In this case is proposed a heat reservoir based on a phase change material instead of using an electrical system.Container consists of a low-cost polymer (pp) filled with any of the following compounds: petrolatum, CaCl2.6H2O, Na2SO4.10H2O or Na2CO3.10H2O, whose melting points are between 29 and 33 degrees celsius. The container is covered with a flexible insulating material like polyethylene foam.For its use, it is introduced into a microwave oven (we considered a 1000 W oven) for 1 minute and 40 seconds aprox., which is reached at about 30 degrees celsius. Thereafter is positioned over the can of polyurethane foam for application.The proposed system avoids any overheating so it is safe to use, it is inexpensive to produce and can be cleaned easily.