diff --git a/utils/replay_sim.py b/utils/replay_sim.py
new file mode 100644
index 0000000..0670b2a
--- /dev/null
+++ b/utils/replay_sim.py
@@ -0,0 +1,205 @@
+#!/usr/bin/env python3
+"""Replay a BrewPi log through the temp controller with candidate PID params.
+
+Feeds real temp_ist readings from a log back into a fresh TempController
+(Normal mode) tick by tick and compares the resulting power output to the
+original logged values.  Because the real sensor measurements are injected at
+each tick, no plant model is needed: the replay shows exactly how a different
+set of PID gains would have responded to the same temperature trajectory.
+
+Usage:
+    python utils/replay_sim.py <date_time> <sud_name>
+    python utils/replay_sim.py <date_time> <sud_name> \\
+        --heat-kp 0.1 --heat-ki 0.03 --heat-kt 1.0
+    python utils/replay_sim.py <date_time> <sud_name> --config alt_config.json
+
+By default reads PID params from config.json in the repo root.  Any --<section>-<gain>
+flag overrides just that one value; the rest come from the config.
+"""
+
+import argparse
+import copy
+import json
+import sys
+from pathlib import Path
+
+import matplotlib
+matplotlib.use("TkAgg")
+import matplotlib.pyplot as plt
+from matplotlib.ticker import AutoMinorLocator
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+from components.pid.temp_controller import TempController
+
+
+def _apply_gain_overrides(params, args):
+    p = copy.deepcopy(params)
+    for section, prefix in (('Hold', 'hold'), ('Heat', 'heat'), ('Cool', 'cool')):
+        for gain in ('kp', 'ki', 'kd', 'kt'):
+            val = getattr(args, '{}_{}'.format(prefix, gain))
+            if val is not None:
+                p[section][gain] = val
+    return p
+
+
+def _infer_max_power(samples):
+    """Largest discrete heater step seen in the log — used to normalise power_set."""
+    return max((s['power_eff'] for s in samples), default=1.0) or 1.0
+
+
+def _infer_heatrate_soll_set(samples):
+    """Heat-rate setpoint from the log: max rate_soll seen while power > 0.
+
+    rate_soll = heatrate_soll_set * pid_hold.get_y(); when the hold PID is
+    saturated at 1.0 (full-power heating) the two are equal, so the maximum
+    over all heating samples is a good estimate of heatrate_soll_set."""
+    candidates = [s['rate_soll'] for s in samples if s['power_set'] > 0]
+    return max(candidates, default=1.0) or 1.0
+
+
+def run_replay(samples, pid_params, heatrate_soll_set, dt):
+    tc = TempController(dt)
+    tc.set_params(pid_params)
+    tc.set_enabled(True)
+
+    out = []
+    for s in samples:
+        tc.set_theta_ist(s['temp_ist'])
+        tc.set_theta_soll(s['temp_soll'])
+        tc.set_heatrate_soll(heatrate_soll_set)
+        tc.process()
+        out.append({
+            'power': tc.get_power(),
+            'state': tc.state.name,
+            'heatrate_ist': tc.get_heatrate_ist(),
+            'heatrate_soll': tc.get_heatrate_soll(),
+        })
+    return out
+
+
+def _style_axis(ax):
+    ax.set_facecolor('white')
+    ax.spines['top'].set_visible(False)
+    ax.spines['right'].set_visible(False)
+    ax.xaxis.set_minor_locator(AutoMinorLocator())
+    ax.yaxis.set_minor_locator(AutoMinorLocator())
+    ax.tick_params(which='major', direction='out', labelsize='small')
+    ax.tick_params(which='minor', direction='out', length=2)
+    ax.grid(which='major', linestyle='-', linewidth=0.5, alpha=0.3)
+    ax.grid(which='minor', linestyle=':', linewidth=0.5, alpha=0.15)
+
+
+def plot_replay(samples, replay, name, max_power, params_label):
+    t = [s['t'] for s in samples]
+
+    fig, (ax_temp, ax_rate, ax_power) = plt.subplots(3, 1, sharex=True, facecolor='white')
+    fig.suptitle('{} — replay simulation'.format(name), fontsize='small')
+
+    # Temperature: identical in both runs (real sensor readings are injected)
+    ax_temp.plot(t, [s['temp_ist']  for s in samples], '-b', linewidth=1, label='theta_ist')
+    ax_temp.plot(t, [s['temp_soll'] for s in samples], '-r', linewidth=1, label='theta_soll')
+    ax_temp.set_ylabel('Temp [°C]')
+    ax_temp.legend(loc='upper left', fontsize='small')
+
+    # Heat rate
+    ax_rate.plot(t, [s['rate_ist']  for s in samples],       '-b',  linewidth=1, alpha=0.6, label='rate_ist (log)')
+    ax_rate.plot(t, [r['heatrate_ist'] for r in replay],     '--',  color='C1', linewidth=1, label='rate_ist (replay)')
+    ax_rate.plot(t, [s['rate_soll'] for s in samples],       '-r',  linewidth=1, alpha=0.4, label='rate_soll (log)')
+    ax_rate.set_ylabel('Rate [K/min]')
+    ax_rate.legend(loc='upper left', fontsize='small')
+
+    # Power (both normalised to 0–1)
+    log_norm   = [s['power_set'] / max_power for s in samples]
+    replay_pwr = [r['power'] for r in replay]
+    ax_power.plot(t, log_norm,   '-b',        linewidth=1, label='power (log, norm.)')
+    ax_power.plot(t, replay_pwr, '-', color='C1', linewidth=1, label='power ({})'.format(params_label))
+    ax_power.set_ylabel('Power [0–1]')
+    ax_power.set_xlabel('t [s]')
+    ax_power.legend(loc='upper left', fontsize='small')
+
+    for ax in (ax_temp, ax_rate, ax_power):
+        _style_axis(ax)
+
+    ax_temp.label_outer()
+    ax_rate.label_outer()
+    fig.tight_layout()
+    return fig
+
+
+def main():
+    repo_root       = Path(__file__).parent.parent
+    default_config  = repo_root / 'config.json'
+    default_log_dir = repo_root / 'logs'
+
+    parser = argparse.ArgumentParser(
+        description=__doc__,
+        formatter_class=argparse.RawDescriptionHelpFormatter,
+    )
+    parser.add_argument('date_time', help='Timestamp part of log filename, e.g. 20260628T184903')
+    parser.add_argument('sud_name',  help='Sud name part of log filename, e.g. Sud-0010')
+    parser.add_argument('--log-dir', default=str(default_log_dir),
+                        help='Log directory (default: %(default)s)')
+    parser.add_argument('--config', default=str(default_config),
+                        help='Config file for base PID params (default: %(default)s)')
+    parser.add_argument('--rate-soll', type=float, default=None,
+                        help='Heat-rate setpoint [K/min] fed to the replay controller '
+                             '(default: inferred from log as max rate_soll while heating)')
+
+    for section, prefix in (('Hold', 'hold'), ('Heat', 'heat'), ('Cool', 'cool')):
+        for gain in ('kp', 'ki', 'kd', 'kt'):
+            parser.add_argument(
+                '--{}-{}'.format(prefix, gain),
+                type=float, default=None,
+                dest='{}_{}'.format(prefix, gain),
+                metavar='VAL',
+                help='Override TempCtrl.{}.{}'.format(section, gain),
+            )
+
+    args = parser.parse_args()
+
+    log_path = Path(args.log_dir) / 'log_{}_{}.json'.format(args.date_time, args.sud_name)
+    if not log_path.exists():
+        print('Error: log file not found: {}'.format(log_path), file=sys.stderr)
+        sys.exit(1)
+
+    config_path = Path(args.config)
+    if not config_path.exists():
+        print('Error: config file not found: {}'.format(config_path), file=sys.stderr)
+        sys.exit(1)
+
+    with open(log_path) as f:
+        log = json.load(f)
+    with open(config_path) as f:
+        config = json.load(f)
+
+    samples    = log['Samples']
+    name       = log.get('Name', '{}_{}'.format(args.date_time, args.sud_name))
+    pid_params = _apply_gain_overrides(config['TempCtrl'], args)
+    dt         = (samples[1]['t'] - samples[0]['t']) if len(samples) > 1 else 1.0
+    max_power  = _infer_max_power(samples)
+    rate_soll  = args.rate_soll if args.rate_soll is not None else _infer_heatrate_soll_set(samples)
+
+    any_override = any(
+        getattr(args, '{}_{}'.format(p, g)) is not None
+        for p in ('hold', 'heat', 'cool')
+        for g in ('kp', 'ki', 'kd', 'kt')
+    )
+    params_label = 'candidate' if any_override else 'config'
+
+    print('Log:    {} ({} samples, dt={:.1f} s)'.format(name, len(samples), dt))
+    print('Power:  max={:.0f} W'.format(max_power))
+    print('Rate:   heatrate_soll_set={:.2f} K/min'.format(rate_soll))
+    print('Params: {}'.format('overridden' if any_override else 'from config'))
+    print()
+    for section in ('Hold', 'Heat', 'Cool'):
+        p = pid_params[section]
+        print('  {}: kp={kp}  ki={ki}  kd={kd}  kt={kt}'.format(section, **p))
+
+    replay = run_replay(samples, pid_params, rate_soll, dt)
+    plot_replay(samples, replay, name, max_power, params_label)
+    plt.show()
+
+
+if __name__ == '__main__':
+    main()