Changing the cell format to [H] really doesn't make any difference for your SUM, that only changes the way the Data is displayed in the cell, so it Only matters on how you want to See your data, but does nothing for the SUM formula.
If you have formulas putting the "" in the cells, that will give you the #VALUE! error, there are different ways to work around it.
One way, if you have Headers for the cells you want to sum, this also simplifies the formula and makes it easier to maintain, you can try something like,
I used "Hours" as the Header, please show what your actual Header is, so the formula can be adjusted accordingly:
=SUMIF(G1:BU1,"Hours",G7:BU7)*24/80