This is a Opentherm slave HAT to upgrade a HeishaMon PCB to add opentherm communication to a opentherm compatible thermostat. This HAT is only needed to upgrade the previous small square (4x4cm) Heishamon and not for the HeishaMon Opentherm versions, which already has the HAT integrated on the PCB. Also the newer large 10x10cm Heishamon (which adds ethernet and relays) has already Opentherm on the board.

The HAT is installed on top of the HeishaMon PCB from version 3.2 and higher. See the 2nd image for an example how the HAT is mounted on top of a HeishaMon PCB. This HAT is useless without a HeishaMon PCB.

If you have HeishaMon PCB 4.1 and heigher it will fit directly. For HeishaMon PCB 3.2 until 4.0 you need to solder 2x 5-header pin on the board first. If you don't have these headers yourself, you can buy them as an option.

For older HeishaMon PCB there is no option to add this opentherm HAT.