Water Heater Tuner

Overview

The water heater setpoint is a three-way trade almost nobody optimizes: scald safety (kids and elders burn at 49°C in seconds), legionella risk (the bacteria that causes Legionnaires' disease colonizes stagnant warm water 20–45°C and dies slowly below 50°C, fast above 60°C), and energy cost (every 10°F / ~5.6°C of setpoint costs roughly 3–5% of water-heating energy via standby loss). This skill computes the trade for YOUR household: who uses the hot water, how big the tank is, what climate zone the tank sits in — and returns a setpoint verdict, a pasteurization plan if you go low, exact vacation protocols, and recovery-time math for "can three people shower back-to-back at 6 am".

When to Use

• "What temperature should my water heater be set at?" — the classic
• New baby / elderly parent moves in → re-evaluate scald risk
• Cutting energy bills: savings estimate per degree + payback of a blanket/ heat-pump upgrade
• Tank has been lukewarm / unused (vacation, vacancy, temp lowered) → legionella flush protocol
• Sizing questions: "will 150 L recover fast enough for our mornings?"
• Vacation/away mode decisions (off vs pilot vs vacation setting)

Don't use for: tankless/on-demand sizing (different physics — inlet temp and flow rate dominate), boiler/radiator central heating, or solar thermal controllers.

The Physics & Standards (what the script computes)

1. Scald times (Moritz & Henriques burn data, the basis of UL/CPSC guidance): full-thickness burns at 49°C in ~10 min... no — at 52°C in ~90 s, 55°C in ~17 s, 60°C in ~3 s, 68°C in ~0.5 s. Children's skin burns faster (~1/4 the time). Verdict thresholds: with infants/elders, deliver ≤49°C at the tap (anti-scald valve or setpoint ≤49°C).
2. Legionella (WHO guidance): multiplies 20–45°C, dormant <20°C, dies at 50°C slowly (hours), 60°C in ~32 min, 70°C in ~seconds–minutes. A tank held ≥60°C keeps itself sanitized; below 50°C stored water needs periodic thermal disinfection.
3. Energy: standby loss scales with (tank_temp − ambient_temp). Tank in a cold basement loses proportionally more; lowering 60°C→50°C saves roughly 4–8% of water heating energy (bigger effect for older tanks, cold rooms, low hot-water-use households).
4. Recovery: gas recovers ~2× faster than electric per tank size (typical first-hour ratings: gas 40-gal ≈ 60-70 gal/hr; electric ≈ 45-55). Script converts tank size + fuel + setpoint into "showers until cold".
5. Pasteurization protocol: if operating low (<60°C storage), a weekly thermal purge — heat to 60°C+, hold ≥30 min (WHO: 60°C/32 min), then flush every tap/shower 5 min at high temp — keeps the system clean.

Numbered Workflow

1. profile — household: ages of users (scald class), tank liters/gallons, fuel (gas/electric/heat-pump), location (basement/garage/conditioned), current setpoint if known.
2. setpoint — verdict: the recommended storage temperature + tap delivery strategy (anti-scald mixing valve at 49°C lets you STORE at 60°C — best of both worlds; note it in the output).
3. energy --to 50 — estimated annual savings for a setpoint move, plus tank-blanket / pipe-insulation / heat-pump-upgrade deltas.
4. recovery --showers 3 — first-hour and consecutive-shower math at your setpoint and inlet temperature (winter inlet matters).
5. pasteurize — dated protocol card for low-setpoint operation or after any stagnation (vacation >1 week, vacancy, temp was ever <50°C).
6. vacation --days 14 — away plan: vacation mode vs off, what to do on return (flush protocol), what NOT to do (don't drain a gas tank's element... fuel-specific notes).

Commands

python3 scripts/heater_tuner.py profile --tank-gal 40 --fuel gas \
  --users adult,adult,child2 --location basement --setpoint 130
python3 scripts/heater_tuner.py setpoint
python3 scripts/heater_tuner.py energy --to 120 --price 0.20
python3 scripts/heater_tuner.py recovery --showers 3 --inlet-c 9
python3 scripts/heater_tuner.py pasteurize
python3 scripts/heater_tuner.py vacation --days 14

Store: heater_profile.json next to the script (--store to override).

Pitfalls

1. Testing temperature at the tank, not the tap — tank says 52°C, the upstairs shower delivers 47°C after pipe losses; legionella decisions are made at the FARTHEST tap, scald decisions at the NEAREST one. Measure both (run 30 s, thermometer in stream).
2. Lowering setpoint without a flush plan — going 60→49 saves energy and adds safety, but stored water at 46-49°C is prime legionella habitat: pair any low setpoint with the weekly pasteurization cycle or a mixing valve + 60°C storage.
3. Trusting the dial — heater dials are unlabeled/±10°F fantasy. Verify with a thermometer at the tap (30-second run), then mark the dial.
4. Forgetting the vacation return — stagnant tepid tank = bacterial culture. Run the pasteurize protocol after any week+ absence before letting immunocompromised users shower (healthy users: flush 5 min hot).
5. Mixing valve ≠ setpoint license — an anti-scald valve at 49°C does NOT sanitize the tank; keep storage ≥60°C for legionella control and let the valve do the safety. The valve is the strategy; the setpoint serves the tank.

Example

Family with a 2-year-old, gas 40-gal tank in a cold basement set to 140°F: verdict — store at 60°C/140°F with a 49°C anti-scald valve (legionella-safe storage + child-safe delivery), savings vs "just lower to 120°F" are equal but with better sanitation; without a valve: 49°C delivery cap → weekly pasteurize protocol printed. Recovery math says 3 morning showers fit with 8 min spacing at 140°F (larger draw before mixed-cold), only 2 back-to-back at 120°F — the morning schedule decides the strategy as much as safety does.