Monitoring accurately the temperature in your home is definitely one of the best way to save on your energy bill. At the same time you want to feel well in a warm home during winter time.

My current thermostat only allows a static programmation:

I can define a day temperature (around 19°/20° to not heat too much) and a night (or no one at home) temperature (16°)
For each weekday, I can define time range to apply day temperature and time range to apply night temperature)
Additionally, I can manually adjust the temperature that will be taken in account until the next time range is reached

Not so bad but

I cannot monitor the temperature remotely: specially when coming from vacation, I am not able to warm the home before we arrive.
I cannot stop heating automatically when there is no one at home.
I cannot take in account other parameters that impacts home temperature like sunshine during the day (warming home), winds (cooling home)...
I cannot have control on temperature set by my wife, overheating all day at 25°C
...

Step 1: Step 0 - Prerequisites

You know how to use Arduino IDE
You have installed ESP8266 board manager: http://arduino.esp8266.com/stable/package_esp8266com_index.json
Thus you are able to upload some code on the NodeMCU and to run it
You have installed the following libraries
- U8g2lib - OLED display
- DHTesp - DHT for ESP
- ESPAsyncTCP - Manage asynchronous HTTP request, install from github
- ArduinoJson - process JSON response from HTTP request
- ESP8266WiFi - included in ESP8266 board
- Ticker- included in ESP8266 board
You have some device or software like a smart home box that will allow to monitor and interact with this thermostat (in my case, I own an eedomus box but you can easily adapt the code to suit your needs)
You know how to solder

Step 2: Step1 - Bill of Materials

NodeMCU - 2€
OLED screen 128x32 -1.50€
DHT-22 - 2.50€
Relay 3.3V - 1.40€
2 push buttons - 2x0.02€
2 resistors 1kΩ to 10kΩ (I used 220Ω) 2x0.01€
Prototype board - 0.16€

Total = 7.62€

All prices are based on AliExpress with free delivery.

Additionally we need materials to solder:

Soldering iron
Soldering wire - I use 0.8mm
Soldering flux - I use it to solder NodeMCU pins
Kynar jump wire or other cable to wire the components on the prototype board
Wire ball - to clean iron
Desoldering wick - Useful to remove soldering when you messed up

Step 3: Step 3 - Cabling

The pictures show the details on the cabling.

You can possibly modify the pin to connect components. However note that the relay cannot be connected to any pin of the NodeMCU. Connecting the relay to some pin does not allow the NodeMCU to boot.

The NodeMCU is powered from a USB cable (5V) connected the Vin pin of the NodeMCU that is connected to the onboard power regulator. Do not power supply another pin of the NodeMCU with 5V unless you want to blow it up.

Step 4: Step 4 - Principles for the Program

The principles of the program are quite simple and are drawn on the first picture.

We have two variables to hold the requested temperature and the measured temperature
Regularly we read the measured temperature and we display it on the display
We also read the requested temperature from our smart home box (eedomus)
According to these 2 temperatures we compute if we should heat or not
Additionally the user could use the buttons to adjust the requested temperature

Additionally we need:

Manage conflicts between requested temp adjusted by buttons and requested temp adjusted by reading from eedomus
Make sure we do not adjust heating to often. The heater may not appreciate and it heats more efficiently when heating on a rather long period. As a rule of thumb we decided to not adjust heating more often than each 15 min
Need to debounce the reading from the buttons. See arduino examples: Digital → Debounce
Smooth the reading of the measured temperature: The raw reading alternate with a gap of over 1°C making single readings hard to rely on
Calculate an hysteresis in order to keep the temperature as stable as possible. A home has an inertia that implies that temperature will continue to rise shortly after we cut the heating. On the other hand the temperature will continue to decrease even after we started heating. Thus we have to anticipate the point when we cut heating or we start heating. This is shown in the second picture.

Step 5: Step 5 - Arduino Code

I just put in production my thermostat. My current code is messy and contains a lot of messages for debugging. I plan to rewrite a clean program shortly. I will update as soon as available and validated on my thermostat.

Ticker are the procedures that are repeated regularly
We use many global variables because we cannot pass parameters to ticker procedures
The AsyncClient allows to handle asynchronous HTTP request
We attach interrupt procedure to handle the press to the buttons
All the code is event driven, the Arduino loop is empty
displayTemp - Manage the display of the requested temperature on 2 digits and the measured temperature with on 2 digits plus one 1 digit after decimal point. The display moves up when heating, moves down when not heating and bouncing up and down when both temp are equal (in this case not heating)
getMeasuredTemp - We read the temp every minute and calculate an average of every 5 readings. We then keep an history of 5 average reading to calculate the vector of the curve which allows to estimate the next reading.
getRequestedTemp - Read value defined in our smart home box.
incTemp/decTemp - procedure called by interrupt when buttons are pressed
setHeating - Turn on/off the relay to monitor the heater. In our case the relay is put to HIGH when no heating and to LOW for heating. Maybe this is weird but we made this choice because there is a red LED that is turned on when the relay is closed (set to HIGH).