Hazel McCready's air exchanger

Hazel McCready’s air-exchanger is a converted furnace turned into a small atmosphere-manufacturing factory, and its importance is ultimately measured in lives: it produces the air the Tommyknockers need to breathe when Haven’s ordinary environment can no longer sustain them. Built from household infrastructure and improvised power components, it becomes both a refuge for the town’s survivors and a vulnerable piece of equipment that must be protected.

Design and Operation

The device sits in Hazel McCready’s backyard beneath a large five-man tent, like a one-ring circus under cover. A furnace supplies its central structure, while galvanized aluminum pipes that once led to the McCready house’s room ventilators now point skyward. The arrangement turns familiar domestic equipment outward, into a system for manufacturing and distributing a different atmosphere.

Twenty-four truck batteries power the factory. They rest on two plywood ramps protected from the elements by silvery netting like that lining the trench where the buried ship lies. The construction is conspicuously makeshift, but its purpose is exact: when switched on, the furnace-derived apparatus makes air—specifically, Tommyknocker air.

Protection and Place in Haven

The air-exchanger is treated as a critical safety device, not as an ordinary household invention. The Shed People designate it as the safety net and insist it be guarded against anyone who might break through Haven’s perimeter. Force-shields surround it, while other residents are assigned to protect the factory in case of accident or a rapid, organized response from the outside world.

Its role is distinct from that of the transformer in the shed. The two devices can perform many of the same functions, but the backyard factory has a capability the transformer does not: it makes breathable air for the Tommyknockers. That difference turns the exchanger into infrastructure for the transformed community, something on which their continued presence in Haven depends.

The Fire and the Survivors

After the ship departs and the becoming ends, Haven is devastated. By nightfall, fewer than two hundred people remain alive, and much of the township’s wooded western half has burned or is still burning. The surviving Tommyknockers gather in Hazel McCready’s yard, where their breathing has become difficult in the changing air.

Bryant Brown and Phil Golden get the large air-exchanger running. The survivors gather around it as people might once have gathered around a stove on a bitter night; as the device supplies the atmosphere they require, their breathing gradually eases. The scene gives the factory its clearest practical test: protected beforehand as a contingency, it becomes a working refuge amid the fire’s consequences.

Capture, Study, and Significance

The exchanger’s importance persists after Haven is invaded. The outside air is lethal to the changed residents, while the air inside Haven cannot be breathed by the soldiers entering the town. The machinery that produces the changed atmosphere is carefully studied, and its failing batteries are replaced. Scientists at MIT, Cal Tech, Bell Labs, and the Shop in Virginia begin studying its underlying principles; as Bobbi Anderson had predicted, the device’s mechanics do not remain beyond their understanding for long.

The exchanger is therefore both a practical life-support system and a hinge between Haven and the institutions beyond it. Its improvised assembly of furnace, pipes, batteries, protective barriers, and human caretakers shows how survival depends on a network of materials and labor, not on a single isolated invention. Yet the knowledge it embodies also helps make the surviving Tommyknockers available for study: the factory is protected because it keeps them alive, then examined as part of the response that carries them away from Haven.