LP_Infeasible in guests preset: terminal DHW floor (53°C) conflicts with K2 pin (45°C)

[albinati]

Symptom

LP solver returns Infeasible on every solve when OPTIMIZATION_PRESET=guests AND DHW_FIXED_SCHEDULE_ENABLED=True AND the LP horizon ends inside the evening shower window (20:00-22:00 BST).

Observed in prod run 1154 at 2026-05-27 19:55 UTC. Error:

LP returned Infeasible; defensive fallback held previous schedule; appliance-drop retry also infeasible (0.75 kWh)

Reverting OPTIMIZATION_PRESET to normal makes solves Optimal again. Pre-fire reconciliation + _check_tank_target_drift kept the tank safe; no hardware impact, only loss of LP-driven optimisation while in guests.

Root cause

PR #406 (K2) introduced DHW pinning: when DHW_FIXED_SCHEDULE_ENABLED=True, e_dhw[i] and tank[i+1] are pinned by equality to the dhw_policy.forecast_dhw_load_per_slot trajectory. K2 correctly added not _dhw_pinned guards on the soft shower floor (src/scheduler/lp_optimizer.py:842) and on the legionella floor (line 912), but missed the terminal floor at line 1067.

In guests:

• dhw_policy pins tank[n] == DHW_TEMP_NORMAL_C = 45.0
• t_min_dhw = TARGET_DHW_TEMP_MIN_GUESTS_C = 55.0 → terminal_dhw_floor = 53.0 when last slot is in shower window
• LP adds prob += tank[n] >= 53.0 → contradicts tank[n] == 45.0 → Infeasible

In normal the analogous chain is tank[n] == 45 (warmup window) AND tank[n] >= 43 → feasible, hence the bug never fired before the user toggled to guests.

Bisection (replay of run 1154)

Scenario	Status
guests + pinning ON (prod)	Infeasible
guests + pinning OFF	Optimal
normal + pinning ON	Optimal
TARGET_DHW_TEMP_MIN_GUESTS_C=47 → terminal floor 45	Optimal
TARGET_DHW_TEMP_MIN_GUESTS_C=48 → terminal floor 46	Infeasible

Fix

Mirror the K2 guard pattern from the shower floor — skip the terminal DHW floor when _dhw_pinned is True. The pinned trajectory fully owns tank[n].

PR: