OpenTherm Thermostat

Posted on Tuesday, July 17, 2018 at 12:00 AM 4049

OpenTherm Thermostat

OpenTherm Thermostat is based on OpenTherm Adapter, OpenTherm Library and ESP8266 controller (WeMos D1 Mini). So you can control your boiler remotely over WiFi via your smartphone.
Using OpenTherm protocol you will be able to:

  • get boiler status
  • set water temperature
  • switch on/off central heating
  • switch on/off hot water
  • get gas modulation level
  • ...and many other commands supported by OpenTherm protocol
Remote control feature is very useful when you returning back home after some holiday or just weekens and you don't want to get into cold home, so you can set temperature on the way home.

In addition to remote control feature there are much more benefits of using OpenTherm thermostat:

  • modulation increases the efficiency of the heating system, reduces environmental effects, as well as increases comfort
  • increases the life of the boiler, reduces boiler start/stops
  • can provide diagnostic information / boiler errors / fault codes
  • lowers fuel bills

To build simple interface between boiler and ESP8266 controller I decided to make stackable OpenTherm Shield compatible with OpenTherm Adapter.

OpenTherm Shield:

OpenTherm Shield

I used RJ12 phone socket as enclosure for OpenTherm Thermostat and to simplify connection to boiler. Made two holes for micro USB charger and for WeMos D1 Mini led. Also I rearranged inner screw connectors. As temperature sensor I used DS18B20, because its small, accurate and easy to use.

Thermostat enclosure:

OpenTherm Shield Enclosure
OpenTherm ESP8266 Adapter Enclosure
OpenTherm Thermostat Enclosure


OpenThrem Shield Schematic


OpenThrem Shield PCB


  • DS18B20 Temerature Sensor 1$
  • 8 Pin 2.54mm Stackable Female Pin Header x 2 1$
  • 2 Pin 2.54mm Male Single Row Header x 3 0.30$
  • 1/4 Watt 5% Resistor 4k7 Ohm 0.01$
  • WeMos D1 mini 5$
  • OpenTherm Adapter 15$
  • EMT Electronics RJ12 Phone Socket Enclosure 1$


Smartphone Web Interface:

OpenTherm Thermostat Interface

Sample Code:

#include <ESP8266WiFi.h>
#include <WiFiClient.h>
#include <ESP8266WebServer.h>
#include <ESP8266mDNS.h>

#include <OneWire.h>
#include <DallasTemperature.h>

#include <OpenTherm.h>

//OpenTherm input and output wires connected to 4 and 5 pins on the OpenTherm Shield
const int inPin = 4;
const int outPin = 5;

//Data wire is connected to 14 pin on the OpenTherm Shield
#define ONE_WIRE_BUS 14

const char* ssid = "Please specify your WIFI SSID";
const char* password = "Please specify your WIFI password";

ESP8266WebServer server(80);
OneWire oneWire(ONE_WIRE_BUS);
DallasTemperature sensors(&oneWire);
OpenTherm ot(inPin, outPin);

float sp = 23, //set point
pv = 0, //current temperature
pv_last = 0, //prior temperature
ierr = 0, //integral error
dt = 0, //time between measurements
op = 0; //PID controller output
unsigned long ts = 0, new_ts = 0; //timestamp

const char HTTP_HTML[] PROGMEM = "<!DOCTYPE html>\
	<meta name=\"viewport\" content=\"width=device-width, initial-scale=1\">\
		window.setInterval(\"update()\", 2000);\
		function update(){\
			var xhr=new XMLHttpRequest();\\"GET\", \"/temp\", true);\
			xhr.onreadystatechange = function () {\
				if (xhr.readyState != XMLHttpRequest.DONE || xhr.status != 200) return;\
<body style=\"text-align:center\">\
    <h1>OpenTherm Thermostat</h1>\
    <font size=\"7\"><span id=\"temp\">{0}</span>&deg;</font>\
    <form method=\"post\">\
        Set to: <input type=\"text\" name=\"sp\" value=\"{1}\" style=\"width:50px\"><br/><br/>\
        <input type=\"submit\" style=\"width:100px\">\

void handleInterrupt() {

float getTemp() {
	return sensors.getTempCByIndex(0);

float pid(float sp, float pv, float pv_last, float& ierr, float dt) {
	float Kc = 10.0; // K / %Heater
	float tauI = 50.0; // sec
	float tauD = 1.0;  // sec
	// PID coefficients
	float KP = Kc;
	float KI = Kc / tauI;
	float KD = Kc*tauD;	
	// upper and lower bounds on heater level
	float ophi = 100;
	float oplo = 0;
	// calculate the error
	float error = sp - pv;
	// calculate the integral error
	ierr = ierr + KI * error * dt;	
	// calculate the measurement derivative
	float dpv = (pv - pv_last) / dt;
	// calculate the PID output
	float P = KP * error; //proportional contribution
	float I = ierr; //integral contribution
	float D = -KD * dpv; //derivative contribution
	float op = P + I + D;
	// implement anti-reset windup
	if ((op < oplo) || (op > ophi)) {
		I = I - KI * error * dt;
		// clip output
		op = max(oplo, min(ophi, op));
	ierr = I;	
	Serial.println("sp="+String(sp) + " pv=" + String(pv) + " dt=" + String(dt) + " op=" + String(op) + " P=" + String(P) + " I=" + String(I) + " D=" + String(D));
	return op;

void handleRoot() {
	digitalWrite(BUILTIN_LED, 1);

	if (server.method() == HTTP_POST) {
		for (uint8_t i = 0; i<server.args(); i++) {
			if (server.argName(i) == "sp") {
				sp = server.arg(i).toFloat();

	String page = FPSTR(HTTP_HTML);
	page.replace("{0}", String(getTemp()));
	page.replace("{1}", String((int)sp));	
	server.send(200, "text/html", page);
	digitalWrite(BUILTIN_LED, 0);

void handleGetTemp() {
	digitalWrite(BUILTIN_LED, 1);	
	server.send(200, "text/plain", String(getTemp()));
	digitalWrite(BUILTIN_LED, 0);

void setup(void) {
	digitalWrite(BUILTIN_LED, 0);
	WiFi.begin(ssid, password);

	// Wait for connection
	while (WiFi.status() != WL_CONNECTED) {
	Serial.print("Connected to ");
	Serial.print("IP address: ");

	if (MDNS.begin("thermostat")) {
		Serial.println("MDNS responder started");

	server.on("/", handleRoot);
	server.on("/temp", handleGetTemp);	

	Serial.println("HTTP server started");

	//Init DS18B20 sensor
	sensors.setWaitForConversion(false); //switch to async mode
	pv, pv_last = sensors.getTempCByIndex(0);
	ts = millis();

void loop(void) {	
	new_ts = millis();
	if (new_ts - ts > 1000) {		
		//Set/Get Boiler Status
		bool enableCentralHeating = true;
		bool enableHotWater = true;
		bool enableCooling = false;
		unsigned long response = ot.setBoilerStatus(enableCentralHeating, enableHotWater, enableCooling);
		OpenThermResponseStatus responseStatus = ot.getLastResponseStatus();
		if (responseStatus != OpenThermResponseStatus::SUCCESS) {
			Serial.println("Error: Invalid boiler response " + String(response, HEX));

		pv = sensors.getTempCByIndex(0);
		dt = (new_ts - ts) / 1000.0;
		ts = new_ts;
		if (responseStatus == OpenThermResponseStatus::SUCCESS) {
			op = pid(sp, pv, pv_last, ierr, dt);
			//Set Boiler Temperature
		pv_last = pv;
		sensors.requestTemperatures(); //async temperature request
	server.handleClient(); //handle http requests

Sample sketch implements simple http web server to be able control boiler using smartphone over WiFi.
Main web page shows current room temperature and allows to set up desired target temperature.
Current temperature constantly refreshes using asynchronous requests.
Sketch uses only two OpenTherm commands: to get/set boiler status and to set water temperature.
In main loop there is logic to read room temperature, calculate required boiler temperature using PID Controller, and to set boiler temerature.

Serial Output:

Connected to Ihor Melnyk WIFI
IP address:
MDNS responder started
HTTP server started
sp=23.00 pv=22.69 dt=1.00 op=3.19 P=3.12 I=0.06 D=0.00
sp=23.00 pv=22.69 dt=1.00 op=3.25 P=3.12 I=0.13 D=0.00
sp=23.00 pv=22.69 dt=1.00 op=3.31 P=3.12 I=0.19 D=0.00
sp=23.00 pv=22.69 dt=1.00 op=3.38 P=3.12 I=0.25 D=0.00



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