Turndown ratio
Turndown ratio is the span between a heater's maximum and minimum steady output — a stove rated 12,000–48,000 BTU/h has a 4:1 turndown. A wide turndown lets one stove burn cleanly in both mild October weather and a January cold snap.
Turndown is where combustion technologies differ most: catalytic stoves hold clean fires at very low output, pellet stoves modulate their feed rate automatically, and non-catalytic stoves have a narrower clean-burn range and would rather run medium-hot.
The EPA database's output range (lower and upper BTU/h) is measured during certification, so it reflects clean operation — not the theoretical minimum a damped-down stove can smolder at.