An EVI (Enhanced Vapour Injection) compressor adds a second refrigerant injection port fed by an economizer heat exchanger, cooling the compression process mid-stroke. This lets a heat pump hold heating capacity and safe discharge temperatures under extreme conditions — sustaining operation down to -25 °C outdoor air and flow temperatures of 60 °C or more without handing the load to an electric backup heater.
How does an EVI compressor work?
An EVI compressor works by injecting medium-pressure refrigerant vapour into the scroll partway through compression. A small fraction of the liquid refrigerant leaving the condenser is expanded to an intermediate pressure and evaporated in an economizer heat exchanger, which simultaneously subcools the main liquid line. The resulting cool vapour enters the injection port, lowering the temperature of the gas being compressed and adding mass flow through the condenser.
The result is a two-stage compression effect from a single compressor: more heat output at the top of the cycle, a colder and safer compression process in the middle, and a subcooled liquid line that extracts more heat per kilogram of refrigerant at the evaporator.
Why do standard heat pumps struggle in deep cold?
Standard single-injection compressors lose capacity as outdoor temperature falls because suction pressure and gas density fall with it — less refrigerant mass flows per revolution, so output shrinks exactly when the building needs more. At the same time the compression ratio climbs, driving discharge temperatures toward limits that degrade lubricant and shorten compressor life.
A conventional air-to-water unit therefore fades below about -15 °C and leans on its electric backup — turning a COP 3–4 machine into a partial resistance heater precisely during the most expensive hours of the year.
What does EVI change in practice?
| Criterion | Standard compressor | EVI compressor |
|---|---|---|
| Usable operating range | To about -15 °C | Down to -25 °C |
| Capacity in deep cold | Falls steeply | Roughly 20–30% higher below -15 °C |
| Discharge temperature | Rises toward safety limits | Held down by injection cooling |
| Flow temperature at low ambient | Limited | 60 °C or more sustained |
| Backup heater reliance | High in cold snaps | Substantially reduced |
The numbers are indicative — the certified figures for any model come from EN 14511 test points and the manufacturer's operating envelope — but the pattern is consistent: EVI buys capacity and flow temperature exactly where standard cycles surrender them. Spare capacity also shortens recovery after defrost cycles: the unit rebuilds flow temperature in minutes instead of limping back while rooms cool.
Where is an EVI heat pump the right choice?
EVI earns its cost wherever high temperature lift is routine: continental and mountain climates with sustained frost, radiator retrofits that need 55–65 °C flow in midwinter, and buildings where the electric backup would otherwise dominate January bills. In mild maritime climates a good inverter compressor unit covers the load and EVI adds little. In bivalent layouts the practical effect is a balance point pushed far below the design norm — the boiler or electric stage becomes rarely used insurance rather than a midwinter workhorse.
The market context favours cold-climate capability: European heat pump sales grew 13% in 2025 to 2.88 million units, and Germany — a country with real winters — grew 50% (EHPA). Solimpeks equips its Varm Up Series with EVI DC inverter compressors rated for operation down to -30 °C, reaching COP up to 4.92 at A7/W35; whether your climate justifies EVI is answered honestly in Do heat pumps work below freezing?.
Frequently asked questions
Is EVI the same as an inverter compressor?
No — they solve different problems and are often combined. The inverter varies compressor speed to match demand; EVI adds a vapour injection stage that preserves capacity and safe discharge temperatures at extreme temperature lifts. A cold-climate unit ideally has both.
Does EVI help in mild climates?
Marginally. The benefit concentrates at high temperature lifts — very cold air or very hot water. Above roughly 0 °C outdoor temperature with low-temperature emitters, a standard inverter unit performs comparably and costs less.
What flow temperature can an EVI heat pump deliver?
Sustained 60 °C or more even at strongly negative outdoor temperatures, which is what makes EVI units suitable for radiator retrofits and domestic hot water without constant electric boosting.
Does an EVI heat pump still need a backup heater?
Usually a small one remains in the design as insurance, but the bivalent point moves sharply downward. Where a standard unit hands over to electric backup at -10 °C, an EVI unit keeps carrying the load toward -25 °C.