Yes, there is a function in Excel that can replace null values with zero. You can use the IF function with the ISBLANK function to achieve this. Here are the steps:

1. Select the range of cells that you want to replace null values with zero.
2. In the first cell of the range, enter the following formula:
   Formula:

   =IF(ISBLANK(A1),0,A1) 

3. Press Enter to apply the formula to the first cell.
4. Copy the formula by selecting the cell and pressing Ctrl+C.
5. Select the range of cells that you want to apply the formula to.
6. Paste the formula by pressing Ctrl+V.

This will replace all null values in the selected range with zero. The IF function checks if the cell is blank using the ISBLANK function. If the cell is blank, it returns zero. If the cell is not blank, it returns the value of the cell.