Below is a list of Hot Water Boilers manufacturers from Monrovia, California. Hot water boilers are tanks or vessels that transfer thermal energy from a heat source to water that is then circulated to provide residential, commercial and industrial heating. Hot water boilers are an important part in hydronics, the use of water as a heat-transfer medium in heating and cooling systems, because they heat the water that circulates through a series of pipes and into radiators, baseboards and other heat exchangers.