Description:

• Can't pipe lava into the heat generator. This seems like an oversight, or a weird choice. I like to automate everything, so would like to see this added, please.
  Reason:
• Because all machines should be able to input stuff. Why have machines at all if not in service to automation?

The Heat Generator does support fluid input, make sure you are configuring it correctly using a Screwdriver and if you're using pipes, make sure you're using the correct servos.