All that heating does is it helps to dissolve the heroin. I really doesn't do anything else. For heroin # 4 better know as heroin hcl , which is the most common heroin on the east coast especially here in Jersey and NYC, there is no need to heat it because the dope readily dissolves with water because white heroin # 4 has HCL. Moreover, if you have somekind of cut in your dope and you heat it, the cut will dissolve into the water and then you will be shootting the cut and the dope. For No. 4 heroin it is better not to use heat because the dope dissolve with the water with a little stirring so heating it will cause the cut to dissolve. If you don't heat the No. 4 the cut will be filter in the cotton, so you wont be shotting the cut. However, some cuts are water soluble and you're gonna shoot it anyways, but it is better to not heat because you'll shoot less cut
For other types of heroin usually addiding acid like citric acid and heating helps to dissolve the dope. It is possible to use just heat because anytime you heat stuff it dissolves.
Below is a video on youtube about adding acid and heating heroin NO.3 in the UK . Note: in the jersey nyc area we have NO.4 heroin so it doesnt require heat or acid. By the way in the video they pay 10 Pound or 20 dollars for 130 mg of 75% dope so basically 98 mg of pure dope. I gotta say not a bad price
http://www.youtube.com/watch?v=e_hlxbW1v20