The SENZOMATIC MHT04 is a combined sensor designed for continuous moisture monitoring in timber building structures. It measures wood moisture content via stainless steel electrode screws, air humidity and temperature both inside the building cavity and in the ambient environment, dew point temperatures, and absolute humidity. The sensor communicates via Modbus RTU over RS-485 at 19200 baud with even parity.
The TapHome template reads 8 measurement values and 3 service attributes from holding registers using BigEndianFloat encoding. Each sensor has a unique Modbus address printed on its label. TapHome connects to the sensor directly via the Multi IO Box RS-485 interface, bypassing the proprietary SENZOMATIC central unit (CU07/CU08) and cloud platform.
Hardware connection
The MHT04 sensor uses a proprietary 4-wire Senzomatic cable with a black snap-in connector on the sensor end and free wires on the other end. For TapHome integration, the RS-485 data lines from the Senzomatic cable connect directly to the TapHome Multi IO Box.
Cable color coding
| Wire Color | Function |
|---|---|
| Red | Power supply (Vcc, 5–12 V DC) |
| Black | Ground (GND) |
| Green | RS-485 B- |
| Yellow | RS-485 A+ |
Wiring to TapHome Multi IO Box
Connect the Senzomatic cable wires to the Multi IO Box RS-485 terminals:
- Yellow (A+) → Multi IO Box A+
- Green (B-) → Multi IO Box B-
- Black (GND) → Multi IO Box GND
- Red (Vcc) → external 5–12 V DC power supply positive terminal
- Power supply GND → shared with Multi IO Box GND
The sensor requires an external 5–12 V DC power supply (typical consumption < 5 mA, max 25 mA). The Multi IO Box does not supply power on the RS-485 bus — a separate power source is required.
In the standard SENZOMATIC installation, all sensor cables are joined color-to-color using WAGO splicing connectors in the switchboard. For TapHome integration, the same principle applies — connect the yellow (A+) and green (B-) wires to the Multi IO Box RS-485 terminals instead of to the CU07 central unit. Keep cables at least 10 cm apart from power cables to avoid interference.
Configuration
Modbus RTU parameters
| Parameter | Value |
|---|---|
| Baud rate | 19200 |
| Data bits | 8 |
| Parity | Even |
| Stop bits | 1 |
| Default Slave ID | 1 (per sensor label) |
Each MHT04 sensor has a unique Modbus address printed on its label. Set the Slave ID in TapHome to match the address on the sensor.
Poll interval
The template uses a 5-minute (300 000 ms) poll interval with a 4-second read/write timeout and 5-second delay between requests. Building moisture conditions change slowly, so 5-minute polling is sufficient for continuous monitoring.
Device capabilities
Ambient environment monitoring
The template provides three sensors for the ambient environment surrounding the sensor:
- Ambient Temperature & Humidity — reads temperature in °C and relative humidity in %RH from the ambient environment. Humidity is converted from raw register value by dividing by 100.
- Ambient Dew Point — dew point temperature of the ambient air in °C, indicating the temperature at which condensation would form.
Cavity monitoring
Four sensors monitor conditions inside the building structure cavity (the space within the wall or floor assembly):
- Cavity Temperature & Humidity — temperature in °C and relative humidity in %RH inside the cavity. Humidity converted from raw value by dividing by 100.
- Cavity Dew Point — dew point temperature inside the cavity in °C.
- Cavity Absolute Humidity — absolute humidity of air in the cavity in g/m³, providing a moisture content measure independent of temperature.
Material moisture and electrode resistance
The core measurement capability of the MHT04 sensor:
- Material Moisture Content — wood moisture percentage measured via stainless steel electrode screws driven into the timber. Raw register value divided by 100 to produce percentage. This is the primary indicator of structural moisture risk.
- Resistance Positive Direction — electrode resistance in MΩ measured with positive polarity current flow through the wood.
- Resistance Negative Direction — electrode resistance in MΩ measured with negative polarity current flow. The two resistance readings together provide a more reliable moisture assessment by compensating for electrode polarization effects.
Service diagnostics
Three service attributes provide sensor identification:
- Sensor Type — sensor model identifier
- Sensor Serial — unique serial number of the sensor unit
- Chip — sensor chip identifier
Additional capabilities
The MHT04 sensor also supports flood detection via an optional flood cable that connects to the white connector on the sensor. This capability is not included in the current template and can be added in a future update.
Troubleshooting
No communication with sensor
- Verify the Slave ID in TapHome matches the address printed on the sensor label
- Check RS-485 wiring — yellow wire to A+, green wire to B-
- Confirm Modbus parameters: 19200 baud, 8 data bits, even parity, 1 stop bit
- Ensure the sensor has external power (5–12 V DC on the red wire)
- Check that GND is shared between the power supply and the Multi IO Box
Humidity readings appear incorrect
- Humidity values from the sensor are raw values multiplied by 100 — the template automatically divides by 100. If values appear 100x too high, verify the template is correctly applied
- Ensure the sensor is not in a condensing environment (operating range: 0–80 %RH non-condensing)
Unstable or noisy readings
- Keep the Senzomatic cable at least 10 cm away from power cables
- For long cable runs, ensure proper RS-485 termination on the last device
- Verify electrode screws are firmly attached to the timber with no play — loose screws cause unreliable moisture and resistance readings
