Water coming from the cold water tap can be pretty chilly during the winter (or year-round if you have a well). According to washing machine manufacturers, if the water is colder than 65 degrees F, the additives in laundry detergent won't work as well—and powder detergents won't fully dissolve.
Cold water for washing should be in the 65- to 85-degree range or clothes won't get completely clean. To find out if your clothes are getting a good wash, check the water temperature with a cooking thermometer (one that registers low temperatures) when you do a cold, a warm and a hot wash. If the water temperature is below 65 degrees for cold water washes, boost it by selecting warm water for part of the initial fill cycle. If the warm water wash is below 85 degrees (a common problem during winter or when the washing machine is at the opposite end of the house from the water heater), try the hot water setting instead for all or part of the wash cycle. Or run the hot water tap into the laundry tub until it gets hot, then turn on the washer.
Always leave rinse settings on cold, no matter what washing temperature you choose. Cold water rinses are just as effective as warm water rinses, and you'll save a lot of energy.
Source: The Family Handyman