Starting near the bottom , slide the opening tool in between the plastic halves of the heater's dial.

Once the tool is in between the two plastic halves, slowly slide the tool up until either the halves separate or you hit a plastic depression clamp.

If you have hit the depression clamp, slightly bend the half to see if you can detach the clamp. If you cannot, slightly pull out the opening tool just enough to slide it above the depression clamp while prying the plastic halves apart.

Push downward and at a slight angle to detach the depression clamp and the plastic cover.