: What well the Flush Fit Register Booster do for me? : The Flush Fit Register Booster is designed to be placed in your existing register. It will replace a standard 4" x 10" register grill and boost the airflow from that register. By boosting airflow, it will warm up rooms that are chronically cold in the winter and cool down rooms that are chronically too hot in the summer. : Does the Flush Fit Register Booster heat or cool the air? : No. The Flush Fit Register Booster neither heats nor cools the air.