An appliance used to heat water for radiant panel and domestic hot water heating systems. Sometimes referred to as a hot water heater, it differs dramatically from the common domestic hot water heater used to heat service water. This is a somewhat antiquated term in the context of radiant heat because the water temperature supplied by a “boiler” to a radiant system should never exceed 180�F. Steam is not used and therefore no water need ever be boiled. Modern boilers are, in essence, on demand water heaters usually fired by gas, fuel oil, electricity, or wood.