A rock or sand heat store is an insulated bed of dry stone or sand charged with hot air from a fire or solar collector and discharged by passing room air through it back out. Dry rock and sand tolerate hundreds of degrees, so they hold far more heat per kilogram of swing than water can at pipe-safe temperatures, but the bed must be charged with clean hot air or a heat exchanger, never with smoke, and heated gradually so trapped moisture cannot burst the stones.
Part of Heat Storage Systems
A rock or sand heat store keeps heat in a bed of solid grains, using air as the carrier to carry heat in and out.
Why Use A Bed Of Stone
Water is easy to move but limited to under 100°C in an open tank and needs a watertight container. Stone and sand are free, never leak and can be run at much higher temperatures. The cost is lower heat per cubic metre at the same swing, and a slower exchange with the air passing through them. A bed with air gaps stores 1.37 MJ/m³·K against water’s 4.18 (Heat Capacity of Common Materials), but stone can swing perhaps 100 K or more where water is practical over 40 K.
Two designs:
- Rock bed with air gaps of 30-40% so air can pass through (Rock Bed Design).
- Sand store, a solid mass heated by embedded pipes or elements (Sand Battery Principles).
Both depend on moving air through channels well (Airflow Channels).
What You Need
- Dry stone of uniform size (rock bed), or dry clean sand (sand store)
- An insulated container with an air inlet at top and bottom
- A heat source that delivers clean hot air: a solar air heater or the exhaust of a stove through a heat exchanger
- A thermometer at the top and bottom of the bed
Method: Charge And Discharge
Step 1: Dry the stone in the sun for days, or warm it gently, before first use.
Step 2: Charge slowly: let the temperature rise over hours, not minutes.
Step 3: Charge with hot air from the top so the hot front moves down; discharge by pulling room air in at the bottom (Airflow Channels).
Step 4: Read the top and bottom temperatures. A bed that is hot at the top and cool at the bottom is working as intended.
Common Mistakes
- Wet stone. River stones hold water inside and burst when heated fast.
- Using exhaust gas directly. Smoke and carbon monoxide would end up in the room.
- No insulation. The bed loses its heat to the ground.
Safety
Carbon monoxide, bursting stone and hot surfaces
Stove exhaust contains carbon monoxide, which is colourless, odourless and deadly: never blow flue gas through a bed in an occupied room; use a heat exchanger or a solar air heater that gives clean hot air, and seal the exhaust to the outside (Rocket Mass Heater explains why a long duct needs a sealed exhaust). Damp or river stone heated fast can burst with force, so use dry stone. A bed at several hundred degrees burns on contact and ignites nearby combustibles; keep wood, straw and cloth clear of the container, and use fireproof insulation near the hot side.
What’s Next
- Learn next: Rock Bed Design — the bed in detail
- Combine with: Rocket Stove Design — a heat source that fits it