I aim to use a Raspberry Pi to provide fine and remote control of my central heating and hot water, in order to improve the user experience as well as reducing energy wastage.

• Raspberry Pi x1 -- I used a Zero for its size and price, since it will only serve as the central heating controller. It will be a waste and overkill to use a £30 worth full size RPi 2, for example.
• USB WiFi Dongle x1 -- for network connectivity
• Female USB Socket x1 -- to connect the USB WiFi dongle.
• 2-Switch Relay Module x1 -- there are many versions of 5V relays that look very similar. The cheap ones I initially used does not support 3.3V signal. I could make it work with additional transistors and resistors, but that's just too messy. So make sure to pick one that supports 3.3V signal (5V for power is okay) to keep the wiring simple and clean. You can tell by the extra 4-leg chip on the relay.
• RTC Module x1 -- it's important to keep track of time in case there no network to sync with an NTP server.
• DHT22 x1 -- as a backup on-board temperature sensor. It will use a remote temperature sensor over WiFi in normal operation. Should it fail to contact the WiFi sensor for any reason, e.g. sensor failure, router failure, signal interference, it will use its backup on-board sensor.
• Micro USB Cable x1 and USB Charger x1 for power
• some wires
• something non-conductive to house the controller

(Download and install)[http://abyz.co.uk/rpi/pigpio/download.html]

simple control commands

• CH and HW pins use WiringPi index.
• Local DHT sensor pin uses BCM index.
• Remote sensor is a URL for reading data.

timed switches

to view room temperature, timer/temperature settings, overrides, etc.

After some research, I chose nginx over Apache for web server for its lightweight and performance.

sudo apt-get install nginx

I normally change the site root to /home/pi/www, so I'll use this path through out the documentation.

I'm going to use Python for CGI, so here's how to set up uwsgi with nginx to handle .py scripts.

sudo apt-get install uwsgi
sudo uwsgi --version

sudo vim /etc/uwsgi/apps-available/uwsgi_config.ini

# make it looks like the block below
[uwsgi]
plugins = cgi
socket = 127.0.0.1:8000
cgi = /home/pi/www   # this should match your site's root
chdir = /home/pi/www
cgi-allowed-ext = .py
cgi-helper = .py = python
uid = pi
gid = pi
die-on-term = true

sudo ln -s /etc/uwsgi/apps-available/uwsgi_config.ini /etc/uwsgi/apps-enabled/uwsgi_config.ini
sudo service uwsgi restart

# Config nginx to use uWSGI for Python CGI

# edit nginx site config
# add a block as below to handle .py files
location ~ \.py$ {
    uwsgi_pass 127.0.0.1:8000;
    include uwsgi_params;
    uwsgi_modifier1 9;
}

sudo service nginx restart

# Testing: add 4 lines of code below into /home/pi/www/hello.py

#!/usr/bin/env python
import cgi
print "Content-Type: text/html\n"
print "<h1>Hello World from Python CGI</h1>"

# make it executable
chmod  x /home/pi/www/hello.py

Now visit http://rpi_ip/hello.py and expect to see some hello world texts.

This is done on your router. Basically, map an external port to your RPi's port 80 for external web access; and map an external port to your RPi's port 22 for external SSH access in case some manual work is needed.

Warning on Security

• add some password protection to your web app for obvious reasons if you allow external access to your home central heating and hot water controller.
• do not map 80 for 80 and 22 for 22. Choose some other random ports to avoid some attacks.
• install fail2ban to block attempts to hack in. I only allow 2 failed attempts before banning an IP for 24 hours or longer. If you do so, make sure you don't lock yourself out. Better to use password-less login on your own computers or mobile phones using rsa key.

In case RPi silently crashes and freezes, watchdog service will auto-reboot it and hopefully things will be back on track. Here's how to set up watchdog.

# add this line to /boot/config.txt
dtparam=watchdog=on

# reboot; check watchdog device is enabled
ls /dev/watchdog*

/dev/watchdog /dev/watchdog0

# install package
sudo apt-get install watchdog
# ignore a few lines of warnings and ignores

# edit config file: /etc/watchdog.conf
# uncomment
watchdog-device = /dev/watchdog
max-load-1 = 24

# add below or you get "cannot set timeout 60 (errno = 22 = 'Invalid argument')"
# because the default 60 seconds is an invalid option!
watchdog-timeout=15

# start watchdog service
sudo /etc/init.d/watchdog start

# check watchdog status
sudo systemctl status watchdog

# now make watchdog service start at boot
# add below to /lib/systemd/system/watchdog.service
[Install]
WantedBy=multi-user.target

# then run
sudo systemctl enable watchdog