Node-HIU Switch

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A walk-through of the the Node-HIU Dashboard on a real world development.

The Node-HIU Switch is used to connect HIUs to a Local Area Network (LAN) to provide remote connectivity, and implement control strategies through distributed logic.

It guarantees efficient and reliable heat network operation, and is used by a growing number of house builders and local authorities.

Not just any BMS motioning system - the most advanced HIU management system in existence, from the manufacturers of the most efficient HIUs in existence.

The Node-HIU Switch Provides

  • 100% accurate commissioning of every single HIU on a heat network
  • Real-time, one second resolution data, so every event is captured
  • Remote commissioning
  • Remote user control
  • Warning and alarm detection, so no problem goes unrecognised
  • Routing of alarms to personnel, so no problem goes unreported
  • Performance monitoring and tracking
  • Reports from various stages of installation through commissioning, to to hand-over and beyond, uploaded to a shareable Dropbox folder
  • A connection between BMS systems in plantroom and HIUs as a Modbus slave
  • The means to implement peak load (capital expense) saving strategies, such as rapid load shedding
  • Additional sensor options for differential pressure and temperature sensing on network
  • Plantroom BMS functionality, including pump and boiler control as a Modbus master
  • End to end functionality on the same software platform
  • Device communications over RS485, RS232, Modbus, OneWire & Relays
  • Network communications over Ethernet, WiFi, GSM, or USSD
  • Built on unified open-source technologies

Fully automated, exceptionally efficient heat networks, with a resilient architecture based on distributed nodes that monitor performance and route alarms when they appear.

An IoT network for a heat network.


Network Dashboard

The Network Dashboard is run from one or two Node-HIU Switches on a network, having a fixed network address, to provide access to a browser based dashboard system.

This dashboard reads and filters all HIU traffic to present important information in the most useful way.

System Summary

To provide installation teams and site management with a simple visual indicator of a heat network's health and performance.

The number of warnings and alarms are clearly indicated, as well as the current system demand (litre/minute), and VWART (Volume Weighed Average Return Temperature).


As new warnings and alarms appear, they are forwarded via email to the relevant persons. The Node-RED programming environment allows alarms to be routed based on any logic required. Typically alarms will go to front-line engineers first, and if the persist after a specified time will go to a second tier of management, and so on.

Historic Loads and VWARTs

The summary page provides a link to the recorded load history, at 5 second resolution.


Commissioning Dashboard

The HIU Commissioning Dashboard provides access to data from all HIUs on the network.

Changes in HIU data are updated every second.


The dashboard shows current HIU readings, as well as settings, and operational parameters and checks. Alarms (need attention now) and warnings (need attention when convenient) are presented in the form of coloured boxes, giving a clear indication of what needs attention.

The following problems can be automatically identified by the system and presented as alarms:

  • Incorrect HIU settings
  • Low central heating pressure and leaks on heating circuit
  • Low differential pressure on network
  • Low temperature on heat network
  • Crossed pipes
  • Bypass flow on network
  • Blocked strainers and pipework
  • High/low/failure on central heating pump
  • Excessive delays in generation of DHW (hot water to taps)
  • Unbalanced radiators
  • HIU power off (loss of communication to single HIU)
  • Node power off(loss of communication to Node-HIU Switch)
  • Ethernet down (general loss of communication)

The data is also summed to drive operational statistics, such as the current and historic VWART - average return temperature weighed by volume.

Engineer Screen

The Engineer Screen provides more detailed data on selected HIUs, and allows HIU settings to be changed.


The engineer screen provides links to one second resolution recordings of all recent hot water events.


Other Dashboards

Plantroom control panel displayRemote DP & Temperature SensorPlantroom control systemDiversified load analysisM-BusData optionsHIU readings

Network Topology

Mobile Infrastructure



A temporary infrastructure enables the full system to be deployed without any cabling, using GSM communications to a cloud server.

Data charges for each system are approximately £1.50 per month per HIU, making hard-wired infrastructure preferable for long term use on large numbers of dwellings.

Wired Infrastructure




Extended to Fibre


Integral Ethernet

The following layout is for HIUs with onboard Ethernet (build in Node Switch).



The following layout is for HIUs with onboard Modbus.


MQTT Infrastructure

  • One to eight HIUs (Hydraulic Interface Units) connecting to a single Node-HIU Switch, over RS485. Cat6 cable (4 twisted pairs) is used to also convey M-Bus data, and two other.
  • One to eight (or more) Node-HIU Switches connect to an Ethernet Switch, typically using PoE (Power Over Ethernet) to obtain power as well as a network connection. Cat6 network cable is used for connection. From the Nodes, the M-Bus network is run separately, in parallel, in 2.5mm twin core cable.
  • Network Switches daisy chain (or loop) to the Internet Router / Modem / default gateway. Cat6 network cable is used for connection.
  • One or two Node-HIU Switches on a network are given a fixed address and host the key services with redundancy:
    • MQTT Messaging Service (can optionally be a cloud based service)
    • User Dashboard
    • VPN Service, for secure remote connection to dashboard


  • A Node-HIU Switch provides a Modbus Interface to BMS.
  • One or more Node-HIU Switches with attached differential pressure & temperature sensors to feed back riser data for pump control.
  • A port on the local network (LAN) is forwarded (by the default gateway) to the designated Node-HIU Switch providing the local networks MQTT services. This is to enable connections to be established from a plantroom, or for protected user connectivity via MQTT enabled phone apps.
  • A port on the local network (LAN) is forwarded (by the default gateway) to a designated Node-HIU Switch providing
    • HTTPS Connection, for encrypted remote connection
    • Reverse Proxy Hosting, for addressing devices running browser based services

And where cabling is not possible the GSM route is available.

Compatible HIUs

The Node-HIU system is designed to work with our full range of electronic HIUs, known across the industry as the most efficient systems ever tested independently to the BESA standards, as referred to in the latest codes of practice.

The SLIM HIUSingle Plate HIU for DHW or UFH.More...The DIGI HIUSingle Plate Electronic HIU for DHW with Direct Heating.More...The DATA HIUTwin Plate Electronic HIU.More...The SLIM Extra HIUSingle Plate Electronic HIU.More...The DATA SPLIT HIUTwin Plate Electronic HIU.More...The DATA Extra HIUTwin Plate Electronic HIU with 70kW Heating OutputMore...

The system can also interface with any HIU that uses Modbus RTU for communications.

Hardware Specification


Model Node-HIU Switch M8
HIU Connections 8 x RS485
8 x M-Bus (separate network)
Network Connections 1 x Ethernet
2 x M-Bus
Bluetooth Low Energy (BLE)
Modbus RTU (utilises one of the RS485 ports)
Power Standard: 5v 2.5A Mains Plug Adapter
Option: Power Over Ethernet (PoE)
Processor Quad Core 1.2GHz Broadcom BCM2837 64bit CPU
Memory 1GB RAM
Expansion 2 x USB Ports
40 Pin GPIO Header
Optional Peripherals Temperature Sensors (daisy chain of up to 10)
Differential Pressure Sensor
Flash Data Storage Drive

Software Specification

Logic Control Node-RED
Communications Protocols MQTT
M-Bus (Master)
Modbus (Master/Slave)
HIU Serial (Thermal Integration)
User Defined Serial
Hosted Services Node-RED Dashboard
Mosquitto MQTT Service
VPN Host
HTTPS Reverse Proxy (certificates required)
Client Services Heatweb Cloud Storage, for data logging
Dropbox, for shared data storage and retrieval
Email, for reporting and alarms
Software Platforms Node.js (Node-RED)


MQTT Topic Structure

We use a common MQTT topic structure across all projects.

Topics are split into 5 levels, separated by a slash. They are case sensitive and should be free of spaces or special characters.

network / node / device / group / key

These levels allow us to organise data in a standard structure, and to set permissions accordingly.

Level Description.
network A unique identifier for the heat network.
node An unique (for each network) identifier for each system running Node-RED on a network, MAC address, or a fixed LAN ip address.
device An identifier for the connected device (e.g. HIU serial number) or sub-system.
group The type of data. This is used for basic grouping, and for controlling data access.
key An identifier for the data.

As an example, the following would be a typical topic referring to temperature data:


Standard MQTT Topics for HIUs

The following table shows the structure of standard topics available for each HIU.


Group Key Description Units
dat tCHo Temperature of flow to central heating °C
dat tH Temperature of primary flow °C
dat tHoCH Temperature of primary return on central heating °C
dat tCo Temperature of DHW flow to taps °C
dat tHoDHW Temperature of primary return on DHW °C
dat last_tCo Last recorded tap use, DHW supply temperature °C
dat last_tH Last recorded tap use, primary supply temperature °C
dat last_tHoDHW Last recorded tap use, primary return temperature °C
dat TTdhw Setpoint temperature for DHW °C
dat fHCH Flow rate on primary for central heating ltr/min
dat fC Flow rate of DHW to taps ltr/min
dat fHDHW Flow rate on primary for DHW ltr/min
dat dpH Differential pressure on primary circuit bar
dat pCH Central heating pressure bar
dat stat Call for central heating (room thermostat) on volt free wiring connection 0 / 1
dat pump Pump activated 0 / 1
dat xCH Central heating valve position 0-270
dat xDHW DHW valve position 0-270
dat st System state
dat eS Error state
dat eT Time in error state seconds
dat vH Volume drawn on primary since boot litres


Group Key Description Units
settings setpoint Setpoint temperature for DHW °C
settings WarmupBoost Boost flow rate when flushing cold primary feed ltr/min
settings EcoMode Keep warm mode eco / comfort
settings PressureMode Differential pressure calculation mode auto / bar
settings PowerUpCounter Number of times the HIU controller has been powered
settings ErrorCounter Time in error state seconds
settings DHReturnLimit Central heating primary return limit °C
settings HESupply Maximum central heating supply temperature °C
settings PumpTimeout Central heating pump over-run seconds
settings PumpProtection Pump low pressure protection enabled 0 / 1
settings HEProtectionMode Central heating return limit enabled 0 / 1
settings DHWExchangerAntiLegionella Anti-Legionella cycle enabled 0 / 1
settings DHWExchangerEcoHeatTemperature Keep warm temperature °C
settings DHWExchangerEcoHeatTime Keep warm timer minutes
settings CentralHeatingControl Central heating enabled 0 / 1
settings PrePayShutOff HIU enabled (for prepay control) 0 / 1
settings BUILDDATE Firmware build date
settings BUILDTIME Firmware build time


Group Key Description Units
stat vC DHW accumulated total litres
stat DHWCount Number of tap uses
stat e_TEMPERATURESENSORS Error relating to temperature sensors
stat e_FLOWSENSOR Error relating to DHW flow sensor
stat e_PRIMARYCONTROL Error relating to valves
stat e_DISTRICTHEATING Error relating to heat network
stat e_CHRETURNTEMP Error relating to central heating return sensor
stat e_DHWTEMP Error relating to DHW temperature sensor
stat e_DISTRICTHEATINGTEMP Error relating to heat network temperatures
stat e_DHRETURNTEMP Error relating to DHW return sensor
stat e_CENTRALHEATING Error relating to central heating
stat e_WARMUPTIMER Error relating to warm-up times
stat e_CHTEMPWARNING Warning relating to central heating temperatures
stat e_CHTEMPERROR Error relating to central heating temperature sensor
stat tis Time in current state seconds
stat beacon HIU alive beacon

System Variables

Group Key Description Units
system node A unique identifier for the Node-HIU Switch
system network A unique identifier for the network
system Serial The serial number of the HIU
system lastcontact The time the HIU last made contact
system time The unix time the HIU last made contact
system property The property location
system Firmware HIU firmware version
system CPUcore_1 Node processor 1 usage  %
system CPUcore_2 Node processor 2 usage  %
system CPUcore_3 Node processor 3 usage  %
system CPUcore_4 Node processor 4 usage  %
system CPUMemory Node memory usage  %
system iplan IP address on local network
system mac MAC address
system ipwan IP address on internet
system tempid Temporary (weekly) id for HIU
system usb USB port identifier for HIU
system Nodeware Node software version

Node Settings

Group Key Description Units
local n_tempid Temporary id
local n_logtosd Log data to SD card
local n_logtoheatweb Log data to
local n_logtopool Log DHW data to public data pool under temporary (anonymous) id
local n_ptype Property type
local n_flow Node-red flow template
local n_network Network id
local n_rolemodel Node-red flow template author system
local n_SYSid id
local n_node Node id
local n_readonly read only file system
local n_email Administrator email address
local n_runcount Node power-up counter

MQTT is a publish/subscribe system, and devices can subscribe to topics using wildcards.

Description MQTT Topic
All network data (admin level) network/#
All data from a unique node +/node/#
All differential pressure data on the network network/+/+/+/dpH
All DS18B20 temperature sensors on network network/+/+/ds18b20/#
All Alarms from network network/+/+/alarm/#

Standard MQTT Topics for Heat Meters

The following table shows the structure of standard topics available for heat meters.


Note that HM stands for Heat Meter. Cooling meters would have a CM instead.

In the topic structure, the Node level represents the unique identifier (serial number) of the M-Bus Gateway the meter is connected to.

Group Key Description Units
dat tHMf Temperature of primary flow °C
dat tHMr Temperature of primary return °C
dat kwhHM Energy used to date kWh
dat kwHM Curent power kW
dat fHM Current primary flow rate litres/minute
dat vHM Volume used to date litres

Stored Data

Where data is stored it is in JSON format, the IoT standard.

The topic CO102XW/b4hdn76s/d234ab3c7800fee1/dat/tH would form part of a data object with the following structure:

CO102XW.b4hdn76s.d234ab3c7800fee1.dat.tH = 75.3

The output JSON would look like:


For multiple devices this may look like:




Modbus Slave Request Log


Modbus Setup

Address 17
Baud 9600
Data Bits 8
Parity None
Stop Bits 2
Data Type 16INT
Function Codes 3 (read), 6 (write)

Modbus Registers

Description Register Units
Lowest differential pressure 1 kPa [x10]
Age of Lowest 2 seconds
Highest differential pressure 3 kPa [x10]
Age of Highest 4 seconds


Node-RED is an open-source visual programming system, developed by IBM, based on JavaScript code and nodes that are programmed and wired together in a browser based editor. It can run on just about any computer system, from the humble Raspberry Pi to cloud based platforms.


It is used at every level of the Node-HIU system, and not just on our own systems, but also those of the mobile providers and IT equipment manufacturers. For the first time, all software logic is unified under one roof, making Node-RED a viable alternative to established BMS technologies.

  • HIU interface
  • Modbus interface to plantroom
  • Network DP & Temperature sensors
  • Cloud services on our Heatweb
  • Thingstream API, for logic routing of USSD messages over mobile network
  • Multitech LoraWAN wireless gateways, for logic routing network traffic

These virtual 'wiring diagrams' are known as a flows, with a number of flows combining to form the overall package. The flow file is saved in a json format that can be imported into other systems.

There are a huge array of existing nodes to be dropped into flows, with more web based service providers, OEMs, and collaborative groups, launching their own. The array and flexibility of these nodes puts existing BMS systems to shame.


The function node allows user defined software code, written in JavaScript, to create more complex functionality.

Related Equipment

Node-HIU GSM Switch


  • Sends data from anywhere there is any mobile signal on any network
  • Just requires power, via a standard plug socket or USB power source
  • From one to ten daisy-chained temperature sensors, 2 as standard.
  • Optional FTDI cable to connect to HIUs
  • Optional DP/Temp sensor

Node-HIU Touchscreen Panel

Plantroom Control Panel - Cover Off.jpg

The heart of the system.

Plantroom Control Panel - Cover On.jpg

  • Touch screen display
  • GSM modem
  • 2 year SIM contract with 5GB/m and fixed public IP address
  • Power supplies for controls
  • Node-RED controllers
  • MQTT server
  • VPN server
  • Valid HTTPS certificate
  • Reverse proxy
  • Pre-commissioned before arriving on site
  • Optional Modbus I/O module(s) for
    • analogue (0-20mA / 0-10v) inputs and outputs
    • digital inputs and outputs
    • SSR relay outputs
  • Optional M-Bus module(s) for meter reading
  • All network variables, inc. HIU data, plantroom data, meter data, and operational data presented over standardised MQTT data structure
  • Alarm detection and reporting via email, SMS, automated voice call or postal letter

See Also