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 180F. 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.