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.
| 2020 NSPS Step 2 limit | 0.10 lb/MMBtu heat output, weighted average; no burn rate above 0.32 |
|---|---|
| Currently certified | 34 of 167 hydronic records in the EPA database |
| HHV efficiency | 58–90% among currently certified units; median 81% |
| Thermal storage sizing | Roughly 800–1,000 gallons per full firebox load of a mid-size gasifier |
| Supply water temperature | Typically 160–180 °F; radiant floor loops mix down to 90–120 °F |
| Seasonal fuel use | Commonly 4–10 cords for a whole house, depending on climate and envelope |
Gasification, and why storage is not optional
A modern cord-wood boiler is a downdraft gasifier. Wood in the upper chamber is pyrolysed by heat rather than burned outright, a fan drives the resulting gas down through a refractory nozzle, and it ignites in a secondary chamber that runs at 1,800–2,000 °F. Burning the gas in a dedicated ceramic-lined chamber rather than in the same space as the fuel is what gets these units to 0.10 lb/MMBtu — and it only works when the boiler is running hard.
That is the design tension. A boiler sized for the coldest week of the year is oversized for most of the season, so without somewhere to put surplus heat it spends its life idling — damping the fan, cooling the secondary chamber, and producing exactly the smoke the technology exists to avoid. Idling is what gave outdoor boilers their reputation, and it is still the fastest way to ruin a good one.
Thermal storage solves it. Run the boiler flat out for one full load, dump the heat into a large insulated tank, then let the house draw from the tank while the fire goes out. The sizing arithmetic is simple: 1,000 gallons of water is about 8,340 lb, and a 60 °F swing between 200 °F and 140 °F stores roughly 500,000 BTU — close to the useful output of one full firebox in a mid-size gasifier. That is where the “roughly 800–1,000 gallons per load” rule of thumb comes from.
Outdoor, indoor, and the reputation problem
The classic outdoor wood boiler is a firebox surrounded by a water jacket in a shed-roofed steel cabinet, connected to the house by buried insulated PEX. It has real advantages: the mess, ash, and fire risk stay outside, the firebox is big enough to take unsplit rounds, and it can heat several buildings plus domestic hot water. Uncertified versions were also notorious — a smouldering water-jacketed firebox with a short stack, sited near neighbours, produces ground-level smoke that generates complaints and, in a number of states and municipalities, outright bans or strict setback and stack-height rules.
The 2020 NSPS brought them under caps for the first time, and the certified survivors are almost all gasifiers or pellet boilers. If you are considering an outdoor unit, check local rules before anything else: setback distances from property lines and neighbouring dwellings, minimum stack heights, and in some jurisdictions a prohibition on new installations regardless of certification.
Indoor gasification boilers avoid the siting problem and the underground heat loss, at the cost of putting a large appliance, a storage tank, and a wood pile in a basement or outbuilding. Pellet boilers are the automated option: hopper-fed, thermostatically modulated, and the strongest performers in the certified data, with efficiencies at the top of the 58–90% range.
Distribution, controls, and the real workload
Heat leaves the tank through circulators to whatever emitters the house has — baseboard, radiators, a radiant slab, or an air handler or plenum coil that turns the water back into forced air. Zones with different temperature needs get mixing valves; a radiant floor wants 90–120 °F water while cast-iron baseboard wants 160–180 °F. Domestic hot water comes off an indirect tank or a heat exchanger, which is often the feature that sells the system.
Every part of that is electrically dependent. Combustion fans, circulators, and controls all need power, and unlike a wood stove a boiler does not keep working through an outage — worse, a hot boiler with dead circulators can boil, which is why thermal dump zones, gravity loops, or a battery backup are standard practice rather than optional extras. Buried supply lines lose heat continuously, so insulated PEX and short runs matter; a long trench to a distant outbuilding can cost a meaningful fraction of the boiler’s output all winter.
The workload is a step above a stove. Expect to load once or twice a day through the heating season, clean the secondary chamber and heat exchanger tubes on a schedule measured in weeks rather than months, monitor water chemistry and pressure, and burn 4–10 cords a year for a whole house. A hydronic system is the right answer for a large or multi-building property with cheap wood; it is a poor answer for anyone who wanted one appliance in one room.
Frequently asked questions
What is the difference between a wood boiler and a wood stove?
A stove heats the room it stands in by radiation and convection. A boiler heats water and pumps it to radiators, radiant floors, or an air handler elsewhere in the building, so one fire can heat a whole house evenly plus domestic hot water — and can sit outside or in a basement. The trade-offs are cost, complexity, electrical dependence, and a much larger annual wood requirement.
Do I need a storage tank with a wood gasification boiler?
Effectively yes. A gasifier only burns clean when it runs hard, and without storage it idles for most of the season, which smokes, fouls the heat exchanger, and shortens its life. Size storage at roughly 800–1,000 gallons per full firebox load — 1,000 gallons cycled between 140 °F and 200 °F holds about 500,000 BTU. Many manufacturers require storage as a condition of the warranty.
Are outdoor wood boilers legal?
It depends entirely on where you are. Some states and municipalities ban new installations outright, and many others impose minimum setbacks from property lines and neighbouring homes plus minimum stack heights. On top of that, only appliances meeting the 2020 NSPS limit of 0.10 lb/MMBtu may be sold new. Check with your state air agency and local zoning office before buying.
How much wood does a wood boiler use?
Commonly 4–10 cords a season for a whole house, against 2–4 for a wood stove heating part of one. The range depends on climate, insulation, whether it also makes domestic hot water, and above all on whether the system has thermal storage — an idling boiler without storage can burn a third more wood for the same delivered heat.
What happens to a wood boiler in a power outage?
It stops distributing heat, because circulators, combustion fans, and controls all need electricity. That is more dangerous than it sounds: a boiler with a live fire and no circulation can overheat or boil. Systems are normally built with a thermal dump zone, a gravity-fed loop, or battery/generator backup for exactly this case. Confirm which one your installer is providing.