One thing that I found when I started is that the quality of the silica wick most companies use is not very good. Even with diligent cleaning and dry burning, it gets hard to really dry burn properly. Low quality silica will tend to burn when dry burning and then the taste never gets better, it will taste "off" or burned.
If you dont want to rebuild, then you have to replace the coils/ heads more often especially if you use sweet or dark juices.
A good way to test if your silica is good , is to take one of your old heads, remove the wick , use a pair of pliers that you dont mind getting discolored and take a torch, butane or propane and heat it up till it gets red. If its bad it will get brownish, beacause its burning. If its good silica it will go back to white, even if you use molasses like tobaccos. If it gets in the least bit singed DO NOT USE IT.