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The honest verdict: cold-water immersion reliably eases short-term muscle soreness and gives a brief lift to mood and alertness, but most of the “boosts metabolism, immunity, heart health, longevity” claims remain unproven in humans. One finding is well-supported and awkward, and this page states it up front rather than burying it: plunging right after weight training blunts the muscle growth you just trained for. Cold-shock cardiac risk is real, documented, and mechanistic — not marketing caution.

Maintenance is a cadence, not a one-time task — and this house runs both hot and cold, so the cadence covers both sides. Cold plunges need water changes keyed to what's protecting the water: every 3–5 days with no filtration, 2–4 weeks on a basic filter, out to 2–3 months — sometimes longer — with a chiller plus managed sanitation. Filter cartridges swap every 3–4 weeks. Saunas need a wipe-down after every session, a deeper clean a few times a year, annual stone re-stacking, and a bench re-oil once or twice a year. Cloudy water or wood that stops beading overrides any schedule.

There’s no single correct protocol for either modality — only sourced ranges, each labeled by how firm the evidence actually is. For post-exercise soreness, cold-water immersion studies cluster around 11–15°C for 11–15 minutes; novices should start far shorter, around 1–3 minutes. Traditional Finnish sauna runs roughly 80–100°C, with one long-running cohort averaging about 14 minutes a session. Contrast therapy is typically heat-then-cold for 2–4 rounds — a common convention, not a tested protocol. And the popular “11 minutes a week” cold target traces to a podcast naming, not a study finding — a plausible extrapolation, not a proven dose.

Avoid cold plunge, or get medical clearance first, for a known arrhythmia, severe aortic stenosis, a recent ICD shock, or ischemic heart disease — the higher-risk modality of the two. Pregnancy, uncontrolled epilepsy, and blood-pressure or thermoregulation medications call for caution. Never combine either with alcohol. Enter gradually, control your breathing, and never plunge alone in deep or very cold water.

Both — depending on your goal and timing. After resistance training, an immediate cold plunge reliably reduces soreness (well supported) but attenuates muscle-fibre growth in controlled trials — contradicting the “boosts recovery” pitch. For endurance work, adaptations appear unaffected. Same tub, opposite verdicts; timing is everything. If you lift for size, don’t plunge in the hours right after.