Description
Same rock, bigger pieces. Large heaters and hard-fired wood stoves want mass at the bottom of the basket rather than surface area: bigger stones hold heat longer and leave wider air channels, which is what keeps a big pile from choking itself.
Check the fraction your heater specifies. The Harvia Virta and Virta Pro line takes this 10–15 cm stone — it is what our Virta cards add when you choose stones (+$68.89 a box). Wall heaters like the KIP, the Finlandia FIN/FLB and most 6–8 kW units take the smaller AC3000 instead. Putting the wrong fraction in a heater leaves gaps that let radiant heat straight out, or packs it so tight the elements cook.
Many builds use both on purpose: large stones low in the basket, the smaller fraction on top where the water actually lands.
