Abstract: Thermal covers for backflow prevention assemblies disclosed to prevent freeze-ups have a cover body with a thermal top wall, opposed thermal side walls that flex toward and away from one another, opposed normally closed ends, a variable shaped bottom opening and an adjustable closure for said bottom opening. The side walls spread apart and said cover body fits down over said backflow prevention assembly with said bottom closure open. The bottom opening is closed by the bottom closure to a reduced size and fits around and against at least one of the vertical pipes connected to the backflow prevention assembly. A second embodiment has cover ends that are selectively opened and closed by adjustable end closures similar to the adjustable bottom closure. Opposed velcro-type co-operative first and second fastening members are used. The second embodiment opens up into a flat blanket-like configuration.