There are two different boards with one being 50x50mm that would have the relay mounted off the board as shown in the picture to the right. The two sensors are also mounted off the board as is the buzzer (upper right of photo).
The other board configuration is 50x100mm and has room for the relay on the PCB. The relay could be part of a power supply circuit (DC or even AC) but the PCB is really designed to be part of an PS_ON circuit of an ATX power supply as it has an bypass switch that can be used to close that circuit.
Both of the boards are available in a kit form as there are a wide variety of ways that they could be mounted and as many ways to connect the board to sensors.
The PCB also has a connection point for the serial port of the Arduino for diagnostic information output by the sketch that drives the monitor.
As stated above, the monitor was designed for a 3D Printer but it could be used to monitor just about anything for heat and gas where a relay needs to be triggered. I used the MQ-2 sensor looking for smoke but other MQ sensors could be used for other gas types.
Below is a documentation index for the Heat and Gas Monitor / Switch PCB:
Part 1 - Introduction
Part 2 - Hardware Configuration Options
Part 3 - Operation
Part 4 - Software Configuration
Part 5 - Assembly