I have LPL 4550 XLG witch has a fan. I print 6x7. The megatives still pop. Wih color I pre-heat them until they pop. Also, doing that gives more consistent color balance since lamp color changes depending on whether enlarger's lamp was pre-heated or not. With B&W this does not always work. Even if I pre-heat, the film may still pop after that or pre-heat time becomes very long. Glass negative holder fixed it all.