In theory, water in a covered pot should boil faster because as water changes to steam, it absorbs energy, which it carries away from the pot as it vaporizes. In a closed pot, most of the steam is trapped so it condenses (or turns back to water) on the lid and releases its captive energy inside the pot. To find out if the lid made a perceptible difference in practice, we did two tests. In the first, we put 2 quarts of water in each of two identical pots, submerged temperature probes in each, and then put a lid on one. We heated both pots over high heat on identical burners with timers running. The water in the covered pot took 10 minutes and 30 seconds to come to a boil (212 degrees), while the uncovered water took 13 minutes and 20 seconds—about 25 percent longer. We repeated the test with 2 gallons of water in 3-gallon stock pots to see if more water made more of a difference. It did. The covered water boiled in 26 minutes and 20 seconds, while the uncovered water took 34 minutes and 42 seconds—about 30 percent longer. Problem solved.
THE BOTTOM LINE: Water boils faster in covered pots. The more water you have in the pot the more of a difference the lid will make. When boiling water in a pot for which you have no lid, cover it with a baking sheet or aluminum foil.
Water boils about 30% faster with a lid.
Source: Cook’s Country April/May 2012