Hydronic heater (wood boiler)
A hydronic heater is a wood- or pellet-fired appliance that heats water rather than air, distributing heat through radiators, radiant floor loops, or a heat exchanger in a furnace plenum. They include outdoor wood boilers, indoor cord-wood gasification boilers, and automatic pellet boilers.
Because the heat moves as hot water, a hydronic heater can serve a whole house (and domestic hot water) from one fire, often located outside the living space. Older uncertified outdoor boilers earned a smoky reputation — and local bans; the 2020 NSPS brought hydronic heaters under emission limits, and certified gasification designs burn dramatically cleaner.
Many certified hydronic heaters pair with thermal storage tanks: the boiler runs hot and clean at full output to charge the tank, and the house draws from storage — avoiding the smoldering idle that made old boilers dirty.