By Eamonn Ryan
CAREL, on 2 December 2025, hosted a live webinar on the topic ‘How STone Virtual Loop can streamline the developers working life in the HVAC/R industry’ presented by Matteo Galenda, CAREL application specialist HVAC residential group solution marketing, and Simone Armano, CAREL SSD HVAC and food retail manager – digital department. This is part one of a three-part series.
The introduction. © RACA Journal
CAREL’s STone Virtual Loop is emerging as one of the most important digital tools in HVAC/R software development, bridging the long-standing skills gap between programmers and thermodynamics specialists. Introduced alongside the broader STone ecosystem, the Virtual Loop leverages digital twin technology to emulate heat pumps, chillers, air conditioners and other HVAC/R systems – enabling developers to test control logic with no need for a physical prototype or laboratory setup.
In an industry where prototype availability, lab bottlenecks, and repeated testing cycles often slow down product development, STone Virtual Loop offers a compelling alternative: software-driven validation that operates like a real unit but runs entirely on a PC. CAREL reports up to 20% reduction in project development time and laboratory costs, a figure consistent with digital engineering benchmarks across the HVAC sector.
A unified ecosystem for modern HVAC/R development
The Virtual Loop is part of the broader CAREL STone ecosystem, which supports manufacturers through the entire HVAC unit lifecycle, from design and development to commissioning and field service. Key components include:
- STone Programming Tool: A structured-text (ST) programming environment with rich libraries, modular architecture, and version-control support – ideal for distributed teams and agile development workflows.
- Simula: A virtual CPU that runs CAREL applications entirely in software, allowing rapid, hardware-free testing.
- SparkLE (Production Tool): Used on manufacturing lines for software upload, variable writing, and end-of-line unit checks.
- STone Gate: Tools for commissioning, maintenance and secure remote access.
Together these tools eliminate the fragmentation that historically plagued HVAC software development, providing a consistent workflow from code creation to in-field operation.
What STone Virtual Loop actually does
The STone Virtual Loop is essentially a digital emulation environment containing mathematical models of HVAC/R systems. It connects to either:
- A virtual controller (via Simula), or
- A physical CAREL controller (via USB/RS485)
The Virtual Loop feeds real-time thermodynamic data – pressures, temperatures, flows, compressor speed, valve positions – into the control software. The software then responds exactly as if connected to a real machine.
Current HVAC/R models available
As of the latest release, CAREL offers models for:
- Residential heat pumps (including domestic hot water)
- Commercial chillers (up to four circuits, with free-cooling capability)
- Two-circuit air conditioners (airflow and humidity control)
Additional models, including refrigeration cabinets and cold-room applications, are planned for 2026.
These models are simplified – not full performance digital twins. Their purpose is not precise thermodynamic prediction, but accurate logic interaction, allowing developers to test safety routines, defrost cycles, compressor management, fan control and alarm conditions.
