Wood burns fast and at high temperature in a refractory firebox. Instead of heading straight for the flue, the flue gases travel a long circuit of channels that gives up their heat to the mass. The fire lasts one to two hours; the mass radiates all day.
Checked on 31/08/2026
Three stages, a single fire
Everything happens in one burn. The wood burns fast and hot in a refractory firebox; instead of heading for the flue, the flue gases pass through the masonry and leave it their heat; the mass, once charged, radiates for eight to twenty-four hours — twelve to twenty-four, and sometimes more, on the heaviest site-built masonry heaters; eight to twenty-four on a more responsive heat-storage stove. The fire lasts one to two hours. The rest of the time, the appliance heats without a flame.
Every part of the heater serves one of these three stages.
The firebox: burning fast and hot
The firebox is a chamber of firebricks, or a manufacturer’s cast-iron firebox lined with refractory. Its walls reflect the heat back onto the fire, so the temperature rises quickly and combustion is complete: the gases the wood releases burn in the firebox instead of leaving as smoke. That is what makes the masonry heater so frugal, and its flue gases so clean.
For that, the fire needs plenty of air, and a firebox that does not try to slow it down. A masonry heater is never run ‘turned down’: the burn is bold, or it is nothing.
The flue path: where the heat stays
At the firebox outlet, the flue gases are very hot. They enter the accumulator: a circuit of channels — baffles, columns, hollow slabs — formed within the masonry or made up of ceramic elements. At each pass, the material takes up the heat and the gases cool. They reach the flue far colder than they left the firebox: the difference has stayed in the heater.
The length and cross-section of this circuit are calculated. Too short, and the heater lets the heat escape; too long or too narrow, and the gases no longer flow and the fire smothers. That is the purpose of the design calculation.
The mass: store, then release
Brick, grog, stone, stove tiles: the heater’s mass stores the heat of the flue gases during the burn, then gives it back to the room as radiant heat. It warms bodies and walls rather than the air, which explains the particular feel of a masonry heater — that of the sun, not of a fan heater. The page on radiant heat describes this effect in detail.
The mass regulates itself: the warmer the room, the less it gives off; when the room cools, it gives more. That is thermal inertia, and it is why a masonry heater needs no thermostat.
The flue and the air: what lets the fire breathe
A masonry heater works thanks to draught: the suction created by the flue, which draws air into the firebox and pushes the flue gases through the accumulator. The flue is therefore calculated together with the appliance (NF EN 13384-1) — height, cross-section, insulation. A poorly matched flue is the leading cause of a heater that smokes or heats badly.
Combustion air comes from outside for preference, through a dedicated duct: the house is not depressurised, and the fire does not consume the room’s warm air. In an airtight house or one with mechanical ventilation, this external combustion air supply is not optional.
On some appliances, an electronically controlled air-inlet damper closes the heater by itself at the end of combustion; a second damper, at the end of the circuit, cuts off the residual draught to keep the stored heat in. The page on regulation explains these two dampers.
Inertia is also a way of running the heater
One to two burns a day, dry split wood, lighting from the top: a masonry heater is run, not adjusted. The heater manual gives the steps; your project study gives the figures for your house.
Key point — Brisk combustion, storage in the refractory mass, slow release as radiant heat: three stages that make for steady, frugal heating.
Page reviewed on 31/08/2026 by Thermasse — stove builder and founding member of the AFPMA. These general principles of the site-built masonry heater are made specific, with figures to back them up, during your project study.