code: public function setDualFuelBreakpoint($breakpoint, $serial_number = NULL) {
          $serial_number = $this->getDefaultSerial($serial_number);
          if (!is_string($breakpoint)) {
              $breakpoint = $this->temperatureInCelsius($breakpoint, $serial_number);
              $data = json_encode(array('dual_fuel_breakpoint_override' => 'none', 'dual_fuel_breakpoint' => $breakpoint));
          } else {
              $data = json_encode(array('dual_fuel_breakpoint_override' => $breakpoint));
          }
          return $this->doPOST("/v2/put/device." . $serial_number, $data);
      }
docs: Change the dual-fuel breakpoint temperature.
  
  @param float|string $breakpoint    DUALFUEL_BREAKPOINT_ALWAYS_PRIMARY, DUALFUEL_BREAKPOINT_ALWAYS_ALT, or a temperature: thermostat will force usage of alt-heating when the outside temperature is below this value.
  @param string       $serial_number The thermostat serial number. Defaults to the first device of the account.
  
  @return stdClass|bool The object returned by the API call, or FALSE on error.