The client’s long-term strategic objective was to expand their existing heating systems by introducing advanced energy management concepts and integrating new energy sources.As a first step toward this vision, the client decided to integrate heat pumps into an already deployed heating infrastructure based on conventional systems such as wood and pellet fired boilers.

The goal was to extend the system into a hybrid heating and cooling solution capable of combining multiple thermal and electrical energy sources.

This required the integration of:

• multiple thermal energy sources (traditional boilers and heat pumps)
• thermal energy storage
• controlled energy distribution throughout the building
• electrical energy sources, including grid electricity with tariff awareness and solar panels

While system-level architecture and product requirements were defined by the client, we were responsible for implementing, stabilizing, and validating specific firmware modules within a complex and evolving system architecture.

At the time we joined, development was already in progress. However, the client recognized that meeting aggressive project deadlines and maintaining system stability would require additional firmware engineers with experience in complex, reliability critical heating systems.

System Overview

The system is designed to provide both heating and cooling for residential and light commercial buildings.

By integrating a heat pump into the existing heating system, the client’s goal was to enable:

• heating and cooling using the same infrastructure
• flexible operation where the heat pump can act as either a primary or secondary energy source
• dynamic switching between energy sources based on availability, efficiency, and cost

A core aspect of the system is energy management. Since heat pumps operate on electricity while traditional boilers rely on solid fuels, the system continuously evaluates:

• electricity tariffs
• availability of solar-generated power
• current heating or cooling demand

Based on these inputs, the system dynamically selects the most efficient and cost-effective energy source at any given moment, while maintaining user comfort and system stability.

Challenges

• System Complexity and Specification Variability

The primary challenge was the overall system complexity. The interaction between multiple thermal and electrical subsystems made it difficult to define a final specification early in the project.

As development progressed, integration testing revealed additional requirements and edge cases that are typical for a complex, multi-source heating system. In several cases, reported issues helped clarify how the system was expected to behave. This required continuous clarification and refinement of expected behavior, particularly in non-ideal and transitional operating states.

• Limited System-Level Visibility

From our side, an additional challenge was testing. We were responsible for individual firmware modules but did not always have access to the complete physical system. This limited system-level visibility made it challenging to validate behavior across all operating scenarios without dedicated simulation and testing tools, especially for scenarios involving multiple interacting subsystems.

• High Cost of Failure

In systems like this, failures are critical and costly:

• incorrect energy distribution can significantly increase energy consumption
• instability directly affects user comfort
• late-stage defects can have serious financial consequences for the manufacturer

These constraints required a development approach focused on robustness and early risk reduction.

Our Contribution

Specification Refinement

To address specification-related challenges, we worked closely with the client to:

• analyze and refine existing requirements
• discuss real-world operating scenarios
• clearly communicate how changes in one subsystem impact the entire system

When certain scenarios were not fully specified, we proactively identified safe and predictable behavior and implemented it to ensure overall system robustness.

Firmware Architecture and Implemented Modules

The system was based on a firmware platform with more than 10 years of continuous evolution. Our task was to extend this platform with new functionality required for hybrid energy management.

We developed firmware for several critical modules:

1. Auxiliary Buffer Module for Heat Pump

A core module responsible for ensuring continuous and stable heat pump operation. Without proper load balancing, the heat pump would frequently stop, restart, or enter error states under fluctuating load conditions.

2. Auxiliary Electric Heater Module

An electric heater responsible for:

• freeze protection
• heating water to temperatures required for safe defrosting of the heat pump
• enabling reliable startup of the heat pump in cold operating conditions

3. Room Cooling Control Module

Firmware responsible for initiating and controlling cooling operation, ensuring correct and safe behavior when switching between heating and cooling modes.

4. Energy Distributor Module

A central control module that manages:

• distribution of energy requests
• coordination between available energy sources
• safe and prioritized energy delivery across the system

5. Electrical Energy Management Module

A higher-level module that dynamically adjusts system parameters based on:

• electricity tariffs
• solar energy availability
• current heating or cooling demand

This module continuously optimizes temperature setpoints and operating modes to maximize efficiency while minimizing operating costs.

Testing Strategy

Automated Testing

To identify risks early, we implemented a custom automated test.

Key characteristics included:

• execution of production firmware code without modification
• automated execution of several hundred test cases within minutes
• clear differentiation between firmware logic issues and simulation artifacts

In addition to firmware, we delivered test scripts, testing frameworks, and configuration tools that enabled the client to independently reproduce results and significantly reduce manual testing effort.

Also, to speed up development, we executed unit testing, integration testing, and regression testing in parallel, which resulted in:

• very few defects reaching the client before their formal testing phase, with the system already stable at the beta stage
• no need for on-site emergency engineering support
• predictable and stable deliveries despite late-stage change requests

Results

The result was a highly interconnected system in which thermal and electrical energy flows had to be coordinated reliably, efficiently, and safely under varying operating conditions.

The client achieved:

• a fully integrated hybrid heating and cooling system
• stable and reliable heat pump operation
• an effective and adaptive energy management solution
• on-time realization of their product roadmap

Most importantly, the system reached a high level of stability, which is critical in heating applications where failures are costly and directly impact end users.

This level of stability also allowed the client to implement late change requests, even shortly before release, without risking overall system quality.