Space heating dominates residential energy demand and carbon emissions; manual thermostatic radiator valves (TRVs) often deliver poor zoning and unnecessary heating. This paper presents a retrofit, low-cost Internet-of-Things (IoT) thermostatic radiator valve (IoT-TRV) that can be deployed across typical dwellings (8–12 radiators) without replacing radiators. Each IoT-TRV integrates an ESP32 microcontroller, temperature–humidity sensing (DHT11), passive infrared occupancy sensing, a window-open sensor, local display, and a rotary actuator to adjust valve position in five discrete settings. A smartphone interface provides real-time monitoring, manual override, notifications, and schedule configuration. To reduce dependence on in-home Wi-Fi coverage and support rapid multi-room coordination, the system uses a hybrid communications architecture: ESP-NOW wireless communication protocol enables router-free, low-latency peer-to-peer exchange of sensor data and control commands among valve nodes, while a designated responder bridges aggregated data to Wi-Fi and cloud services. Local access-point provisioning is implemented to avoid hardcoding network credentials. Prototype testing demonstrates reliable multi-node operation and improved responsiveness for remote monitoring and control, achieving sub-400 ms end-to-end updates over an approximately 150 m range, compared with 0.8–1.2 s baseline implementations. An indicative bill of materials suggests a component cost of about £21 per valve node for an 8-radiator configuration. The proposed approach supports zonal heating, occupancy-aware setback, and window-open suppression, offering a scalable pathway to reduce avoidable space-heating demand and associated household costs.