Retrofit kit for switch box cooling for the cooling circuit controller of the M Flex indoor unit to prevent overheating of the electrical components at higher room temperatures. Installation is mandatory if ambient temperatures of +25 °C (daily average temperature) are to be expected. The fan mounted on the switch box cover in the retrofit kit switches on automatically via a thermostat and thus ensures that the electrical and electronic components are cooled. When using the extension kit, temperatures in the installation room of up to +35 °C (daily average temperature) are permissible.