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EMS-ESP has a command API which can be used to read and write values to EMS device and call specific commands.

There are 3 methods commands can be invoked:

  • via the Console using Telnet CLI
  • via HTTP using RESTful HTTP calls
  • via MQTT via topics their payloads


First some important definitions.


  • <device> is the short-name. It can be either
    1. an EMS Device and supported devices include: boiler thermostat mixer heatpump solar gateway switch controller pump generic heatsource ventilation
    2. the EMS-ESP system itself identified as system
    3. the Dallas temperature sensors as temperaturesensor
    4. any custom Analog sensors as analogsensor
    5. any EMS telegram custom entities as custom
  • <command> is the name of either
    • a generic command, or
    • an EMS device entity also referred to as an <entity>. See the Supported Devices page for the complete list
  • <id> is an optional identifier and has different meanings depending on the context
  • <data> is used to represent the value to read or write. It can be either a single value as any type (integer, float, string or boolean) or a JSON object {} string containing multiple key/values pairs as:
    • "cmd" is the <command>. The key "cmd" may also be substituted for "entity".
    • "value" is the value and can be either a text string in quotes, an integer, float of boolean. "data" is an alias that can also be used instead for the key.
    • "hc", "wwc" and "id" are all are used to represent a value or in the context of an EMS Device a heating or warm water circuit.

Note: A boolean value can be represented in many forms: - as a True value, "TRUE", "yes", true, "true", "on", 1 - as a False value, "FALSE", "no", false, "false", "off", 0


  • Commands can be executed using the call command
  • You need to be admin to use the call command. First su and enter the password
  • For a list of all available commands you can use show commands
  • The syntax is call <device> <command> <data> <id>

For more information on how to use the Telnet Console see the Console section.


Things to note:

  • The REST API follows the OpenAPI Specification
  • The URL path always starts with http://<hostname>/api/
  • <hostname> is either an IP address or the mDNS name, which default is ems-esp.local
  • Some commands require can be dangerous and require security authentication, unless disabled via an EMS-ESP setting. The authentication is in the form of an Access Token which is generated from the WebUI's Security->Manage Users and then clicking on the key button for the admin user. The 152 character long string must be included in the HTTP header like "Authorization: Bearer {ACCESS_TOKEN}". The tokens have no expiry date
  • With HTTP PUSH/PUT/PATCH commands an HTTP body may be required. This can be in the form of either plain text or as a JSON object <data>
  • HTTPS with self-signed certificates are not yet supported

Reading and Writing EMS Device information~

The URL path is http://<hostname>/api/<device>/

endpoint HTTP method action authentication required? body
info GET outputs current EMS device information in verbose no
values GET outputs current EMS device information in short format no
(empty) GET same as values above no
commands GET lists the available command entities to call no
entities GET lists all enabled entities no
{entity} GET outputs details of a specific entity, for reading no
{entity}/{hc} GET same as the read command above but for a specific heating circuit no
{entity} POST updates a entity value, for writing yes <data>
{entity}/{hc} POST same as the write command above but for a specific heating circuit yes <data>


URL body action
http://ems-esp.local/api/thermostat/temp 22 sets the selected room temperature of the master thermostat on heating circuit 1
http://ems-esp.local/api/thermostat {"cmd":"mode", "data":"auto"} sets the thermostat mode to auto for heating circuit 1
http://ems-esp.local/api/thermostat {"cmd":"seltemp", "data":23, "hc":3} sets the room temperature to 23 degrees for for heating circuit 3
http://ems-esp.local/api/thermostat/hc2 {"cmd":"seltemp", "data":20.5} sets the room temperature to 20.5 degrees for for heating circuit 2

Fetching custom entities (EMS telegrams)~

The URL path is http://<hostname>/api/custom/

endpoint HTTP method action authentication required? body
info or blank GET outputs all custom entities and their values no

Fetching Temperature Sensor information~

The URL path is http://<hostname>/api/temperaturesensor/

endpoint HTTP method action authentication required? body
info or blank GET outputs connected Dallas temperature sensors and readings no

Controlling the Analog Sensors~

The URL path is http://<hostname>/api/analogsensor/

endpoint HTTP method action authentication required? body
info or blank GET outputs configured analog sensors and readings no
commands GET lists the available system commands no
setvalue POST set value/offset of counter or output pin, +/- sign corrects value yes {"value":<val>, "id":<gpio>}

System Commands~

The URL path is http://<hostname>/api/system/<endpoint>

endpoint HTTP method action authentication required? body
info or blank GET outputs current system information no
fetch GET forces at refresh of all device values no
restart GET restarts EMS-ESP yes
commands GET lists the available system commands no
allvalues GET lists all connect devices and sensors and their entity values no
send POST send telegram to the EMS bus yes "XX XX...XX"
publish POST MQTT publish all values, and optional HA-configuration or specific for a device no [ha] \| [device]
watch POST watch incoming telegrams no <on \|off \| raw \| <type-id(hex)>


...via the command line~

# Most GETs do not need authentication
% curl http://ems-esp.local/api/thermostat/seltemp

# POSTs (with -d) need authentication tokens
% curl http://ems-esp.local/api/thermostat/seltemp \
  -H 'Content-Type: application/json' \
  -H 'Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ8.eyJ1c2VybmFtZSI6ImFkbWluIiwiYWRtaW4iOnRydWUsInZlcnNpb24iOiIzLjEuMWIwIn0.qeGT53Aom4rDYeIT1Pr4BSMdeWyf4_zN9ue2c51ZnM0' \
  -d '{ "value" : 22.5 }'

# GET with authentication using query parameter with token
% curl http://ems-esp.local/api/system/publish\?access_token\="eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ8.eyJ1c2VybmFtZSI6ImFkbWluIiwiYWRtaW4iOnRydWUsInZlcnNpb24iOiIzLjEuMWIwIn0.qeGT53Aom4rDYeIT1Pr4BSMdeWyf4_zN9ue2c51ZnM0"

# GET to restart EMS-ESP
curl http://ems-esp.local/api/system/restart -H 'Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ8.eyJ1c2VybmFtZSI6ImFkbWluIiwiYWRtaW4iOnRydWUsInZlcnNpb24iOiIzLjEuMWIwIn0.qeGT53Aom4rDYeIT1Pr4BSMdeWyf4_zN9ue2c51ZnM0'

...using a Python script~

import requests
import json

url = "http://ems-esp.local/api/thermostat/temp"
token = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ8.eyJ1c2VybmFtZSI6ImFkbWluIiwiYWRtaW4iOnRydWUsInZlcnNpb24iOiIzLjEuMWIwIn0.qeGT53Aom4rDYeIT1Pr4BSMdeWyf4_zN9ue2c51ZnM0"
body = json.dumps({ "value": 22.5 })
headers = { 'Content-Type': 'application/json', 'Authorization': 'Bearer ' + token }

response =, headers=headers, data=body, verify=False)



MQTT uses the same format as the API.

The topic matches the URL path except the <hostname> is replaced with the MQTT Base name as defined in the Settings (default is ems-esp).

The payload is the same as the <data> as described above and used in the API.


topic payload action
ems-esp/system/send "XX XX...XX" send raw ems-command
ems-esp/thermostat/seltemp fetches the seltemp entity values and publishes it in the topic ems-esp/response
ems-esp/thermostat/seltemp 23 change the selected setpoint temperature to 23 degrees on the master thermostat on hc1
ems-esp/thermostat/mode "auto" sets the thermostat mode to auto for hc1
ems-esp/thermostat {\"cmd\":\"mode\",\"value\":\"heat\",\"id\":1} sets the thermostat mode to heat for hc1

Publishing Commands~

EMS-ESP will MQTT 'subscribe' to specific topics depending on the EMS devices attached. For example boiler, thermostat etc.

Commands can be sent to EMS-ESP via these MQTT topics using the payload format:

{"cmd":"<cmd>", "data":<data>, "id":<n>}


  • cmd is one of the commands listed in the Commands and must be enclosed in quotes as a String. The key entity may also be used instead of cmd.
  • data (or value) holds the value for the command, and can be either a String or numeric value.
  • id is used as an generic indicator. hc, wwc, ahs and hs are other suppoerted aliases. For example with hc it is used to indicated a specific Heating Circuit. A numeric value or String can be both used.

With Home Assistant, Thermostat commands can also be sent to control individual heating circuits via sending a mode string or temperature number to a topic thermostat_hc<n>.

Depending on mqtt-settings there are also direct subscriptions for each value like boiler/wwtemp, thermostat/hc1/daytemp, etc. Thermostats which supports only a single heating circuits will subscribe to /thermostat/daytemp.

You can also use MQTT to send a specific read request and the telegram response will be returned in a topic called response. For example sending the payload {"cmd":"send", "data":"0B 88 19 19 02"} to ems-esp/system will result in a topic response being published with the data {"src":"08","dest":"0B","type":"19","offset":"19","data":"7D 00","value":32000}.