the hydraulic network side, a saturated sludge trap and a clogged heat exchanger end up costing more than cleaning them.

Good practice for 2026: safety, compliance and client expectations on heat pumps

Handling refrigerants and traceability: what matters in 2026

On a heat pump, safety starts with safe practices on the refrigerant circuit. Only work with the appropriate qualification and capacity certificate. Systematically record the type of refrigerant, the quantities added or recovered, and keep the tracking documents. In 2026, clients and inspections expect clean traceability, from the quote to the commissioning report.

Maintenance and advice for the client: preserving the heat pump and its components

Explain regular maintenance in concrete terms. Cleaning the filters and units, clearing the grilles, monitoring noise and cycles, and reminding them of the mandatory inspections based on the power and type of equipment. Give a simple sheet with 5 actions, and note down the settings delivered (setpoint, heating curve, modes). A well-set heat pump ages better and consumes less. To frame the requirements and check points, rely on the mandatory inspections and the associated good practice.

On-site talking points: explaining the expansion valve's role simply

The expansion valve is the "tap" that doses the refrigerant to the evaporator. Its fine adjustment avoids overheating, limits frost, and stabilises performance. Tell the client simply. If this dosing is off, the heat pump strains, starts more often, and wear speeds up.