Edit: According to Rinnai, the minimum flow rate is 0.26 GPM.

I just replaced an old water heater tank with a Rinnai RU199 tankless unit. A dedicated 1/2 return line was ran through the attic to the master shower; the RU199 specifies a "compatible" pump to have recirulation work, so I used a Grundfos GTK15 pump. This pump is meant to be used with 3/4 pipe, but the 3/4 lines are under a slab, and breaking concrete isn't doable, so 1/2 PEX was used.

The unit is throwing a code 63, indicating low flow on the recirculation line. The GTK15 has a head height of 17 feet, which I suspect isn't enough. I had planned on getting a "non-compatible" pump with a higher head height; the RU199 PCB has a header that powers the GTK15's pump, so I'm sure I could either splice a connector for that header, or run a relay, which should work just like the GTK15. The pump I'm looking at has a head height of 30 feet.

Below is a rough diagram of the current piping:

enter image description here

While this may work, I'm sure variables like friction loss factor into this - I'm just not sure which variables I should be looking at when it comes to sizing the pump correctly.

  • I know this is obvious, but have you tried asking Rinnai?
    – KMJ
    Commented May 16, 2023 at 23:47
  • @KMJ I actually haven't - at least about this particular issue. I'll try reaching out to them.
    – tripleblep
    Commented May 17, 2023 at 0:09
  • So you ran a (new?) dedicated line, but made it the wrong size? Or you had an existing dedicated line of the wrong size?
    – Ecnerwal
    Commented May 17, 2023 at 1:04
  • @Ecnerwal This was professionally installed: the plumbers dictated 1/2 line based on the length of the run. There was no existing dedicated line, so I had one added.
    – tripleblep
    Commented May 17, 2023 at 4:05

1 Answer 1


You need whatever the minimum flow required to avoid the low flow error is, which may or may not be in the manual, or Rinnai might tell you.

Since you provided no info on the supply lines length, I just ran 92 feet of pipe for the return through a head calculator and get 17 feet of head on 3/4" pipe at 7-8 GPM, depending on pipe material (thus, friction) which you don't specify. Perhaps 5.8 with an additional 80 feet of 3/4" as a guess.

The same flow rates in the same length of 1/2" pipe require 96-126 feet of head. 30 feet of head would only manage 3.3-4 gallons per minute. 17 feet of head yields 2.4-2.5 GPM for that pipe.

In reality, you have additional length of supply piping, but that should put the problem in perspective for you. Say you had 80 feet of 3/4 as I guessed above, and your 92 feet of 1/2" - for 5.8 GPM you'd need 76 feet of head, or 30 feet would do 3.14 GPM, or 17 feet would be doing 1.92 GPM.

Looks like buying 100 feet of 1" or perhaps 3/4" PEX to replace the undersized return line would be a smart move here rather than a new pump, to me.

  • Just a comment, not an answer, I personable have found that tank less WHs don't play will with re-circ systems. better to have have a tank type for re-circ. Commented May 17, 2023 at 3:35
  • I updated the original post to specify PEX for the return line; Rinnai stated that 0.26 GPM is the minimum required flow.
    – tripleblep
    Commented May 17, 2023 at 4:26
  • Then I'd have to suspect that your professional plumbers screwed something up, and should fix it.
    – Ecnerwal
    Commented May 17, 2023 at 11:12

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