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Calcimator

Thermal Mass Heater Calculator

Calculate thermal mass requirements, heat storage, release time, and room heating capacity for masonry and rocket mass heaters.

Inputs

lb
lb
%
°F

Results

Heat release duration

17.7 hours

Mass temperature rise152 °C
Peak mass surface temp172 °C
Average heat output1,996 W
Room area heated40 m²
Stored energy127.5 MJ
Material NameBrick
How to Use This Calculator
  1. Enter the total thermal mass weight in kg and select the material type.
  2. Set the wood burned per firing in kg and combustion efficiency percentage.
  3. Enter current room temperature.
  4. The calculator shows temperature rise, peak surface temperature, average heat output in watts, room area heated, and heat release duration.
  5. 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 weightHeat release duration
1,00917.8 hours
3,50727 hours
6,50433.1 hours
9,00136.9 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
0 = Brick, 1 = Stone, 2 = Cob/Adobe. 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

  1. Identify Input Parameters
    4 parameters
    Thermal mass weight = 1000, Mass material = 0, Wood per burn = 10, Combustion efficiency = 85 = 5 input(s) provided
  2. Calculate Heat release duration
    Heat release duration = (tau * 2.3) / 3600
    17.7 = 17.7
  3. Calculate Mass temperature rise
    Mass temperature rise = usefulEnergyKJ / (massKg * specificHeat)
    152 = 152
  4. Calculate Peak mass surface temp
    Peak mass surface temp = roomTempC + tempRiseC
    172 = 172

Engine last updated . Checked against 1 independently-derived test — how we verify calculators.

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