Thermal Mass Heater Calculator
Calculate thermal mass requirements, heat storage, release time, and room heating capacity for masonry and rocket mass heaters.
About this calculator
A thermal mass heater — a masonry heater or rocket mass heater — deliberately burns fuel hot and fast, then stores that heat in a large mass of brick, stone, or cob to radiate slowly over many hours, and this calculator walks through both halves of that process. On the storage side, it applies the basic heat-capacity relationship Q = m × c × ΔT: wood's energy content (about 15 MJ per kg, air-dried) is multiplied by combustion efficiency to get useful energy transferred to the mass, then divided by the mass's weight times its specific heat (0.84 kJ/kg·°C for brick, 0.79 for stone, 0.93 for cob/adobe — cob stores noticeably more heat per kilogram) to find how many degrees the mass rises per firing. On the release side, it models the mass as a cube to estimate surface area from volume (assumed density of 2000 kg/m³ for brick and stone, 1600 for cob), then applies Newtonian cooling: a heat-transfer coefficient of roughly 8 W/(m²·K) for natural convection off a warm surface combines with the mass's heat capacity and surface area to produce a time constant, and the time to release 90% of stored heat works out to about 2.3 times that constant.
Average heat output over the release period, divided by a rough 50 W/m² heating benchmark for a well-insulated room, estimates how much floor area the heater can keep warm. Because the surface-area model assumes a simple cube and the convection coefficient is a generic estimate, real heaters with more complex geometry, added surface texture, or forced airflow will release heat faster or slower than predicted — always check the peak surface temperature output against safe touch limits before finalizing a design meant for a space people will be near.
Inputs
Results
Heat release duration
17.7 hours
How to Use This Calculator
- Enter the total thermal mass weight in kg and select the material type.
- Set the wood burned per firing in kg and combustion efficiency percentage.
- Enter current room temperature.
- The calculator shows temperature rise, peak surface temperature, average heat output in watts, room area heated, and heat release duration.
- Use peak surface temperature to ensure it stays within safe touch limits and heat release hours to plan firing frequency.
How the result changes with Thermal mass weight
| Thermal mass weight | Heat release duration |
|---|---|
| 500 | 14.1 hours |
| 750 | 16.1 hours |
| 1,500 | 20.3 hours |
| 2,500 | 24.1 hours |
What each input means
- Thermal mass weight
- Total weight of the thermal storage mass (bricks, stone, cob). Typical rocket mass heater bench: 500–2000 kg.
- Mass material
- Thermal mass material. Affects heat capacity.
- Wood per burn
- Weight of air-dried firewood loaded per firing session (~20% moisture content).
- Combustion efficiency
- Heat transfer efficiency. Rocket mass heater ≈ 80–90%, traditional fireplace ≈ 10–30%.
- Room temperature
- Desired room temperature / starting temperature.
What each result means
- Heat release duration
- Estimated time for the thermal mass to release 90% of stored heat.
- Mass temperature rise
- How much the thermal mass temperature increases from one burn.
- Peak mass surface temp
- Estimated peak surface temperature of the thermal mass.
- Average heat output
- Average radiant heat output during the release period.
- Room area heated
- Approximate floor area that can be heated (assumes well-insulated, ~50 W/m²).
- Stored energy
- Total useful thermal energy stored after combustion losses.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersThermal mass weight = 1000, Mass material = 0, Wood per burn = 10, Combustion efficiency = 85 = 5 input(s) provided
- Calculate Heat release durationHeat release duration = (tau * 2.3) / 360017.7 = 17.7
- Calculate Mass temperature riseMass temperature rise = usefulEnergyKJ / (massKg * specificHeat)152 = 152
- Calculate Peak mass surface tempPeak mass surface temp = roomTempC + tempRiseC172 = 172
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does cob store more heat per kilogram than brick or stone, even though it weighs less per volume?
Heat storage per kilogram depends on specific heat, not density — the calculator uses 0.93 kJ/(kg·°C) for cob versus 0.84 for brick and 0.79 for stone, so a kilogram of cob absorbs more energy for the same temperature rise. Cob's lower density (1600 kg/m³ versus 2000 for brick and stone) means a cob mass of the same weight takes up more volume, but per kilogram it's actually the better thermal reservoir of the three.
What does the "90%" in heat release duration actually mean?
The calculator models heat loss from the mass as Newtonian cooling, computing a time constant τ from the mass's heat capacity, surface area, and an assumed convection coefficient of 8 W/(m²·K). The formula releaseTimeHrs = (τ × 2.3) / 3600 uses the fact that τ × ln(10) ≈ 2.3τ is the time it takes to release 90% of the stored heat under this exponential decay model — it's not the time to release all the heat, just the bulk of it.
Why is the thermal mass modeled as a cube, and how might that affect a real design?
The calculator estimates surface area from volume using surfaceAreaM2 = 6 × volume^(2/3), which is the exact surface-to-volume relationship for a cube. A real bench or heater is rarely a perfect cube — a longer, flatter bench has more surface area per unit volume than a cube of the same mass, which would release heat faster than this calculator predicts, while a more compact or insulated shape would release it more slowly.
How is the room area heated calculated, and why might it be optimistic?
roomAreaM2 divides the average heat output in watts (spread evenly over the release period) by a fixed benchmark of 50 W/m², which the calculator's own documentation notes is the requirement for a well-insulated room. A drafty or poorly insulated space needs considerably more watts per square meter to stay warm, so the reported room area should be treated as an upper bound that shrinks for less efficient buildings.
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