I asked this question a week ago. If i understand well, then the cold glow plugs are good for the summer where there are high temps, so the engine is running better. The eact thing about the hot plugs. They are good for winter. An example is the classic OS P3 and P7 glow plugs. The P3 is ultra hot and the P7 is cold. The P3 i'm using it all the time in the past. I notice though that in Summer i have a few problems because of VERY high temps here in Cyprus (this island is like desert ), so this summer i will test a P7 or either a P8 if you asking me and see the difference. They also told me that with a very hot glow plug running in Summer (high air temps), the engine might "stall".