Heating Systems
Part of Permanent Shelter
In any climate with cold winters, heating is not a comfort — it is survival. A shelter without effective heating is just a wind break. Understanding how heat moves, how different heating systems compare, and where to position your heat source will determine whether your fuel lasts the winter and whether your family stays warm or slowly freezes. This guide covers the principles and practical options for indoor heating with pre-industrial materials.
How You Lose Heat
Before you heat a space, understand how it loses heat. Every heating system is fighting these four mechanisms:
| Mechanism | What Happens | How to Reduce It |
|---|---|---|
| Conduction | Heat flows through solid materials (walls, floor, roof) from warm side to cold side | Thicker walls, insulating materials (straw, wool, air gaps) |
| Convection | Warm air rises, escapes through gaps; cold air flows in to replace it | Seal gaps around doors, windows, roof joints; use airlocks |
| Radiation | Warm surfaces emit infrared energy toward cold surfaces | Reflective surfaces behind heat source; position heat near occupants |
| Infiltration | Cold outdoor air enters through cracks and openings | Chinking, weatherstripping, draught excluders |
The Biggest Heat Thief
In most primitive shelters, infiltration accounts for 40–60% of heat loss. Before improving your heating system, seal every crack and gap you can find. A well-sealed shelter with a small fire is warmer than a drafty shelter with a roaring one.
Heating System Comparison
Not all heating methods are equal. The key metric is how much of the fuel’s energy actually heats the living space versus going up the chimney or radiating into the walls.
| System | Efficiency | Heat Type | Fuel Consumption | Complexity |
|---|---|---|---|---|
| Central open fire (fire pit, smoke hole) | 5–10% | Radiant + smoky convection | Very high | Minimal |
| Open fireplace (with chimney) | 10–15% | Mostly radiant | High | Moderate |
| Enclosed fireplace (Rumford-style, tall and shallow) | 15–25% | Radiant + convective | Moderate | Moderate |
| Masonry stove (Russian/Finnish style) | 70–85% | Thermal mass radiation | Low | High |
| Metal stove (cast iron or sheet metal) | 40–60% | Radiant + convective | Moderate | Requires metalwork |
| Heated floor/wall (Korean ondol, Roman hypocaust) | 60–80% | Radiant (floor) | Low-moderate | High |
The Open Fire Illusion
A central fire pit with a smoke hole in the roof feels warm when you sit next to it, but it heats almost nothing else. Most of the heat goes straight up with the smoke. The smoke hole also acts as a chimney for your warm air — every fire sucks heated room air out through the roof. An open fire can actually make the far corners of a room colder by increasing the airflow of cold outside air through gaps.
Radiant vs. Convective Heat
Understanding these two types of heat transfer changes how you design your system.
Radiant heat travels in straight lines from a hot surface to whatever it hits, like sunlight. It warms people and objects directly without warming the air in between. A fireplace radiates heat — you feel warm facing it, cold with your back to it.
Convective heat warms the air itself, which then circulates through the room. A metal stove primarily works by convection — it heats the air touching its surface, that air rises, and room air circulates past the stove continuously.
Thermal mass heat is stored radiant heat. A massive stone or clay structure absorbs heat from a fire over several hours, then radiates it slowly for 12–24 hours after the fire goes out. This is the principle behind masonry stoves, heated floors, and heated walls.
For Sleeping Comfort
Radiant heat from thermal mass is vastly superior for overnight warmth. Build a fire in the evening, let the thermal mass absorb the heat, and it radiates gently all night. A convective system (metal stove or open fire) stops heating the moment the fire dies.
Positioning Your Heat Source
Where you place the heating system determines how effectively it heats the space.
Central Placement
- Best for: Round or square single-room structures.
- How it works: Heat radiates outward equally in all directions.
- Problem: Requires a smoke hole or central chimney, which is harder to weatherproof than a wall-mounted chimney.
Against an Interior Wall
- Best for: Multi-room structures.
- How it works: The thermal mass of the fireplace heats two rooms simultaneously — one by direct radiation, the other through the warmed wall.
- Problem: Takes up wall space; chimney runs through the roof rather than up an exterior wall.
Against an Exterior Wall
- Best for: Single-room structures, ease of chimney construction.
- How it works: The chimney runs straight up the outside wall.
- Problem: The back of the fireplace radiates heat into the exterior wall and outside — wasted energy. Mitigate by making the back wall of the firebox thick (30+ cm) to slow heat loss.
Best Practice
Place the heat source against an interior wall whenever possible. The heat that escapes through the back of the fireplace warms the adjacent room instead of the outdoors. Reserve exterior-wall fireplaces for single-room buildings.
The Masonry Stove Principle
The masonry stove is the most fuel-efficient pre-industrial heating system ever developed. The concept is simple: instead of letting hot exhaust gases go straight up the chimney, route them through a long, winding channel inside a massive stone or brick structure. The masonry absorbs the heat from the gases. By the time the exhaust exits the chimney, it is barely warm.
How It Works
- A hot, fast fire is burned for 1–2 hours (not a slow smoulder).
- The hot gases travel through 3–5 metres of internal channels (baffles) inside the masonry mass.
- The stone/brick/cob mass absorbs 70–85% of the heat energy.
- The mass then radiates that heat into the room for 12–24 hours.
- The chimney exhaust is cool enough to touch — nearly all useful heat has been extracted.
Key Dimensions
| Parameter | Guideline |
|---|---|
| Mass | At least 800 kg for a small room (15 m2); 1,500+ kg for larger spaces |
| Channel length | 3–5 metres total (folded back and forth inside the structure) |
| Channel cross-section | 15 × 20 cm minimum to avoid blockage |
| Firebox size | Large enough for a vigorous 1–2 hour fire |
| Burn cycle | One or two fires per day, not continuous |
Chimney Basics
Any enclosed fire needs a chimney to exhaust smoke. A chimney works by the “stack effect” — hot gases are lighter than cold air, so they rise. The column of hot gas in the chimney creates a draft that pulls air into the firebox and pushes smoke out the top.
Chimney Rules
- Height: The chimney must extend at least 60 cm above the roof peak. A taller chimney draws better.
- Cross-section: The flue opening should be roughly 1/10 the area of the firebox opening. Too small chokes the fire; too large loses too much heat.
- Straight and vertical: Bends and horizontal runs reduce draft. If you must offset the chimney, keep the angle above 60 degrees from horizontal.
- Material: Stone, brick, or thick clay/cob. The chimney gets extremely hot — combustible materials within 15 cm of the flue will eventually ignite.
- Chimney cap: A flat stone or clay slab on supports at the top, leaving open sides for exhaust. This keeps rain out without blocking airflow.
Safety
Indoor heating kills more people than cold does — usually through carbon monoxide poisoning or house fires. Every heating installation must address both risks.
Carbon Monoxide
Carbon monoxide (CO) is produced by incomplete combustion. It is odourless and colourless. Symptoms begin with headache and drowsiness and progress to unconsciousness and death.
- Never close the damper or block the chimney while coals are still glowing. Smouldering coals produce the most CO.
- Always maintain adequate draft. If smoke enters the room, the chimney is not drawing properly — open a window immediately and fix the draft problem before using the fire again.
- Sleep ventilation: Even with a properly drawing chimney, maintain a small ventilation opening in the sleeping area. A 10 cm gap under a door or a small vent in the wall is sufficient.
Fire Risk
- Hearth clearance: The floor in front of any fire opening must be non-combustible (stone, brick, packed earth) for at least 45 cm in front and 30 cm to each side.
- Wall clearance: Combustible walls must be at least 90 cm from the fire opening. Reduce to 30 cm with a stone, brick, or clay heat shield.
- Chimney clearance: Maintain 15 cm of non-combustible material around the flue where it passes through walls or the roof.
- Creosote: Burning wet or resinous wood produces tar (creosote) that coats the inside of the chimney. This buildup is highly flammable and can cause chimney fires. Burn dry, well-seasoned hardwood. Clean the chimney at least twice per heating season by dropping a bundle of thorny branches or a weighted brush down the flue.
Chimney Fires
A chimney fire sounds like a freight train inside your walls — a deep roaring with intense heat. If one occurs: close all air inlets to the firebox to starve the fire of oxygen, get everyone outside, and watch for embers landing on the roof. After it burns out, inspect the chimney for cracks before using it again. Prevention (burning dry wood, regular cleaning) is far easier than fighting a chimney fire.
Key Takeaways
- Seal your shelter before upgrading your heating — infiltration causes 40–60% of heat loss in primitive buildings.
- Open fires are 5–10% efficient. Masonry stoves reach 70–85% by routing exhaust through long internal channels.
- Thermal mass heating (masonry stove, heated floor) provides overnight warmth long after the fire goes out. Metal stoves and open fires do not.
- Place heat sources against interior walls so waste heat warms adjacent rooms, not the outdoors.
- Every chimney must be tall enough (60 cm above roof peak), sized correctly (flue = 1/10 firebox opening), and kept clean of creosote.
- Carbon monoxide kills silently. Never block the chimney while coals are glowing, and always maintain ventilation in sleeping areas.