I Added a Non-Wi-Fi Mitsubishi AC to Home Assistant

When software engineer Ivan Gomez Arnedo set out to integrate his non-Wi-Fi Mitsubishi air conditioner into his home automation stack, he encountered a systemic problem familiar to thousands of smart home enthusiasts: vendor lock-in and functional obsolescence engineered into consumer hardware. Despite manufacturing some of the most efficient heat pumps and split-system HVAC units on the market, Mitsubishi Electric’s native smart home solutions—specifically their Kumo Cloud and MELCloud adapters—have earned a reputation among engineers for high hardware costs, frequent cloud outages, severe API rate limits, and frustrating operational latency.

Rather than paying hundreds of dollars for proprietary cloud dongles that route local temperature commands through remote corporate servers, Gomez Arnedo leveraged open-source reverse-engineering work to connect his offline AC directly to Home Assistant. By tapping into the internal CN105 serial interface present on the HVAC unit’s control board using a low-cost ESP32 microcontroller, the project demonstrated how low-cost hardware hacks can liberate high-value appliances from proprietary ecosystems. The project quickly sparked widespread discussion across technical communities like Hacker News, highlighting a growing rift between consumer desire for local, private control and appliance manufacturers' insistence on subscription-driven, cloud-dependent platforms.

Key Takeaways

• Direct Board-Level Interface: Non-Wi-Fi Mitsubishi HVAC units contain an unpopulated or internal 5-pin red JST connector (CN105) that exposes a raw UART serial interface for local control.
• Complete Cloud Bypass: Using an inexpensive ESP32 or ESP8266 microcontroller flashed with ESPHome or custom C++ firmware, users can bypass OEM cloud bridges like Kumo Cloud or MELCloud entirely.
• Sub-Second Local Latency: Local integration provides instant bidirectional control over target temperature, mode switching, fan speed, and vane positioning without relying on an external internet connection.
• Broader Smart Home Shift: The viral response reflects an industry-wide pivot toward local-first home automation architectures, driven by security concerns, privacy, and vendor reliability issues.

What Happened?

The core of Ivan Gomez Arnedo’s project relies on a simple technical reality that major appliance manufacturers rarely publicize: most "dumb" or non-Wi-Fi air conditioners share the exact same internal printed circuit board (PCB) architecture as their high-end, connected counterparts. To streamline manufacturing, Mitsubishi Electric equips almost all of its indoor mini-split and ductless HVAC units with a standardized internal serial expansion port labelled CN105 (or CN92 on certain regional board revisions).

Traditionally, Mitsubishi uses this CN105 port to attach their official Wi-Fi interfaces, such as the MAC-568IF-E or Kumo Cloud adapters. These official dongles can cost anywhere from $150 to $300 per indoor unit, require professional installation in many jurisdictions, and force all control traffic through external cloud servers. When those servers experience downtime, or when the manufacturer decides to deprecate older hardware generations, homeowners are left with expensive appliances that suddenly lose their smart functionality.

Recognizing these limitations, Gomez Arnedo synthesized existing open-source reverse-engineering work—most notably the foundational SwiCago/HeatPump library and modern ESPHome components—to build a custom hardware bridge. By wiring a sub-$5 microcontroller directly into the CN105 header on his Mitsubishi unit, he established direct, bi-directional communication between the AC unit's internal microcontroller and his local Home Assistant server via Wi-Fi, completely eliminating the manufacturer's cloud from the loop.

The post gained significant traction on Hacker News, sparking deep technical debates among embedded systems engineers, HVAC technicians, and smart home builders. The discussion highlighted a broader trend: consumer tech enthusiasts are increasingly unwilling to accept cloud-tethered IoT devices that introduce unnecessary latency, security vulnerabilities, and artificial paywalls for basic operational data.

The Technology Behind It

Understanding how this hack functions requires looking into the lower layers of embedded serial communication and microcontrollers. The Mitsubishi CN105 header is a 5-pin JST-PA series connector soldered directly onto the main control board of the indoor unit. This interface exposes five key pins:

• 12V DC / 5V DC Power: Provides operational power directly from the mainboard.
• Ground (GND): Common ground reference.
• Transmit (TX): Serial data output from the AC mainboard (5V TTL logic level).
• Receive (RX): Serial data input to the AC mainboard (5V TTL logic level).

Because modern microcontrollers like the ESP32 or ESP8266 operate on 3.3V CMOS logic levels, connecting an ESP32 directly to the 5V serial lines of the CN105 port risks damaging the microcontroller's GPIO pins. Implementations must incorporate a bi-directional logic level converter (LLV) or a simple resistor voltage divider to step down the 5V TX signal from the AC unit to a safe 3.3V for the ESP board.

"By tapping directly into the CN105 serial interface, developers gain access to raw operating telemetry—including heat exchanger temperatures and exact compressor frequencies—that proprietary cloud apps obscure from the end user."

On the software side, communication over the CN105 port uses a asynchronous serial protocol configured at 2400 baud, 8 data bits, even parity, and 1 stop bit (2400 8E1). The AC unit expects structured packet frames containing control commands (packet type, function code, payload data, and a checksum byte). The microcontroller continuously polls the AC for state updates while translating incoming payload frames into readable metrics like room temperature, operating mode (Heat, Cool, Dry, Fan, Auto), fan speed, and vertical/horizontal vane positions.

When running software frameworks like ESPHome, the microcontroller converts these binary serial packets into Native API calls or MQTT topics that Home Assistant natively understands. This exposes the unit as a standard climate entity within Home Assistant. Commands sent from the Home Assistant dashboard or automation scripts—such as lowering the setpoint by 1°C—are instantly packaged into a 2400-baud serial frame by the ESP32 and transmitted directly over the wires into the HVAC mainboard. The entire round-trip command execution takes less than 100 milliseconds, compared to the multi-second latencies common with cloud-based APIs.

This level of direct hardware interfacing mirrors broader industry efforts where engineers bypass closed proprietary codebases to build resilient, local solutions—a process similar to how modern software teams tackle engineering real-world hardware interfaces and complex legacy enterprise systems without relying on third-party middleware.

Why It Matters & Industry Impact

This project is more than an isolated weekend DIY hack; it represents a functional critique of the modern Internet of Things (IoT) landscape. Over the past decade, major appliance manufacturers have aggressively shifted toward cloud-centric smart home models. While cloud connectivity simplifies initial setup for non-technical consumers, it introduces systemic vulnerabilities that undermine device longevity and user autonomy.

From an enterprise and economic perspective, the success of local-first HVAC hacks highlights four major industry shifts:

1. Decoupling Hardware Lifespans from Software Support: HVAC systems are designed to operate for 15 to 25 years. Cloud services and server infrastructures, however, are routinely deprecated within 5 to 10 years. Local serial hacks decouple the long physical lifespan of heavy appliances from the short operational cycles of corporate software teams.
2. Cybersecurity and Surface Area Reduction: Cloud-connected HVAC units represent potential entry vectors into residential and commercial networks. If a vendor's cloud service is compromised, millions of connected units could theoretically be manipulated simultaneously, potentially destabilizing local power grids. Local-only integrations keep control traffic entirely within the local area network (LAN), isolated behind firewalls.
3. Grid Decarbonization and Virtual Power Plants (VPPs): Effective demand-response programs require precise, low-latency control over energy-intensive appliances. Heat pumps and air conditioners are primary candidates for load shifting. By establishing reliable local interfaces, energy managers and smart home systems can throttle energy consumption dynamically during peak grid stress without relying on fragile cloud APIs.
4. Pressure on Hardware Supply Chains: The availability of inexpensive, open-hardware microcontrollers allows global developer communities to create alternative hardware ecosystems. This dynamics reflects larger shifts in how global technology hardware is sourced and deployed, where low-cost components are altering traditional market dominance—much like how shifts in hardware supply chains and electronic component sourcing continue to reshape global tech manufacturing.

What Experts & Sources Say

The technical community’s reaction on platforms like Hacker News and specialized home automation forums demonstrates strong support for local serial integration, paired with frustration toward traditional OEM software offerings.

Multiple embedded engineers participating in the Hacker News discussion noted that Mitsubishi’s native Kumo Cloud architecture suffers from systemic design flaws, including dropped connections, slow polling rates (often taking minutes to update room temperature states), and restrictive API rate limits that break third-party home automation platforms. One commenter noted: "Kumo Cloud is expensive, unreliable, and requires an internet connection to turn off an AC unit that is ten feet away from me. Flashing an ESP32 with ESPHome via the CN105 port completely fixed every issue I had."

IoT security researchers also point out that local serial hacks return control to the physical owner of the hardware. When an appliance relies on an external API, the manufacturer retains implicit ownership over the device's feature set. Manufacturers can retroactively lock features behind paywalls, introduce subscription fees for scheduling, or shut down servers entirely when hardware reaches end-of-life status. Microcontroller-based local bridges physically prevent remote vendor intervention.

Furthermore, developers building on open platforms emphasize that community-maintained libraries often outperform official corporate software in terms of stability and features. Open-source communities collaborate to patch bugs, add micro-optimizations, and maintain legacy hardware compatibility long after commercial vendors have moved on—a pattern seen across many open-source ecosystems, including initiatives like community-driven open-source mapping projects where decentralized contributors maintain critical shared infrastructure.

What Happens Next?

Over the next 6 to 12 months, the ecosystem surrounding DIY local HVAC automation is expected to mature along several key trajectories:

• Commercialization of Open Hardware Assemblies: While building an ESP32 bridge historically required soldering individual jumper wires and logic level converters, small hardware startups are now selling pre-built, plug-and-play PCB modules equipped with official JST-PA connectors and enclosure cases. Projects like standard ESPHome-based Mitsubishi dongles are moving from niche developer projects to commercial off-the-shelf accessories.
• Expansion to Other Major Manufacturers: The underlying reverse-engineering methodology applied to Mitsubishi’s CN105 port is expanding rapidly across other major HVAC brands. Similar serial header hacks are gaining traction for Daikin (S21 port), Panasonic (CZ-TACG1 interface), Fujitsu, and LG systems, creating a unified blueprint for local HVAC control across brands.
• Manufacturer Pushback vs. API Openness: Appliance manufacturers face a strategic choice. They can either locked-down internal diagnostic ports with encrypted firmware, or recognize that local access is a primary selling point for technical buyers. If OEMs continue to restrict local access, they risk alienating high-value enterprise and residential customers who demand local integration with energy management tools.
• Matter and Thread Adoption: As the Matter smart home standard expands to cover HVAC and climate systems, manufacturers will face pressure to support local IP-based control out of the box. However, until Matter adoption becomes standard across mini-split systems, local microcontroller bridges will remain the primary solution for privacy-conscious users.

Bigger Picture

The success of Ivan Gomez Arnedo's Mitsubishi Home Assistant project is part of a larger trend in technology: the friction between consumer desire for local computing control and corporate efforts to monetize hardware through cloud services. This dynamic extends far beyond air conditioners—it applies to smart TVs, EV chargers, solar inverters, and home robotics.

As home automation shifts toward decentralized, edge-AI models—where local language models and automation engines run entirely on home servers without transmitting data to external clouds—the reliance on cloud-dependent IoT peripherals becomes a clear system bottleneck. A smart home running high-speed local AI models cannot afford to wait five seconds for a cloud API call to complete just to adjust a room's fan speed.

By demonstrating how easy it is to bridge legacy hardware into modern local platforms using basic electronics and open-source software, projects like this establish a clear path forward for computing hardware. The future of smart devices does not lie in routing every byte of data through corporate clouds, but in building reliable, local interfaces that put users back in control of the physical machines in their homes.

Frequently Asked Questions

Is it safe to connect an ESP32 directly to a Mitsubishi AC mainboard?

Yes, provided you use the correct voltage level conversion. The CN105 port exposes 5V logic lines for serial communication, whereas ESP32 and ESP8266 microcontrollers operate on 3.3V logic. Connecting the 5V TX pin directly to an ESP GPIO pin without a logic level converter or resistor voltage divider can damage the microcontroller over time. The AC unit's internal board itself is protected, but proper circuit design is necessary for safe, reliable operation.

Will interfacing with the CN105 port void the manufacturer's warranty?

Plugging a third-party module into an internal expansion port can technically void manufacturer warranties if the device causes physical damage to the board. However, because the CN105 port is designed as a standard plug-and-play diagnostic and expansion header, using non-invasive serial readers does not modify the unit's native firmware or alter its permanent internal settings. If necessary, the ESP module can be disconnected without leaving trace modifications on the PCB.

Can this approach be used on non-Mitsubishi air conditioning brands?

While the CN105 protocol specifically applies to Mitsubishi Electric units, almost all major HVAC manufacturers (including Daikin, Fujitsu, LG, and Panasonic) include similar diagnostic or expansion headers on their internal control boards. Dedicated open-source projects exist for many of these brands, leveraging similar microcontroller setups (such as ESPHome) to achieve local, cloud-free control over serial interfaces.