A masonry heater is fired, not adjusted. One lively, complete burn, once or twice a day, with dry, split wood: the mass does the rest for eight to twenty-four hours. Most draught problems or sooty glass come from the wood or the rhythm, not from the heater.
Checked on 31/08/2026
This manual is the general part, common to all Thermasse heaters. Your heater has its own: the heater sheet, handed over at completion with the component manuals and the handover report. It gives the wood load, the number of reloads, the drying programme and the equipment fitted. When the manual says ‘see the heater sheet’, that is what it means.
The three rules
- No continuous fire. One or two lively burns a day, never a slow fire kept going permanently.
- Do not open the door and do not close the air while it is burning. The air is closed only at the very end.
- Dry, natural, sized wood. Wood is the first cause of every draught, glass and flue problem.
Before lighting
Ash managed according to your firebox: grate cleared and ash pan emptied before it is two-thirds full if the firebox has a grate; a bed of ash one to two centimetres deep kept if it has none. Air intake clear, flue damper open, air control fully open. Cooker hood, mechanical ventilation on high speed and tumble dryer switched off: they put the house under negative pressure and push smoke back into the room.
The draught test, at the first fire of the season, after more than three months without use or in mild weather: open the door slightly and hold a match in the gap, at the top. The flame must be drawn inwards. If it stays upright or comes out towards you, do not light: switch off the extractors, open a window slightly, then pre-heat the flue — through the bypass if your heater has one, otherwise by burning a twist of crumpled paper through the cleaning door closest to the flue. Repeat the test.
Lighting from the top
Lighting from below produces, on its own, most of the fine particles from wood heating. On a masonry heater, you light from the top: the largest logs at the bottom, in crossed layers, a finger’s width apart; two small logs on top, three or four pieces across them, one or two shop-bought firelighters. No more than the load given in the heater sheet, never above the secondary air inlets. Light, close the door straight away, and do not open it again until the embers.
Weigh your wood the first few times, until you can recognise the load by eye. Some Finnish soapstone fireboxes prescribe lighting from the bottom instead: if the manufacturer’s manual says so, that is what applies.
Closing: the critical step
A burn lasts an hour to an hour and a half. When the logs have collapsed into embers and there is no more yellow flame, look right down at the embers, in the dim light: short blue flames are still running there. That is carbon monoxide burning. As long as you can see any, close nothing. When not a single one is left, anywhere: close the air supply, then, a minute later, the flue damper. If in doubt, wait another quarter of an hour.
Closing too early soots up the glass and can send carbon monoxide into the house. Closing too late lets the stored heat escape up the flue — the equivalent of a radiator switched on outdoors. If your heater has no flue damper, the airtight door acts as the cut-off: same criterion.
The rhythm of the seasons
| Season | Rhythm | Load |
|---|---|---|
| Deep cold | two burns a day, morning and evening | full load |
| Ordinary winter | one burn a day | full load |
| Mid-season | one burn a day | half load |
| Mild between-season | one burn every two days | half load |
Never less than eight hours between two burns: firing one after another does not heat faster, it overstresses the masonry. If the house is still cold twenty-four hours after a burn, it is almost never a lack of wood: first check that the air and the damper were properly closed.
The heater is not run by thermostat. You anticipate: the evening burn carries the night and the morning.
The wood
Natural, untreated, dry logs: that is all. Fifteen to twenty per cent moisture, never more — beyond that, the fire spends its energy evaporating water. A moisture meter was handed to you at completion: split a log, push the pins into the heart of the wood, test three logs from the batch, and do it again with each delivery.
Hard broadleaves for winter — oak, hornbeam, beech, maple, ash. Softwoods are accepted because a masonry heater burns hot; they would be dangerous at a slow burn. Pieces from wrist to palm thickness depending on the species, length as given in the heater sheet, never exceeded: a log that is too long rests on the walls and damages them.
Drying: eighteen to twenty-four months after splitting, two years to be safe, on pallets, never directly on the ground, covered on top and open at the sides. Split before drying: a round log hardly dries at all.
Forbidden, without exception: damp, treated, painted, varnished or stained wood, pallets, chipboard, glossy paper, cardboard, waste, coal, and any lighting fluid. This heater is not an incinerator.
Commissioning: drying and running in
A new heater contains several tens of litres of water, brought in by the mortars and renders. Fourteen days minimum without any fire, door, cleaning doors and damper open, room ventilated — the date of the first permitted burn is on the heater sheet. The flue test carried out by the installer during assembly is not a burn and shortens nothing.
Then comes the running in: about ten burns of increasing load, from kindling alone up to the nominal load, at least eight hours apart, most often a day apart. Allow two to four weeks. Drying smells, fine cracks in the render and small white efflorescence are normal; a wide crack, or smoke in the room, is not: stop and call. Rushed heating or overloading during this period causes damage excluded from the guarantee.
Maintenance and obligations
Ashes wait twenty-four hours before being taken out, in a metal container with a lid, never in a bag or a dustbin. The glass is cleaned cold, with a damp cloth and fine ash; a cracked glass is replaced before any further burn. The door seal is a wear part: a sheet of paper trapped in the closed door must resist when you pull it.
Sweeping. The law requires one mechanical sweeping a year, by a qualified professional, with a certificate to be kept — and the annual servicing of the heater. Thermasse asks for two, one of them during the heating season: this is what the French regulation (arrêté) recommends once a heater burns more than six cubic metres of wood a year, and a masonry heater used as main heating exceeds that threshold. The internal channels are cleaned once a year, at the end of the season, through the cleaning doors indicated on the heater sheet. ‘Chimney-sweeping’ logs and powders replace nothing.
What you should find in a well-run heater: dry, powdery soot, nothing else. A sticky, shiny deposit — creosote — points to slow fires or damp wood, and it is the first cause of chimney fires. Correct the firing, have it swept without delay.
What you must never do
- Never a load prepared the night before: embers may remain under the ashes.
- Never close the air while it is burning.
- Never a continuous or slow-burning fire.
- Never open the door during a burn, before the embers.
- Never damp, treated, painted or varnished wood, nor lighting fluid.
In an emergency
A roaring in the flue, flames at the roof, thick black smoke: that is a chimney fire. Call 18 or 112, close all air supplies, leave the door shut, move away anything that burns, and never throw water on it. If the carbon monoxide alarm sounds: open up, get out, call. In both cases, have a professional check before any return to service.
A carbon monoxide alarm complying with NF EN 50291 is essential with a masonry heater — one in the heater room, one in the bedroom. And remember that the heater stays burning hot for twelve to twenty-four hours after the fire, while the firebox is empty: children have no reason to be wary of it.
Your heater’s documents
Four documents make up your heater’s file, and each makes sense only with the others: this manual, the heater sheet, the component manufacturers’ manuals, and the handover report signed on the day of commissioning — after you have yourself carried out the draught test, the load and the closing sequence. If you have mislaid them, write to us: we will find them in your project file.