Choosing a Rheem Mains Pressure Gas water heater will ensure an abundant supply of hot water. Due to Rheem's superior design and manufacturing, it's range of mains pressure hot water heaters are top performers under variable water pressures and conditions. To underline their flexibility, they operate with mains pressure, unequal pressure and also low pressure. The Rheem mains pressure water heaters are lined with vitreous enamel and are recommended for areas of poor water quality where copper cylinders can sometimes corrode.