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Rinnai Tankless Water Heater Will Not Flush

deblaw10 Member Posts: 5
I have owned a Rinnai Tankless water heater for over 10 years. I flush the unit quarterly with vinegar. I have tried to flush the unit several times over the past few weeks and have been unsuccessful. The pump is working, but the unit is acting like the valves are closed. I have never experienced this before.

I spoke with Rinnai and they told me my transfer pump was too powerful. The one I was using was transfering at 25 gallons per minute. I needed one that pumped 3-5 gallons per minute. I found a garden pump and tried, I was unsuccessful. Called Rinnai again and they said my pump isn't powerful enough. I asked for a recommendation for a pump, they do not give recommendations. I went back to my old pump and attempted to pump the unit again with Rinnai suggesting to turn my hot water on and shut the water off from the tank instead of the faucet. This will keep the valve open. This was also unsuccessful. Any ideas why the unit is not flushing?


  • nibs
    nibs Member Posts: 499
    No suggestions about why, but if you can get your hands on a 12V RV water pump (Around $40 new), try using it they put out about 3gpm at up to 40 psi. If you were near, I would give or lend a used one that you could use.
  • deblaw10
    deblaw10 Member Posts: 5

    Thank you! I did purchase a 3gpm pump, but I was told by Rinnai after I purchased that it needs 18foot head. Those are hard to come by this time of year. I don't understand why the pump type I have been using for the past 11 years suddenly doesn't work.
  • Leon82
    Leon82 Member Posts: 684
    Could the impeller be worn?
  • deblaw10
    deblaw10 Member Posts: 5

    I have tried 3 different pumps, all work fine until they are hooked up to the water heater. It's like the valve is closed, but it isn't.
  • Leon82
    Leon82 Member Posts: 684
    Inlet screen clogged?
  • deblaw10
    deblaw10 Member Posts: 5

    No, the inlet screen is clear of debris.
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,957
    Hello, A trick I've used to unclog plumbing is to reverse flow, repeatedly. That way if there is sediment plugging things up, it will eventually get worked loose. I don't know if you have a check valve in there someplace; if so this idea wouldn't work. Still it seems it couldn't hurt to try.

    Yours, Larry
  • deblaw10
    deblaw10 Member Posts: 5

    I have tried to reverse the flow, but no luck, however, I did not try several times. I have never had this problem before, like I said, I flush my unit quarterly due to having very hard water.
  • bobn4907
    bobn4907 Member Posts: 1
    so how did you resolve your issue; I believe I have the same problem
  • Dekito
    Dekito Member Posts: 1
    I have rhe same problem ,done many times now its not working 
    Le John
  • Intplm.
    Intplm. Member Posts: 1,442
    Has anyone called Rinnai customer tech support?
    What did they suggest doing?
  • f0x14
    f0x14 Member Posts: 1
    edited December 2022
    I had this problem twice, tried reversing the flow and that fried my control board. 

    What I got to work just now was with the unit on and having one washer hook up hose that’s about 3/8” in diameter inside as the feed and one thats about 1/2” inside diameter as the discharge. The pump is also a sump pump that pumps 1980 gph 1/4 horsepower so yes definitely overkill but the hoses setup the way I did helped resolve this issue apparently regardless. 
  • hot_rod
    hot_rod Member Posts: 19,261
    edited December 2022
    Does water flow through from your homes water system? Then a pump should flow through it as it has in the past

    its psi not gpm that you need, those sump pumps only pump about 15 psi, so you may need a bit more.

    If the heater operates with 20 psi of water pressure, you need a pump that supplies that.

    You are sure the hose isn’t clogged🥴
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • pedmec
    pedmec Member Posts: 718
    they sell kits for just this purpose. comes with pump and descaling solution

  • mahonri11
    mahonri11 Member Posts: 1
    edited February 7
    I was having this same issue and got it working by doing the following. 1)first reset the code to resume normal function. Press on/off button 5 times for my model. That turns the unit back on for 70 hrs and blinks the set temp and lc code. 2)unplug the water heater, shut off the gas valve, close inflow and outflow water valves. 3) connect hot/cold lines to the respective in/out purge ports and open both in/out purge valves. 4) connect cold input line to the sump pump, put it in the solution bucket 5) put your output line in the basket to complete the loop. 6)Plug in the water heater with the gas valve still off. This will prevent the heater from heating up and turning on completely but will allow the heater to open an internal valve (you should hear the system opening the internal valve) that will allow the solution to flow through the heat exchange. After hearing the system open the valve 7) Plug in the sump pump, solution will start flowing immediately through the system. Hope this works for others as it worked for me. Good luck!