I haven’t fully tested this but I think this should work for you.
You can create a helper using the “Utility Meter” integration. It creates a helper, and you can set it to:
Name: Liters Since Reset (or Gals Since Reset)
Input Sensor: Volume Today
Meter reset cycle: No cycle
Meter reset offset: 0
Net consumption: false
Delta values: false
Periodically resetting: true
Sensor always available: whatever you want (true or false)
Then when you want to reset the helper count, use an automation with an action of:
Add action → Helpers → Utility Meter → Reset
And then choose entity: sensor.liters_since_reset
You can name the automation “Reset Filter Count” and manually run the action.
You can also add an automation “Replace Filter Notification” that triggers on a certain number of Liters (or Gals) accumulated on sensor.liters_since_reset, and set an action to send you a notification to “Replace the Filter”. And then manually reset filter count when you replaced the filter.
Or, you can combine both: set a trigger based on the number of liters (or gallons) you want. When it triggers, it will can do action1 and reset the value to 0 and then do action2 and send you a notification. This assumes you can replace the filter right away.
Edit: Just occurred to me that you may not be able to use an exact value as the increments to liters_since_reset is not guaranteed to be increments of 1, it may jump over your selected value without triggering. You will likely have to use an Above value selection - which may cause multiple notifications. You’ll have to play with this a bit so you don’t create a nuisance set of notifications.
