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Heating a Large Space Efficiently

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Heating a Large Space Efficiently

To heat a large hall efficiently, heat the occupied zone and the building’s mass rather than all its air, because in a well-insulated hall that is ventilated for a crowd, the heat lost to ventilation outweighs the heat lost through the walls and roof. A 120 m² hall with R-30 walls and roof loses about 1.1 kW through its fabric at a 20 K temperature difference but about 3.4 kW to one air change an hour, and 100 people give off roughly 10 kW, so a full hall needs little heating while an empty one needs about 1.8 kW overnight.

Part of Community Gathering Hall

Heating a large space efficiently is the practice of supplying and keeping heat where people are in a hall whose volume and surface area make whole-room heating wasteful.

The Hall’s Heat Budget Has Three Parts

The gathering-hall guide gives three heating strategies: a central hearth with a chimney or smoke hood (radiant heating and cooking, but smoke management is hard); a masonry heater, a massive stove with long winding flues that is burned hot for 2-3 hours and then releases heat for 12-24 hours, very fuel-efficient for a large space and needing real masonry skill; and zone heating, where movable screens or curtains confine heat to the occupied part. The arithmetic below shows why they work.

Steady heat loss has two parts, each proportional to the indoor-outdoor temperature difference ΔT:

  • Fabric loss = U × A × ΔT, with U in W/m²·K and A the area of wall and roof.
  • Ventilation loss = 0.33 × n × V × ΔT watts, where V is the room volume in m³, n the air changes per hour and 0.33 W·h/(m³·K) the heat capacity of a cubic metre of air (1.2 kg/m³ × 1.005 kJ/kg·K ÷ 3.6 = 0.335).

Worked Example: The Heat Budget Of A 10 × 12 m Hall

Inputs: floor 10 × 12 m (120 m²); walls 3 m high; roof ridge 5.5 m (volume 510 m³, wall area 157 m² including the two end gables, roof slope area 134 m²); R-30 for walls and roof, the figure for plastered straw bale in the corpus, which in SI is R = 30 × 0.1761 = 5.28 m²·K/W, so U = 1 ÷ 5.28 = 0.19 W/m²·K (assuming the roof is insulated to the same value); ΔT = 20 K; floor loss ignored; one person gives off about 100 W.

  1. Fabric: 0.19 × (157 + 134) × 20 = 0.19 × 291 × 20 ≈ 1,106 W, about 1.1 kW.
  2. Ventilation at 0.2 air changes an hour (infiltration only): 0.33 × 0.2 × 510 × 20 = 673 W. At 1 air change an hour: 0.33 × 1 × 510 × 20 ≈ 3,370 W. At 3 changes: about 10,100 W.
  3. Empty hall overnight: 1,106 + 673 ≈ 1,779 W ≈ 1.8 kW. For 12 hours: 1.78 × 12 ≈ 21 kWh = 77 MJ (1 kWh = 3.6 MJ).
  4. Wood at about 15 MJ/kg (the typical handbook value the Fuel Savings Measurement guide uses): 77 ÷ 15 ≈ 5.1 kg at 100% efficiency; divide by your stove’s real efficiency for the true figure.
  5. Full hall of 100 people: 100 × 100 W = 10 kW, which already exceeds the 4.5 kW of fabric plus 1 air change an hour (1.1 + 3.4); only at 3 air changes an hour does ventilation loss (10.1 kW) match it.

The 100 W per person is the commonly used figure for a seated adult; it is not stated elsewhere in this corpus and should be checked. The U-value is assumed to hold for the real wall; a poorly sealed or uninsulated hall will lose several times more.

A Masonry Heater As A Heat Store

Illustration, not a design: a heater with 2,000 kg of brick (assumed) warmed on average by 100 K stores 2,000 × 0.84 × 100 = 168 MJ, using about 0.84 kJ/kg·K for brick (a typical value that is not stated elsewhere in this corpus). Released over 12-24 hours that is 168 ÷ 12 ÷ 3.6 ≈ 3.9 kW down to 1.9 kW on average, the same order as the empty hall’s loss of 1.8 kW. This is why a short hot burn can warm a hall overnight.

What You Need

Method

Step 1: Insulate and seal first; it costs less than fuel.

Step 2: Size the heat source to the empty-hall loss, not the full-hall gain.

Step 3: Burn hot and short in a masonry heater, then close the damper only when the fire is out to avoid carbon monoxide.

Step 4: Zone the hall with curtains for small groups.

Step 5: Ventilate full halls with controlled vents rather than opening doors.

Common Mistakes

  • Heating the whole hall for a few people.
  • Closing the damper on live coals to save heat, which traps carbon monoxide.
  • Sealing the hall tight with a central fire and no air inlet.

Safety

Fire, smoke and carbon monoxide in a hall full of people

A central hearth fills a poorly ventilated hall with smoke and carbon monoxide; use a chimney or smoke hood with at least 5 cm of clearance from combustible framing, a non-combustible hearth extending 40 cm in front and 20 cm to each side, and a spark arrestor if the roof is combustible (Fire Prevention in Settlements). Keep the crowd and hangings well clear of the fire, and do not close a damper while embers still glow.

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