Standalone Hot Spring: Hard Numbers

[h1]Hard numbers[/h1]

A hot spring does not require any prerequisite technology, as a welcome relief for long modlists that already have hundreds of added research projects to worry about. Instead, hot springs simply have an extended construction time, require a skilled builder (Construction skill 8) to be worked on, and have a rather steep cost of 500 of any stony Stuff as building material. In-universe, you can think of it as every steam geyser being a unique challenge that requires its own particular approach, puzzled out by the builder pawn on the worksite itself. No two steam geysers are exactly alike deep underground; the nature of the intersection between groundwater and geothermal heat sources is always different, and so every hot spring dug out is an arduous project.

A hot spring built from Vanilla-available stone blocks will take 1500-1800 work to complete. A builder with 8 Construction can finish 1000 units of work in approximately 24 hours of labor, so construction of a hot spring will occupy a typical well-skilled builder pulling ~12 hour shifts for anywhere between 3-4 days, depending on his workspeed, time spent walking, and other typical Rimworld "distractions". Just because this building is available from the start doesn't mean it should be your first priority; if you need to pull your builder off of it mid-construction to focus on building something else, forbid the construction site until you can afford to resume work on the hot spring.

A hot spring is 8 tiles wide and 4 tiles tall (by comparison, a geothermal generator is 6x6). Vanilla mechanics mean that the steam geyser "underneath" the building is never destroyed or removed, so it will continue to release large amounts of heat. If placed outdoors, the hot spring does not block wind turbines. It sits in the earth, after all; it's not an above-ground pool. Because it is fully passable (much like a bed), this means it can (and you will want to) have floor constructed "underneath" it. With 2000 base HP and 0% base flammability, a finished hot spring is unlikely to be destroyed by accident. It can also eat a fair number of mortar shells or drop-podded scyther swings.

A hot spring is pleasant to look at, with a base beauty score of 200 (equivalent to a twice that of a Large sculpture; this will be affected by stone type, though not by builder skill as the building does not have a quality). With a steep building cost of 500, though, I personally recommend saving jade or other precious stones for sculptures. 200 beauty on a single tile is usually not enough by itself to give pawns positive moods; you will still need to the beautify the hot spring's surroundings for maximum effect.

Using a hot spring is a meditative joy source with 140% recreation power; a strong improvement over Vanilla meditative joy sources that (with the exception of a grave/sarcophagus in an impressive tomb) do not scale above 100%. Its joygiver is weighed heavily but not overwhelmingly so; if you have a hot spring, colonists will generally prefer it to Vanilla meditative joy sources. Keep in mind that while you can manually direct colonists to use a hot spring, colonists who are bored of meditative joy will restore very little recreation while doing so. The hot spring is also very comfortable (1.00) and acts as a rest source while in use (Vanilla stone blocks will result in a 100% rest rate), which also means that sick/injured pawns using a hot spring are not "missing out" on (standard) bed rest recovery rate.

The hot spring has two separate hediffs, though both are very similar; one for pawns actively using the hot spring ('Healing waters'), and another that replaces the first hediff once they leave ('Residual rejuvenation'). The only difference between the two is that the former has a generous temperature-insulating effect allowing pawns to use it even in hot or cold temperatures without discomfort (should the hot spring need to be left outdoors in an inhospitable biome, such as an extreme desert) and that it lasts exactly as long as the pawn is using it, while the latter lacks the temperature-insulating effect and lasts for 30 hours. 'Residual rejuvenation' is programmed to be replaced by 'Healing waters' and vice versa; a pawn should never have both hediffs at the same time. (If it happens, please comment below telling me how you managed it so I can investigate further.)

Both hediffs improve the pawn capacity Blood filtration by +8%, improving Immunity Gain Speed equivalent to a Royalty Immunoenhancer (stacking additively with most other Blood filtration buffs). They also apply a 125% modifier to Natural healing factor, equivalent to half the buff from a Royalty Healing enhancer. This buff allow pawns to heal an extra 2 to 5.5 Hitpoints every 24 hours (stacking multiplicatively with an installed Healing enhancer or similar from mods), depending on their activity. While resting in a hot spring is theoretically less effective than doing so in a hospital bed (if one is available), taking the time to leave a hospital bed, pick up the Residual rejuvenation hediff from a hot spring, and then return should always be worth it as long as your hospital is located reasonably close to the hot spring.

[h1]Understanding healing rate[/h1]

Disclaimer: this analysis was performed by a mixture of amateur code-reading, Discord server think-tanking, and pre-existing information available on the Rimworld wiki. It may turn out to be incorrect.

Every 600 ticks, a random (healable) injury on a pawn is selected for recovery. 0.01 x [healing rate] points of damage are removed from the injury (aka healed). With 60000 ticks to an in-game day, this can be generalized as a pawn healing 1.0 x [healing rate] every day. What determines the healing rate of injury, however, is somewhat complicated.

Pawns have a base healing rate of 8. If the pawn is resting on a sleeping spot/the ground, add 4. If they are resting in a (Vanilla) bed or bedroll, add 8. If they are resting in a hospital bed, add 14. These values are determined by bed_healPerDay in a bed's ThingDef .xml entry.

The sum of the pawn's base healing rate and their current activity are modified by Natural healing factor, a hidden stat that was added in Royalty. It is currently only modified (in Royalty) by the Healing enhancer implant, which applies a 150% multiplier. An active pawn will gain an extra +4 healing rate from a Healing enhancer, a pawn resting in a bed/bedroll will gain an extra +8, and a pawn resting in a hospital bed will gain an extra +11.

Tended wounds heal faster on top of this, though the bonus from a wound's tending is not multiplied by Natural healing factor. If the wound is tended, the injury's healing rate increases further on an individual basis, scaling from +4 to +12. A 0% tend adds +4 healing rate to an injury; a 100% tend adds +12 healing rate. In other words, a tend on a wound adds +4 healing rate, with another +2 added for every 25% of tend quality.

[i]If injuries are picked randomly, can't I get screwed by RNG?[/i]

With 100 injury recovery checks being made per day, it is highly unlikely that any single wound will be either focused on or ignored compared to the rest. Additionally, the relevant endgoal is most frequently "full recovery", with every single injury being fully healed, so the order in which the injuries are selected is irrelevant.

The only relevant RNG is each tend's individual quality; you can get unlucky and roll low for a large injury. Even then, the quality of the tend can only contribute a maximum of +8, which contributes a non-majority portion of a typical patient's heal rate (+8 base, +4 for having tended at all; even without bed rest, tend quality only contributes a maximum of 8/20 = 40% of a pawn's sum healing rate; with bed rest, 8/28 = 28.57%; with hospital bed rest, 8/34 = 23.53%; and Natural healing factor buffs further reduce the relative impact of tend quality).