Scalding flour is the process where the flour starch is gelatinised by the addition of very hot water (See https://en.wikipedia.org/wiki/Starch_gelatinization). I'm assuming here that a very similar process occurs when making hot water pastry (using lard or lard and butter plus hot water).
According to Wikipedia, this reaction depends on the starch but occurs between 55-85 °C. If my assumption here is correct, there is no need to boil the mixture for gelatinisation to occur. Chefs seems to be divided on this matter, some say the liquid must boiled, others not.
As bringing a fat and water mix to a boil is potentially very dangerous as the mixture could explode out of the pan if overheated, what is the minimum safe temperature that this reaction will occur at? Or is something different chemically occurring here and the mixture must be boiled for best hot water pastry?