How do you include Support in your Sprint?

Question

Our company moved to Scrum recently on a product that was almost coded by a single person (Joe). We have support to do with our exising customers that we try to integrate in our process.

For now we tried the following approach:

• We do a rotation on a person in charge each week.
• The person in charge for the week may spend up to 8 hours on support.

But we failed to have it work:

• Joe always does the support because he knows the code better, and it's always faster for him to do it than to explain it.
• We (the rest of us) tend to focus on what's on the Sprint board aka new stories instead of support tasks.
• Joe has too much work, he can't handle all the support by himself because he has to do things outside of the Sprint as well.

Have you already faced similar situation? How did you manage to get out of it?

Note: We can't dedicate a single person on the support right now. We hope to in the future.