OK first of all locals do not call it the "Orange Bus" (even not in Thai) some call it "Wan Yen" ("cold sweet" as a dessert). It is an old bus ... the bus we had to Thung Chang was put together with ropes and stickers and had an wooden floor. And in the bus were many unintentional air-flow holes.
As you can imagine it is not an air-conditioned bus, but with open windows and doors you have air-flow in the all bus. And it is traveling in Nan style .... slow. From Nan to Thung Chang will take you about 3 hours, without the waiting time for the bus (time-schedule is flexible). But it is a great experience to see the real way of local living & traveling. And it is cheap, 75 Thai baht per person from Nan to Thung Chang. Well the bus actually goes to Pon, what is about 10 km further at route 101. That is where the mountains begin .... and the bus engine would not be able to push the bus up! So end of the road for the Orange Bus. If you want to continue to Huai Kon you can change transport in Pon. You can stop the anywhere along the route, the bus will stop anywhere! For more information about the beautiful Nan province see Destination Travel Guide Nan, North Thailand.