The heating time for Hydropool hot tubs depends on a number of factors. The temperature outside, how powerful your heating element is as well as if you have the hot tub covered can all factor into the equation.Generally, a hot tub in good working condition will raise the water temperature between 3-6 degrees per hour. To help the hot tub heat quicker, make sure to leave the cover on as heat can escape from the water surface. If the hot tub is located outside, the air temperature also is a factor. Simply put, the warmer it is outside, the quicker the hot tub will heat up. If it is a warm, sunny day and you want your water temperature to reach 100 degrees, it will most likely take approximately 4 hours.