In a tempering furnace glass is on top of rotating ceramic rollers, where it is exposed to radiation emitted by radiant heaters, rollers and other surfaces inside the furnace. Convection heat transfer between hot air jets and glass affects both surfaces. On the lower surface also contact heat transfer between glass and rollers takes place. In order to solve glass temperatures during heating, the problem is to find solving method for radiation heat flux, convection heat transfer coefficients and contact heat transfer coefficient. In the paper the basics of these solving methods are show, and the behaviour of thermal radiation in clear and low-e coated glass in tempering furnace is shown with details. There is quite wide range on different tempering furnace models in the market, in which the devices for creating, focusing and controlling of radiation and convection heat transfer differs. In the paper some of them are theoretically studied to show how glass heats up in them. The portions of the heat transfer phenomena above during heating shown, and theoretically solved glass heating times are compared with realized ones.