The ONLY time you should heat dope is if you have base dope that you want to IV. You add citric acid and heat it up to make it water soluble
Which you don't get in the USA (if you do, it is EXTREMELY rare, as the heroin market is pretty laid out that we've come to learn the even with tar, it's the salt of diacetylmorphine and not the base or base w/ caffeine). The whole idea of cooking shoots is so ingrained in our society that many users feel they must do it with out any real reason or understanding to why. Its seen on TV or in movies, as well as on videos like those on youtube that spread this vicious addict myth. It even spreads to web forums by long time users swearing by it. Once they started doing shots for themselves, they noticed that when they cook their shot, they dissolved everything that was dumped in the cooker, which they felt is important. They don't realize that their dope isn't 99% pure or close, and the stuff that isn't dissolved is the cut. Diacetylmorphine is very highly water soluble that, just some agitation will lead to all of the diacetylmorphine to be in the solution. Cooking the solution will only lead to more cuts dissolving due to the heat raising the solubility of these cuts into water, and potentially lead to a break down of the diacetylmorphine leading to other products than it within the solution.
Even with black tar heroin found in half the country you don't need to heat. Yeah, it helps speed up the process, but it also is creating an unhealthier shot. If they just the time to separate the tar into small chunks (say from a .100 balls), throw them into a shot glass with the appropriate amount of liquid, take a simple object like a key or screwdriver and stir the mixture for 3-5 minutes, and then let settle so there is no frothy/foamy surface before tossing in the cotton for filtering. It may take a few minutes longer than desired, but in many ways you are doing yourself a service. You aren't using a spoon that is easily split (seen/heard of this happening many times to the individuals dismay) and that is covered in small cervices from use which is a breading ground for bacteria. A shot glass is more likely to have less scratches on the surfaces, preventing this, and isn't as easily split (makes a good container for stirring as well) With out heat you are also, you aren't allowing all the real nasty shit that makes up the BTH to be dissolved, or at least more than what already is, and not to mention helps prevent putting a hot solution into your veins.
The actual heroin dissolves very quickly in water from BTH, it is the rest of the junk that doesn't. Also, if you are separeting the BTH into little pieces, you increase the surface area of the BTH, letting the actual diacetylmorphine and 6-monoacetylmorphine be dissolved quickly. Stirring the solution with the tiny chinks of tar and water, you will allow all the heroin to dissolve with out letting in all the extra shit like shoe shiner or whatever it is cut with.