By "heroes," do you mean Heroes & Icons (also known as H&I)?
This is one of those networks that are carried on "subchannels" of other channels (the same with BUZZR, Decades, and a bunch of others). The problem is, with Xfinity's listings, they don't list the channel as "H&I" or "Heroes & Icons," but with the call letters of the station that carries it, usually followed by "DT" and a number.
Try this: go to your online guide, enter 4455, and see if "Hill Street Blues" shows up in the list (HILL is 4455); if it does, click on it, and the guide should go right to the next episode it can find, which should be on your local Heroes & Icons station. Make a note of the channel number.