What is EVI (Evaporative Vapour Injection)
Enhanced Vapour Injection (EVI) is an advanced integrated-circuit technology used in Air Source Heat Pumps (ASHPs) to significantly boost heating capacity and efficiency, particularly in cold ambient conditions.
Engineering Performance, Not Just Promises
For engineers and consultants specifying commercial heating solutions, the performance of an Air Source Heat Pump (ASHP) is defined not by its peak output, but by its stable performance in the midst of a harsh winter. Standard heat pumps often do not meet expectations at this critical time, suffering a rapid and precipitous decline in heating capacity and flow temperature when ambient air temperatures drop below zero. This forces an immediate and heavy reliance on supplementary heating sources, often spiking OPEX and compromising system efficiency.
This is why Strebel’s advanced Enhanced Vapour Injection (EVI) circuitry is a non-negotiable feature for serious specifications. It is a sophisticated and proven engineering solution that ensures stable temperature output and robust capacity exactly when the heating load is at its highest.
The Technical Edge: How EVI Turbocharges the Cycle
The EVI circuit is best understood as a “turbocharged secondary circuit” integrated into the core refrigeration cycle, primarily designed to increase the mass flow of refrigerant within the compressor by means of a partial ‘secondary’ injection.
Key Mechanics:
- The Economiser Injection: After the refrigerant leaves the condenser, it passes through an additional expansion valve, and a small portion is diverted through an additional Plate Heat Exchanger (acting effectively as an ‘economiser’). This process changes the state of the refrigerant into a saturated liquid and intermediate-pressure saturated vapour.
- The Mid-Stage Boost: This vapour is then injected back into the compressor via a specialised mid-stage port. This process effectively boosts the cycle with a second refrigerant pass.
- Performance Result: This injection significantly increases the total refrigerant mass flow through the system, resulting in optimised performance.
In simple terms, EVI allows the heat pump to move a greater amount of energy (enthalpy) through the system per cycle, directly translating to superior capacity in cold ambient conditions as well as improved output and efficiency.
The Specification Advantage: Capacity, Temperature, and Efficiency
For the Building Services Engineer, EVI delivers three core advantages that significantly improve ASHP performance and ensure client satisfaction:
| Performance Metric | Non-EVI Standard ASHP | Strebel EVI Unit | Benefit to Specifier | |
| Output Temperature | Stable up to 68°C
(Falling in colder ambient air temperatures less than A0) |
Stable up to 78°C
(No decline in output down to A-15) |
Constant unchanged temperature output – even at sub-zero temperatures. | |
| Output Power at A-4°C | Significant capacity reduction at design temperature
(circa 30% lower output from A7 to A-4) |
Lower output capacity loss at design temperature
(circa 20% lower output from A7 to A-4) |
Potentially reduces the number of machines required as well as lower reliance on auxiliary heating systems. | |
| SCOP Improvement | Standard performance. | Marginal Improvement (circa SCOP 0.05-0.1) | Reduction in long-term OPEX. |
The Critical Specification Point
Strebel EVI systems—such as the LPi, VP, NTi, NTVi, and NTVMi series—can deliver the required output temperature (e.g., 68-78°C) even at very low air temperatures (e.g., -5-10°C). Non-EVI units often struggle with this – effectively being unable to deliver enough heat when ambient air temperatures fall below zero. This can then lead to a drop in comfortable temperature inside buildings or a requirement for supplemental heat sources.
Air Source Heat Pumps
Air Source Heat Pumps
Air Source Heat Pumps
Air Source Heat Pumps
System Reliability and Longevity
Beyond capacity, EVI is an investment in system reliability. Specifying units with EVI circuitry helps safeguard the entire installation:
- Reduced Thermal Stress: The injected vapour actively helps cool the compressor’s motor and discharge gas. By moderating discharge temperatures, EVI reduces thermal stress on components, potentially extending compressor lifespan.
- Shorter Defrost Cycles: EVI’s higher refrigerant mass flow and temperature stability lead to shorter defrost durations. This improves overall heating continuity, particularly on standalone systems, preventing prolonged interruptions to supply.
- System Stability: The ability to deliver more capacity per cycle means the compressor operates more efficiently, reducing mechanical wear and tear, and mitigating power surges, which is crucial when integrating with existing building electrical supplies.
EVI isn’t really a choice. It’s the sensible thing to specify when selecting an Air Source Heat Pump.
To find the exact EVI-equipped model for your project’s flow temperature requirements, use our ASHP Selector Guide or contact your Area Sales Manager today.
