If the bus you are driving do not get heated indoors even though you have checked all buttons, handles, Webasto, and you have driven long it might be
1. Main switch handle is closed.
2. The filter is dirty so the cooling fluid transporting the heat cannot pass.
Both of them you find near the engine behind a separate hatchet.
First check that the main switch handle is not closed. It is often the largest handle and placed on a thick pipe. Here on the backside of the pipe in the front. If the handle is open it is aligned with the pipe.
Run the engine to see if the cabin get heated.
It might be the heating system filter that is clogged.
First of all. Close the main switch handle. Very important! If you do not close it hot fluid will splash all over you.
Then open the “lid” on the pipe placed on the opposite side of the main switch handle.
Be aware! Some hot cooling fluid will pour out so wear glows and be careful!
Here is a ring holding the filter in place, and behind it is the filter pointing inside.
Remove the ring.
Take the filter out using a finger (keep the glove on).
If it is dirty it might look like this.
Rense it with water.
Put it all back in. And put the “lid” on.
Open the main switch handle.
Start the engine and run it for a while.
Hope you get heat, and if not – you probably need more help from professionals.