A DIY smart thermostat built with an ESP32 and OLED display brings modern HVAC control to 120V fan-coil setups—without needing a 24V system.
Many HVAC upgrades assume you have a standard 24V control circuit. For residents in older apartments and some condos, that’s often not the case.
A new DIY approach from Misryoum addresses that gap: a smart thermostat designed specifically for 120V fan coil systems.. Instead of forcing an off-the-shelf thermostat to fit a wiring reality it wasn’t built for. the project focuses on creating a controller that can actually switch a fan coil unit on demand.
At the center of the build is an ESP32 running ESPHome firmware.. That choice matters because it keeps the device flexible and easy to iterate, while also enabling a clean user experience.. Misryoum notes the project pairs an integrated 2.42-inch OLED screen with automatic brightness adjustment. so the temperature and control parameters remain readable across different lighting conditions—an underrated detail in small wall-mounted devices.
For physical interaction, the thermostat uses a rotary encoder with a built-in button.. The hardware is designed to feel intentional rather than “gadget-like. ” with a front panel and overall enclosure approach meant to look neat in a home environment.. In practice. that kind of interface can reduce friction for everyday use. especially for anyone who doesn’t want to rely only on a phone app.
Cooling and heating control is handled through a relay that switches the fan coil system on and off based on the thermostat logic.. Misryoum highlights that the project uses an SHTC3 temperature and humidity sensor to monitor current indoor conditions.. That combination—temperature plus humidity—can make control feel more responsive than temperature-only approaches. because comfort isn’t just a single number.
The biggest practical implication is compatibility.. Many popular “smart thermostat” options are optimized around 24V HVAC control wiring.. When you’re stuck with 120V fan coil equipment. you’re often left either paying for specialized work or using less capable controllers.. This build shows a third path: bringing smart logic and a modern display to a setup that otherwise wouldn’t be an easy match.
Beyond the obvious convenience, there’s a reliability and safety angle to consider with any DIY HVAC control.. Relays, wiring, and switching circuits have real-world consequences when they’re tied to mains-voltage equipment.. Misryoum’s editorial takeaway is that the technical “how” is only half the story—anyone adapting similar ideas should treat the electrical side as the part that demands careful. methodical verification rather than experimentation.
Still, the broader trend is clear: smart home control is spreading into systems that weren’t originally designed for it.. ESP-based controllers and firmware like ESPHome have lowered barriers for creators who want local displays. physical controls. and automation logic without waiting for a product that matches their exact hardware.. For renters or condo residents. where major HVAC rewiring may not be an option. controller-level upgrades can be the most realistic improvement.
If Misryoum’s takeaway resonates. it’s also because this kind of project points toward a future where “smart thermostat” stops meaning one standardized product and starts meaning a compatible controller tuned to the system you already have.. The comfort gains might be incremental. but the day-to-day usability—clarity on the screen. straightforward knob control. and automatic management of heat and cool cycles—can make a noticeable difference over time.