Hot Tub

The Hot Tub is a recreational building that relaxes Duplicants with jets of heated liquid. It is used as a morale-building amenity rather than a survival necessity, but it has a few unusual thermal behaviors that make it notable among spa-type buildings.

A Duplicant spends 90 seconds using a Hot Tub, which is equivalent to 3.6 schedule cells. While at least one Duplicant is using it, the Hot Tub consumes 116.7 g/s of Bleach Stone. During use, the Duplicant is in a Relaxed state and its stress decreases by 15% per cycle. After finishing, the Duplicant gains Hot Bathed for 7.95 cycles, granting +5 morale, and also receives Chill Resistance for 3 cycles.

The building’s temperature handling is the main constraint on its use. Water entering the Hot Tub must be kept heated to at least 37°C, but the building will not function if it is above 37.7°C. In practice, that means the input water must be warm enough to satisfy the building, while the surrounding environment must be kept cool enough to avoid exceeding the upper limit. Steel construction raises the Hot Tub’s overheat temperature to 237.7°C, which protects the building itself from heat damage and allows it to tolerate much hotter surroundings or inputs, but it does not change the Duplicant-use limit: if the water is hotter than 37.7°C, Duplicants still will not use it.

The Hot Tub also actively manipulates the temperature of its contents. If the building contains water, it will continuously generate heat at a constant rate, regardless of whether it is powered or damaged. At the same time, the water inside will continuously destroy heat at the same rate, cooling itself down to 37.7°C. These two processes are independent. As a result, a sitting Hot Tub with water above 37.7°C behaves as though it is exchanging heat with its contents at a fixed rate; once the water reaches 37.7°C, the effect switches to net heat production. The water also exchanges heat with the tile directly beneath the center of the building’s lower exit, so the floor under it can be affected by the tub’s temperature behavior.

Because the building actively cools its contents to the usable range, the surrounding temperature does not meaningfully increase the water’s heat loss once the tub is operating. That makes the Hot Tub relatively forgiving in some setups, but it still needs careful integration with the colony’s temperature management so that the building stays within its operating range while the water remains warm enough for use.