The Thermostat Wars: a room-aware heating controller

Field: Embedded systems · local IoT · home automation · feedback control

Original article: The Thermostat Wars: Revenge of the Engineer, published October 22, 2026. This page is a technical summary of that complete first-person build story.

The problem: one thermostat, very different rooms

My house had uneven heating. The living-room thermostat could only measure its immediate surroundings, while one bedroom became excessively hot and another stayed relatively cold.

I wanted the heating system to consider room-level measurements—not just the temperature near a single thermostat—without making the house dependent on experimental software for basic furnace operation.

What I built

• Honeywell T6 Pro Z-Wave thermostat installed on an existing two-wire R/W heating circuit.
• Zooz 800 Series Z-Wave USB controller connected to my Ubuntu workstation, with Z-Wave JS UI bridging the thermostat to Home Assistant.
• Two DIY sensing nodes, each combining an ESP8266 D1 Mini and a BME280 temperature/humidity/pressure sensor.
• ESPHome, hosted on Buffy, for firmware, sensor reporting and integration with Home Assistant over Wi-Fi.
• Home Assistant, also running on Buffy, to calculate a weighted room temperature and send thermostat setpoint requests.

There are three temperature inputs: living room (from the Honeywell itself), my bedroom, and my roommate's bedroom.

The engineering chain

Bedroom #1: BME280 → ESP8266 ── Wi-Fi / ESPHome ──┐
Bedroom #2: BME280 → ESP8266 ── Wi-Fi / ESPHome ──┤
                                                   ├→ Home Assistant
Living room: Honeywell T6 Pro ── Z-Wave JS UI ──────┘
                                                       │
                                                       ↓
                                                Weighted temperature
                                                + threshold rules
                                                       │
                                                       ↓
                                            Thermostat setpoint request
                                                       │
                                                       ↓
                                             Honeywell's own HVAC control

Debugging the physical sensors

My early assemblies connected to Wi-Fi but failed to detect their BME280 sensors. ESPHome logs reported BME280: Communication failed; the I²C scanner initially found no devices. After replacing unreliable pin connections with proper soldered wiring, the scanner discovered the BME280 at address 0x76 and sensor readings appeared.

That failure taught me to debug hardware and software separately: working firmware and Wi-Fi do not guarantee a reliable electrical connection.

The actual weighted-temperature design

I used the following weights:

The calculation is:

T_weighted = 0.20 × T_living + 0.40 × T_bedroom + 0.40 × T_roommate

For 69°F, 71°F and 65°F, respectively, this produces 68.2°F. All inputs are converted to consistent units before the calculation.

Heating decisions, hysteresis and boundaries

The starting target was 69°F with a lower threshold of 68.5°F and an upper threshold of 69.5°F.

• Below the lower threshold, the automation can request a higher setpoint.
• Above the upper threshold, it requests the lower setpoint.
• Within the band, it generally leaves the requested setpoint unchanged.
• If my bedroom reaches 72°F, a higher-priority rule asks the system to reduce heating even when the weighted temperature is low.
• Requested setpoints in the initial design were bounded to 68–72°F.
• The supervisory automation evaluates conditions approximately every ten minutes; this is not a guaranteed minimum furnace runtime.

The command is sent through Home Assistant's climate.set_temperature action, which changes the Honeywell thermostat's target. The Honeywell still decides when to call for heat. Lowering its target does not guarantee that the furnace instantly stops in every room.

What happens if something fails?

The thermostat retains standalone operation without Buffy or Home Assistant. Home Assistant validates sensor values and, when running but given invalid measurements, can request a 68°F fallback setpoint. This cannot substitute for a hardware safety system or guarantee that a thermostat will reset itself when the server goes offline.

The completed result

The sensors, ESPHome, Z-Wave integration, weighted calculation and setpoint automation worked together. I confirmed commands through Home Assistant automation traces and the thermostat's reported state. This is a functioning end-to-end integration, not a claim of independently controlled HVAC zones or measured comfort improvements.

Try it: Interactive thermostat controller — change the room temperatures and inspect the resulting example decision.

Read the full story: The Thermostat Wars: Revenge of the Engineer includes the complete process, soldering failures, system setup and reflections.