diff --git a/plugins/host_agent/sensors.py b/plugins/host_agent/sensors.py
index 4b3297b..031f176 100644
--- a/plugins/host_agent/sensors.py
+++ b/plugins/host_agent/sensors.py
@@ -40,6 +40,12 @@ _REFERENCES = (re.compile(r"^Package id \d+$", re.I), re.compile(r"^(Tctl|Tdie)$
 
 # {host name: {sensor label: why it isn't a thermometer}}, from the last refresh.
 _unwired: dict[str, dict[str, str]] = {}
+# {resource: {hour: average}} over the last WINDOW, and the hour the next
+# refresh reads raw samples from. Reading the whole week took over three
+# minutes on the live instance, so it is read once at startup, and each later
+# refresh reads only the hours since the last one.
+_hours: dict[str, dict[datetime, float]] = {}
+_read_from: datetime | None = None
 
 
 def unwired(host_name: str) -> dict[str, str]:
@@ -109,6 +115,21 @@ def judge_hosts(rows) -> dict[str, dict[str, str]]:
     return out
 
 
+def merge_hours(hours: dict[str, dict[datetime, float]], rows, since: datetime) -> None:
+    """Fold (resource, hour, average) rows into `hours`, a newer read of an
+    hour replacing an older one (the hour that was still filling last time),
+    and drop the hours older than `since`."""
+    for resource, bucket, value in rows:
+        if value is not None:
+            hours.setdefault(resource, {})[bucket] = float(value)
+    for resource in list(hours):
+        kept = {h: v for h, v in hours[resource].items() if h >= since}
+        if kept:
+            hours[resource] = kept
+        else:
+            del hours[resource]
+
+
 async def refresh(session, now: datetime) -> dict[str, dict[str, str]]:
     """Re-judge every host's sensors from the last WINDOW of hourly averages,
     and keep the verdicts for ingest and the pages.
@@ -118,8 +139,12 @@ async def refresh(session, now: datetime) -> dict[str, dict[str, str]]:
     averaged from the raw samples here. Retention deletes the raw rows it
     rolls up, so the two never cover the same hour. The sensors are named
     from plugin_metric_latest, so each series is read by its index rather
-    than by a pattern over every host_agent row."""
-    global _unwired
+    than by a pattern over every host_agent row.
+
+    The first refresh reads the whole WINDOW; later ones read raw samples
+    from the start of the hour the last one ended in, so that hour is
+    re-read whole."""
+    global _unwired, _read_from
     from sqlalchemy import func, select
     from steward.models.metrics import PluginMetric as M
     from steward.models.metrics import PluginMetricHourly as H
@@ -130,15 +155,19 @@ async def refresh(session, now: datetime) -> dict[str, dict[str, str]]:
     if not names:
         _unwired = {}
         return _unwired
+    read_from = since if _read_from is None or _read_from < since else _read_from
     hour = func.date_trunc("hour", M.recorded_at)
-    raw = (await session.execute(
+    rows = list((await session.execute(
         select(M.resource_name, hour, func.avg(M.value))
         .where(M.source_module == "host_agent", M.metric_name == "temp_c",
-               M.resource_name.in_(names), M.recorded_at >= since)
-        .group_by(M.resource_name, hour))).all()
-    rolled = (await session.execute(
-        select(H.resource_name, H.bucket, H.value_avg)
-        .where(H.source_module == "host_agent", H.metric_name == "temp_c",
-               H.resource_name.in_(names), H.bucket >= since))).all()
-    _unwired = judge_hosts([*rolled, *raw])
+               M.resource_name.in_(names), M.recorded_at >= read_from)
+        .group_by(M.resource_name, hour))).all())
+    if _read_from is None:
+        rows += (await session.execute(
+            select(H.resource_name, H.bucket, H.value_avg)
+            .where(H.source_module == "host_agent", H.metric_name == "temp_c",
+                   H.resource_name.in_(names), H.bucket >= since))).all()
+    merge_hours(_hours, rows, since)
+    _read_from = now.replace(minute=0, second=0, microsecond=0)
+    _unwired = judge_hosts((r, h, v) for r, pts in _hours.items() for h, v in pts.items())
     return _unwired
diff --git a/tests/integration/test_host_metrics.py b/tests/integration/test_host_metrics.py
index fa29f1f..bf1330e 100644
--- a/tests/integration/test_host_metrics.py
+++ b/tests/integration/test_host_metrics.py
@@ -208,6 +208,16 @@ def test_sensors_are_judged_from_raw_hours_and_rollups_together(app):
                                              metric_name="temp_c", value=f(99), recorded_at=now))
             return await sensors.refresh(s, now)
 
+    sensors._hours.clear()
+    sensors._read_from = None
     got = asyncio.run(_go())
     assert list(got[host]) == ["SYSTIN"]
     assert got[host]["SYSTIN"].startswith("falls as the CPU warms (r -1.00 over 100 h")
+
+    async def _again():                        # the next hour reads only the hour it ended in
+        async with app.db_sessionmaker() as s:
+            return await sensors.refresh(s, now + timedelta(hours=1))
+
+    assert sensors._read_from == now.replace(minute=0)
+    again = asyncio.run(_again())
+    assert again[host]["SYSTIN"].startswith("falls as the CPU warms (r -1.00 over 100 h")
diff --git a/tests/plugins/host_agent/test_sensors.py b/tests/plugins/host_agent/test_sensors.py
index bdb6d1c..b918383 100644
--- a/tests/plugins/host_agent/test_sensors.py
+++ b/tests/plugins/host_agent/test_sensors.py
@@ -72,3 +72,13 @@ def test_a_judged_sensor_is_left_out_of_the_host_temperature_and_listed_with_why
     real, unwired = split_temps("util", {"temp3": 62.0, "Package id 0": 55.0})
     assert list(real) == ["Package id 0"]
     assert unwired == {"temp3": (62.0, "falls as the CPU warms (r -0.81 over 168 h)")}
+
+
+def test_a_later_read_replaces_the_hour_that_was_filling_and_old_hours_drop():
+    hours = {"h:temp:a": {T0: 50.0, T0 + timedelta(hours=1): 51.0}}
+    s.merge_hours(hours, [("h:temp:a", T0 + timedelta(hours=1), 55.0),     # the hour, read whole
+                          ("h:temp:a", T0 + timedelta(hours=2), 56.0),
+                          ("h:temp:b", T0 + timedelta(hours=2), None)], since=T0 + timedelta(hours=1))
+    assert hours == {"h:temp:a": {T0 + timedelta(hours=1): 55.0, T0 + timedelta(hours=2): 56.0}}
+    s.merge_hours(hours, [], since=T0 + timedelta(hours=9))                # a sensor gone quiet
+    assert hours == {}