One of the best ways to control heat in the grow room is to add air conditioning. And, like almost all things horticultural, that requires some basic calculations. Air conditioners are rated based on BTUs, or British Thermal Units, which is a measure of how much heat energy it takes to raise the temperature of one pound of water by one degree Fahrenheit. For reference, a burning match emits about one BTU, while one gallon of gasoline contains 124,238 BTUs. Most air conditioners designed for a conventional 115-volt circuit are generally rated to handle between 5,000 and 12,000 BTUs per hour, and a number of websites let you calculate what size air conditioner you will need based on the dimensions and location of the room you want to cool. But nearly all of them fail to account for the intense heat grow lights and ballasts give off. HID grow lights add 3.41 BTUs per watt, per hour (1,000 watts = 3,400 BTUs). In a sealed, windowless 4-by-8-foot grow room in a sunny apartment, about 3,000 BTUs worth of heat energy will build up over the course of an hour. But with two 1,000-watt lights and two digital ballasts* running in that room, that figure jumps to about 16,000 BTUs per hour, or 14,000 BTUs (20 percent less) if those lights are air cooled. If the ballasts are less-efficient magnetic models, the total BTUs per hour increase by 2,000. A CO2 generator sized for that room will add 600 BTUs, and if the room is being dehumidified by a 40-pint dehumidifier, that will add up to 1,200 BTUs. With all that equipment running, the room will be generating a total of 19,800 BTUs in one hour (15,800 BTUs for air-cooled lights). One ton of air conditioning is equal to 12,000 BTUs, so to determine what size air conditioner you will need to fully counter that heat, divide the total number of BTUs your equipment is generating by 12,000 (19,800/12,000 = 1.65 tons or 15,800/12,000 = 1.31 tons for air-cooled lights).