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Looking at the Navien NPE-240A2 Tankless Recirculating Water Heater Installation Manual there are several parameters of interest to me:

  • Recirc Interval Time: Set the recirculation duration. (Default 20 min)
  • Recirc Sample Time: Set the recirculation interval. (Default: 30 Min)
  • Recirc Off Diff. Temp: Set the Recirculation Off offset temperature. (Default: 9 °F)

The unit is setup for DHW to use the internal recirc pump and an external recirculation loop. Assume the unit is set to “Weekly”, and the current time is within the hours specified (e.g 6am to 10pm). The water temperature is set to 120 °F. The above listed parameters are set to their default values.

My understanding is that the temperature at the hot water return inlet is sampled every 30 mins. If the temperature reads more than 9°F cooler than the desired water temperature setting, the boiler fires and the recirc pump turns on.

Question: Does the recirculation pump run for the full 20 minutes set by the “Recirc Interval Time”? Or does it simply run until the hot water return temperature at the pump reads within the differential (9°F)? Or does it possibly try to reach the actual target temp (120°F)?

If it runs for the full recirculation duration (20 mins), that seems potentially wasteful. OTOH, if it only runs until the temperature at the return inlet reads hotter… what then is the purpose of the “Recirc Duration” setting?

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  • Only makes sense to me if the recirc runs just long enough to get the loop to temp. Should be possible to determine this by observation. Commented Jan 25 at 2:36

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