Ya estufas (jazz-TOO-fass, lit. already stoves) is an expression that denotes strong agreement or implies that a deal is done. It has nothing to do with stoves. It comes from the equally solid but less colorful expression ya estuvo (lit. it already was), with the same meaning as ya estufas.
Maestro albañil: ¿Ya prepararon la mezcla?
Colador: ¡Ya estufas!
Senior construction worker: Is the concrete mix ready?
Junior construction worker: You betcha!