Retrofitting a Multi-Zone Hydronic Heating System with a DIN-Mount Micro-PLC

The Control Problem

Upgrading aging building automation rarely means replacing everything. In this project, a 450 m² residential building had a fully functional hydronic heating system: five underfloor zones, three circulation pumps, a heat pump, and a buffer tank, all wired to a relay box with mechanical timers and wall thermostats.

The hardware was sound. The control logic was not: no fault detection, no temperature logging, no remote access, and no way to know whether a zone valve had actually opened.

The brief was to replace the control logic without touching the field wiring. The chosen controller: the EQSP32 micro-PLC, DIN-rail mounted in the existing panel.

I/O Inventory

Before writing a line of logic, the I/O requirements were mapped the same way you would for any PLC project:

Total hard I/O: 11 discrete outputs (relay), 5 discrete inputs. Well within the EQSP32's 16-port capacity.

Control Architecture: Local Logic First

The fundamental design principle for any safety-relevant installation applies here: the controller must operate correctly regardless of what the network does.

All safety-critical logic runs locally on the EQSP32:

• Valve open-switch confirmation: each zone valve has an end-of-travel open switch wired back to the EQSP32. If the switch does not close within a timeout window after the valve output is energised, the output is cut and a fault is flagged. The EQSP32 will not enable circulation for a zone until it has hardware confirmation that the valve is actually open.
• Pump interlock: circulation pumps are only enabled when at least one zone valve has confirmed open via its open switch.
• Heat pump minimum run timer: once the heat pump enable output is energised, a 20-minute hold-off prevents the output from de-energising, regardless of zone demand. This protects the compressor from short-cycling even if all zone setpoints are satisfied shortly after startup.
• HMI heartbeat watchdog: the HMI server publishes a periodic heartbeat over MQTT. The EQSP32 monitors this heartbeat and, if it is absent for longer than a configured timeout, treats the HMI as offline: all zone valve outputs, circulation pump outputs, and the heat pump enable are de-energised in a controlled shutdown. This is a deliberate design choice: an unresponsive HMI is treated as a loss of supervision, not a safe condition.
• Sensor watchdog: each wireless temperature sensor is monitored for regular heartbeat messages. If a sensor goes silent, the corresponding zone is disabled automatically.

The HMI heartbeat interlock inverts the usual assumption. In many installations the controller keeps running if the HMI drops out. Here, the HMI is considered part of the supervision chain: if the operator layer is no longer responding, the controlled system shuts down rather than running unsupervised.

The HMI Layer: Home Assistant as Open-Source SCADA

With safety logic running on the controller, a separate layer handles operator visibility, scheduling, and remote access. In this project that layer is Home Assistant, an open-source automation platform running on a low-power Linux mini PC backed by a UPS for ride-through during brief power events.

Think of it as a lightweight SCADA: it provides per-zone thermostat setpoints, trend logging, alarm history, and a web dashboard accessible from any browser or mobile device. It does not run any safety logic. The connection between the two layers is monitored in both directions: the HMI publishes a periodic heartbeat that the EQSP32 expects to receive. If the heartbeat stops, the EQSP32 initiates a controlled shutdown of all outputs rather than continuing to operate without supervision.

This separation of controller logic and HMI is standard practice in industrial automation. The difference here is cost: the entire HMI layer runs on open-source software with no per-seat licensing.

MQTT: The Field Bus Between Controller and HMI

Communication between the EQSP32 and Home Assistant uses MQTT, a lightweight publish/subscribe protocol widely used in industrial IoT alongside Modbus TCP and OPC-UA. The EQSP32 handles MQTT natively: broker connection, topic management, and Home Assistant device discovery are all handled automatically once credentials are configured via the EQConnect app.

From the application code perspective, exposing a relay output to the HMI is a two-step declaration:

// In setup() -- declare the output to Home Assistant
createControl_Switch("Zone A Pump", "mdi:pump");

// In the control loop -- update state after each logic evaluation
updateControl_Switch("Zone A Pump", pumpA_state);

The relay then appears in Home Assistant as a controllable switch with real-time state feedback, historical logging, and automation support, with no broker configuration, no topic mapping, and no retain flag management required.

Power Efficiency: Relay Derating Built In

Running 11 relay outputs simultaneously in a compact panel generates heat and draws continuous coil current. The EQSP32 addresses this with an automatic derating feature on relay outputs:

• On activation: full drive current for a configurable period (typically 150 ms), enough to guarantee pull-in under all load conditions.
• After pull-in: automatic reduction to a holding level (e.g. 35% duty cycle via PWM).

The result is roughly one-tenth the steady-state coil power compared to holding at full current. Built-in flyback diodes suppress inductive kick at the terminals, with no external suppression components needed. For a panel with 11 relay outputs running near-continuously during heating season, this is a meaningful efficiency gain and keeps enclosure temperatures down.

Trend Logging and Fault Diagnosis

One of the advantages of pairing a micro-PLC with an MQTT-connected HMI is automatic trend storage. Every sensor value and output state change is time-stamped and stored by Home Assistant's recorder.

In this installation, the trend data surfaced a real fault within the first heating season: one zone was consistently underheating despite the valve showing open. The temperature vs. setpoint graph revealed the zone was heating too slowly, pointing to a sticky valve actuator that was only partially opening. Without trend data, this would have been attributed to a poorly insulated room.

The same trend graphs also provide heartbeat monitoring for each wireless sensor: a missing heartbeat is immediately visible as a gap in the timeline, and the watchdog logic on the EQSP32 responds to it automatically.

Results

The EQSP32 replaced a relay box. What came with it (structured I/O, local safety logic, built-in MQTT, and relay derating) is what makes a micro-PLC the right tool for this job rather than a relay timer panel or a consumer smart home hub.