12 to 18 hours
it should take to increase eggshell temperature from 29°C to 37.8°C  

The time used to warm eggs from the storage temperature to the optimal eggshell temperature of 37.8°C at the start of incubation is often dependent on planning issues such as; the moment eggs arrive and whether the setter is clean and available. Due to a lack of time, too often the setter is started with a quick start: warming eggs as fast as possible to prevent that chicks hatch too late 21 days later. 

However, HatchTech Research team has shown that with a fast warming process hatchability and chick quality are negatively affected. They showed that the especially the part of the warming process in which temperature increases between 29°C and 37.8°C is crucial. A slow warming period (between 12 and 18 hours) for this phase improves hatchability and chick quality in short and long stored eggs.  

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