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0.1.0 • Public • Published


node.js library and command line tool for controlling Heatmiser Modbus Thermostats.

It has been tested with the following devices:


To install in your local project:

npm install heatmiser-modbus

Or to install globally and make easy use of the hmmb command line tool:

npm install -g heatmiser-modbus


To connect a Heatmiser Modbus Thermostat to a computer, you need a RS-485 adaptor.

Heatmiser Edge Connected to USB using RS-485 wiring

I initially had some trouble with connecting using a short (1m) cable during testing, until I discovered that the termination resistor in the Heatmiser Edge had to be switched off for short distances.

The following RS-485 interfaces have been tested with:

There are some very expensive industrial RS-485 interfaces out there. But I have found Waveshare to be a good balance between cost and quality.

To connect everything together, I used a 1 pair Screened 24-AWG RS-485 Cable:


As the Heatmiser devices do not have a RS-485 signal ground terminal, I left them disconnected. But I looped the shielding through between devices and connected the ground on the Waveshare adaptor.

Command Line Tool

The hmmb command line tool is provided to configure and control thermostats, without having to write any JavaScript.

Two things are required to use the tool:

  • The path to the serial port, that the modbus devices are connected to
  • The Unit Identifier (aka Communications ID) of the thermostat to talk to

These can be provided as command line options, or via environment variables. If the Modbus Unit Identifier is not given, it defaults to 1.

Usage: hmmb [options] [command]

Tool for controlling Heatmiser Modbus Thermostats

  -V, --version                      output the version number
  -d, --device <port>                The serial port device to connect to (eg /dev/ttyUSB0) (env: HMMB_DEVICE)
  -i, --id <num>                     The Communications ID of the device to control (1-32) (default: 1, env: HMMB_ID)
  -h, --help                         display help for command

  get-status                         Display thermostat status (including current temperatures)
  turn-on                            Turn on the thermostat
  turn-off                           Turn off the thermostat
  hold <temp> <hours:mins>           Set a different temperature for a desired duration
  set-temperature <temp>             Set the target room temperature
  set-floor-limit <temp>             Set the temperature limit for the floor sensor
  set-frost-temperature <temp>       Set the frost protection temperature (7-17 °C)
  set-switching-differential <temp>  Set the thermostat switching differential (in °C)
  set-output-delay <minutes>         Set time in minutes to delay output switching by
  set-up-down-limit <limit>          Set a limit on the use of the up and down keys
  set-sensors <mode>                 Set the sensor selection mode. See extended help for values.
  set-programme-periods <periods>    Set the number of programme periods for each day
  set-programme-mode <mode>          Set the type of programme / schedule mode. See extended help for details.
  set-units <units>                  Set the temperature units used by the thermostat
  set-time                           Sync the system clock to the thermostat
  set-auto-dst <on or off>           Enable or disable automatic adjustment for Daylight Saving Time
  set-keylock <pin>                  Set a PIN to lock the keypad with
  factory-reset                      Restore thermostat to the default factory settings
  help [command]                     display help for command

Some of the commands have additional help text, for example:

$ hmmb help set-programme-mode
Usage: hmmb set-programme-mode [options] <mode>

Set the type of programme / schedule mode. See extended help for details.

  mode        the programme mode number or name

  -h, --help  display help for command

Programme modes:
  0   5day_2day   One schedule for weekdays, another for weekends (Default)
  1   7day        Different schedule for each day of the week
  2   24hour      Same schedule every day
  3   none        Non-Programmable - temperature control only

JavaScript Example

const {Client} = require('heatmiser-modbus')

// Create a new client with path to the serial port device
const client = new Client('/dev/ttyUSB0')

// Add a thermostat with Communications ID = 1
const thermostat = client.addThermostat(1)

// Connect to the serial port
  .then(() => {
    // Once connected, send a modbus command to set the temperature
    const target = 20.5
    console.log("Setting temperature to: ", target)
    return thermostat.setTargetTemperature(target)
  .then(() => {
    // If successful
  .catch((err) => {
    // If there was an error
    console.error('Error!', err)
  .finally(() => {
    // Always close the serial port afterwards

JavaScript API Overview

Methods on the Client class:

Function Return type Description
constructor (port: string) Creates a new Client with the specified serial port device
addThermostat (id: number, name?: string) Thermostat Add a thermostat to the client
getThermostat (id: number) Thermostat Get an thermostat object
deleteThermostat (id: number) void Deregister / delete a thermosta
getThermostats () Thermostat[] Get an array of all the thermostats
async readStatus (thermostat: Thermostat) Promise<any> Read the status of a thermostat
async readTemperatureUnits (id: number) Promise<string> Get the temperature units (C/F)
async connect () Promise<any> Start a connection to the modbus serial port
close () void Close a connection to the modbus serial port

Methods on the Thermostat class:

Function Return type Description
turnOn() Promise<any> Turn On the thermostat
turnOff() Promise<any> Turn Off the thermostat
holdMode (temperature: number, minutes: number) Promise<any> Set thermostat to a temporary temperature of N minutes
setProgrammePeriods (periods: number) Promise<any> Configure the number of heating periods on the thermostat
setProgrammeMode (mode: number | string) Promise<any> Set the type of programme / schedule mode (0 / 1 / 2 / 3)
setTargetTemperature (temperature: number) Promise<any> Set the target room temperature
setFrostProtectTemperature (temperature: number) Promise<any> Set the frost protection temperature (7-17 °C)
setSensorSelection (mode: number) Promise<any> Set the sensor selection mode
setFloorLimitTemperature (temperature: number) Promise<any> Set the temperature limit for the floor sensor
setSwitchingDifferential (temperature: number) Promise<any> Set the thermostat switching differential (in °C)
setOutputDelay (minutes: number) Promise<any> Set time in minutes to delay output switching by
setUpDownLimit (limit: number) Promise<any> Set a limit on the use of the up and down keys
setTemperatureUnits (units: string) Promise<any> Set the temperature units used by the hermostat (C/F)
setTime (time: Date = new Date()) Promise<any> Set the time. If no date is given, use the current time.
setAutoDST (enabled: boolean) Promise<any> Enable/disable auto Daylight Saving Time mode
setKeylock (pin: number | null) Promise<any> Set a PIN number for the therostat
readFirmwareVersion () Promise<number> Get the firmware version number of the thermostat
factoryReset () Promise<any> Perform a factory reset of the thermostat - warning this switches off Modbus


There are a number of limitations to the current version of this library, including:

  • Turning frost mode on and off
  • No support for setting or getting schedules
  • Wireless Sensors are not tested
  • Setting Holiday / Away Mode
  • Optimum Start

These may be features that are added in the future.


node-heatmiser-modbus is licensed under the terms of the MIT license. See the file LICENSE for details.

This software is independently written and maintained and is not supported by Heatmiser UK Limited.


  • Author: Nicholas J Humfrey

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