Budget around 80 minutes. Example entities: sensor.tank_level_pct, binary_sensor.tank_empty, binary_sensor.tank_overflow, switch.rainwater_pump.

A Problem from Real Life: A Pump Trying to Pump Air

The irrigation script from the previous lesson fires up the rainwater pump. The tank has been empty for a week of drought: the pump runs dry, heats up, and after a few cycles like that, it burns out. A different scenario: heavy rain overfills the tank past its edge, water floods the building's foundation because nobody was watching the overflow. Both cases call for hard, independent safeguards, not just a chart on a dashboard.

Measuring Level: Three Approaches

A sensible combination: an ultrasonic or pressure sensor for a percentage readout on the dashboard, plus a mechanical float switch in the pump's own circuit as the last line of defense, independent of the network and of HA.

A Software Lockout for the Pump at Low Level

automation:
  - alias: "Rainwater: lock the pump at low level"
    id: rainwater_lock_pump_low_level
    mode: single
    triggers:
      - trigger: state
        entity_id: binary_sensor.tank_empty
        to: "on"
    actions:
      - action: switch.turn_off
        target:
          entity_id: switch.rainwater_pump
      - action: input_boolean.turn_on
        target:
          entity_id: input_boolean.pump_locked_low_level
      - action: notify.mobile_app_your_phone
        data:
          title: "Rainwater: tank empty"
          message: "Pump locked out by software. Rainwater irrigation paused."

  - alias: "Rainwater: unlock the pump once refilled"
    id: rainwater_unlock_pump_refilled
    mode: single
    triggers:
      - trigger: state
        entity_id: binary_sensor.tank_empty
        to: "off"
    actions:
      - action: input_boolean.turn_off
        target:
          entity_id: input_boolean.pump_locked_low_level

The irrigation scripts from Lesson 8, if they draw water from the tank, should check input_boolean.pump_locked_low_level as an extra condition before starting the pump: add that condition to the script for whichever zone runs off rainwater.

A Tank Overflow Alert

automation:
  - alias: "Rainwater: tank overflow alert"
    id: rainwater_tank_overflow_alert
    mode: single
    triggers:
      - trigger: state
        entity_id: binary_sensor.tank_overflow
        to: "on"
    conditions:
      - condition: state
        entity_id: input_boolean.outdoor_alerts_enabled
        state: "on"
    actions:
      - action: notify.mobile_app_your_phone
        data:
          title: "Tank: overflow"
          message: "Water level has passed the safe edge. Check the overflow drain."
          data:
            importance: high
            tag: "tank-overflow"

This alert doesn't control anything: the physical overflow (a pipe carrying excess water somewhere safe) has to exist independently of any automation as the base-level installation safeguard. HA only tells you something needs checking, like whether the overflow drain is clogged with leaves.

A Water Dashboard: A Card with History and Thresholds

type: vertical-stack
cards:
  - type: gauge
    entity: sensor.tank_level_pct
    name: "Rainwater level"
    min: 0
    max: 100
    severity:
      green: 40
      yellow: 15
      red: 0
  - type: history-graph
    hours_to_show: 168
    entities:
      - entity: sensor.tank_level_pct
  - type: entities
    title: "Pump status"
    entities:
      - entity: switch.rainwater_pump
        name: "Rainwater pump"
      - entity: input_boolean.pump_locked_low_level
        name: "Low-level lockout"
      - entity: binary_sensor.tank_overflow
        name: "Overflow"

The 168-hour (one week) graph shows the trend of use and refill: a fast drop with no rain means heavy irrigation use or a leak in the system, worth checking before the tank empties out mid-season.

Ultrasonic vs. Pressure Level Sensors: Which to Choose

If you go with an ultrasonic sensor despite its limitations, it's worth fitting the tank with a small vent (a mesh-covered opening) that reduces water vapor condensation inside, which is the most common cause of bad readings from this type of sensor.

Filtration and Rainwater Quality for Different Uses

Rainwater collected off a roof carries pollen, leaves, bird droppings, and sediment, so before using it for irrigation, it's worth adding at least a basic pre-filter (a mesh screen at the tank inlet) to keep pumps and solenoid valves from Lesson 8 from clogging. For more demanding uses, like watering edible vegetables, it's worth considering an additional filter at the pump outlet, protecting plants from contaminants that build up in the tank over time.

Submersible vs. Self-Priming Pumps: Matching the Pump to the Tank

The dry-run protection described in this lesson's main content matters especially for self-priming pumps: running dry destroys the seals much faster than with submersible pumps, which stay partially cooled by the surrounding water even at low levels.

Cleaning and Maintaining a Rainwater Tank

A rainwater tank needs periodic cleaning of the sediment that builds up on the bottom, usually once every one to two years, depending on how much debris comes off the roof. It's worth planning this as part of the quarterly review from the maintenance module, checking the condition of the level sensor (ultrasonic or pressure, per the comparison in this lesson) at the same time, since it may need cleaning of sediment affecting reading accuracy after a while.

Combining Several Tanks into One Coherent System

On a larger property with several rainwater collection points (different roof sections, different outbuildings), it's worth considering a system of several connected tanks instead of one large one, which gives placement flexibility and easier future expansion. Home Assistant can monitor each tank's level as a separate entity, with one overarching automation deciding which tank to draw from at any given moment, based on comparing current levels.

Automatic Top-Off from the Municipal Supply During a Shortage

For irrigation systems fully dependent on rainwater, a long drought means the risk of an empty tank and no way to water at the most critical moment. A solenoid valve connected to the municipal water line, controlled by an automation that only fires when the tank hits a critically low level, can serve as an emergency top-off, keeping rainwater as the primary source while still not leaving the garden dry through a prolonged drought.

Winter Freezing: Why Draining the System Is Essential

How to Test This Lesson

Common Mistakes

Practical Task

Key Takeaways

What's Next

Next lesson: Specialty Outdoor Sensors: Mailbox, Packages, Bins, the Drainage Pit, Frost, and Technical Alerts. Less obvious sensors that genuinely make everyday life easier.