Rinnai Tankless + Taco 008-ct = Pressure Drop & Hot/cold Mix

Discussion in 'Tankless Water Heater Forum' started by dobrocat, May 4, 2016.

  1. dobrocat

    dobrocat New Member

    Joined:
    Mar 27, 2016
    Occupation:
    plumber. musician. alchemist.
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    SF Bay Area
    Hello!

    I installed the Taco 008-CT-USK for a client in an effort to reduce water waste at the fixture furthest from his Rinnai tankless unit. This is the pump that is installed under the sink.

    He is currently experiencing hot water mix when the cold side experiences a pressure drop due to demand, such as watering his yard, or toilet tank refill after flushing, or using two faucets simultaneously in the house.

    Has anyone encountered this before? I reached out to Taco, and they said this was a fairly common yet difficult-to-mitigate problem. (I'm more or less ticked off at Taco for not including that in their lush four-color brochures, but that's a matter for another time.) Their support fella mentioned "maybe a check valve", but I can't visualize how this would help matters

    Any ideas?

    Cheers

    Geoff the Dobrocat
     
  2. Terry

    Terry Administrator Staff Member

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  4. dobrocat

    dobrocat New Member

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    plumber. musician. alchemist.
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    SF Bay Area
    Well, the pump is installed correctly and definitely NOT backwards.

    It works, it has greatly reduced the amount of waste clearing the hot line of cold water.

    Here is the text of the issue from my client

    That's where I'm stumped.
     
  5. Cool Blue Harley

    Cool Blue Harley One Dumb Plumber

    Joined:
    Apr 16, 2016
    Location:
    San Francisco, CA
    When that type of pump is installed in a standard trunk and branch system the cold line becomes heated and acts as a return to your water heater. When cold water is needed, the hot must be displaced first. The reasons I dislike this design are 1)the hot in the cold you are describing 2)hot water in cold piping which introduces bacteria 3)is the tankless heater designed to have this type of circulation? It my be cycling excessively leading to premature failure. Is it a hybrid tankless?
     
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  6. dobrocat

    dobrocat New Member

    Joined:
    Mar 27, 2016
    Occupation:
    plumber. musician. alchemist.
    Location:
    SF Bay Area
    Thanks for the replies, fellas!

    I fail to recall the model of the Rinnai, it seems it's a "standard" tankless. It ISN'T the one that has a pump and integrates a crossover valve.

    Interesting, I'd never considered the potential for bacterial contamination. Thanks for that one, Cool Blue Harley. Just when one thinks one has all the bases covered, suddenly bacteria haha. I'm definitely going to delve deeper here.

    It's not so much excessive cycling, but cycling due to a pressure drop created by sufficient cold demand, like from an outside hose bibb fully open, or a toilet flush.

    So, do you know of a way I can mitigate the pressure drop, or prevent a spike in cold usage from sucking hot through the pump and starting up the Rinnai?
     
    Last edited: May 5, 2016
  7. Harry Dotson

    Harry Dotson New Member

    Joined:
    May 28, 2016
    Location:
    California
    A large cold water demand causes a pressure drop on the cold water side and, if you are using any kind of crossover thermal valve for non-dedicated re-circulation, similar to the Grundfos valve that comes with the Rinnai RUR models, you will always experience hot water being drawn to the low pressure (cold water) side. Although this will trigger the tankless heater to come on, it will turn off when the crossover valve reaches a temperature of about 94 degrees F and closes. This is one draw backs of using crossover re-circulation system with a tankless. A worse case scenario is if the thermal valve fails after aging, it will stay in the open position and water will continue to be drawn to the cold water side and the heater will continue to run as long as there is enough cold water demand to cause a significant pressure drop.

    What is needed is a crossover valve that stays closed until the water gets cold on the hot side, as opposed to stays open until the water gets hot on the hot side and stays closed if it fails. That way, the valve would stay closed during a pressure drop. Does anyone know if this type of "active" crossover valve is available?

    I love my Rinnai RUR98i, but what I do not like, in addition to the problem described above, is how much noise it the Rinnai makes during re-circulation. Has anyone had that problem and found a solution? Could it be air in the internal loop or pump? If so, how would you purge the air?
     
    Last edited: May 28, 2016
  8. dobrocat

    dobrocat New Member

    Joined:
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    Occupation:
    plumber. musician. alchemist.
    Location:
    SF Bay Area
    Taco recommended I add one of their mixing valves to the "mix", installed so it closes the loop at around 85 degrees. Unfortunately the sinks aren't at the end of the line, indeed there are two toilets, a shower, and the dreaded garden hose. Now, the drop due to toilet flushing is minimal and livable. The garden hose though, as he waters the garden the hot water is still a problem. I will be bypassing the hose bibb and moving its source to before the Taco and bathroom sinks...that should end this nightmare.

    The irony is, that Taco pump is supposed to eliminate the need to run additional footage of pipe, and now I'll be running about ten feet or so in the crawlspace.
     
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