1. Use soft water
2. Use the correct LPG pressure, at the burner.
3. If your unit puts chemical through the heating system, make sure it has a corrosion inhibitor, and is not one that will build up in the heating coil.
The failure to abide with these three guidelines does not necessarily mean you will have problems, but it does mean you will have to perform periodic maintenance procedures to keep problems at bay.
Minerals in the water(hardness), will lead to boiler scale that will require periodic descaling to remove. Scale inhibits heat transfer and causes lack of efficiency. It also causes the tubing to have to be heated much higher than normal, reducing the life of the coil.
Correct LPG pressure will maintain a proper flame, and lower sooting. Soot buildup will lower heating efficiency, and also cause the jets to clog. Clogged jets leads to more soot, and loss of heat.
Chemical buildup leads to the same problem as boiler scale. And corrosion will cause thinning of the copper exchanger, leading to failure.
Very few cleaners take heed of these guidelines, and thus have maintenance and service issues that routinely must be implemented.