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Two Rinnai tankless water heaters connected in parallel. Only one ever seems to run....

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Trango
Trango Member Posts: 5
in Domestic Hot Water
How can I force the other unit to run as well? I have tried running ALL the hot faucets in the house, still no luck. There’s a communication cable between the two. I also tried flushing the unit, but No luck. Looks like the water flow servo valve still closed. Thanks in advance.

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  • ratio
    ratio Member Posts: 3,626
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    Are they configured for parallel operation? Do you have sufficient flow to trip them both?
  • the_donut
    the_donut Member Posts: 374
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    Could be a balance issue. Run water. Throttle the one that runs all the time until both units get called at nearly same time.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,524
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    I agree try what @the_donut said
  • Jack
    Jack Member Posts: 1,047
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    Model numbers always help on tech questions. There is a cable (E-Z Connect) that should be installed to operate the two units together. It will regulate the operation of both as to firing rates and rotate the operation so each unit will be primary. If that cable is installed check to see if the connections are secure. Also get into your owners manual and learn how to read flow rates on the controller. There is a ton of information you can get on that control.
    rick in Alaska
  • Trango
    Trango Member Posts: 5
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    I have turned on all faucets in the house plus ran my washer and dishwasher, but the 2nd unit still did not come on. Regardless of flow rates, shouldn’t there be a way to easily switch between the 2 water heaters??

    The Rinnais are model 2520s.
  • ratio
    ratio Member Posts: 3,626
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    I couldn't find programming instructions for that unit, so I have no idea what's involved in programming the units for twinned operation, but it looks like the units are good for over 5 gallon/minute apiece. If they are set up right and you aren't drawing over about 5.3 gal/min, it may be working correctly.

    Is there a reason why you think the second unit should run simultaneously with the first unit? Does the system fail to keep up?

  • the_donut
    the_donut Member Posts: 374
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    I think he is concerned about equal loading of equipment. Unit 2 doesn’t run. Either it’s a control issue or a balance issue. We had an issue with a couple of a.o. Smith masterfit water heaters. Units plumbed in parallel but only unit one would fire up on use. 50 unit building, observed no firing from unit 2 except via heat loss. Pinched down cold water supply valve an eighth of a turn and unit 2 fires now. Basically unit 1 was preferential flow and very little water was fed from 2.
  • ratio
    ratio Member Posts: 3,626
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    One would imagine that, mfgr's instructions aside, paralleled units would be piped reverse return as a matter of course. Then again...

    Perhaps a picture of how the two units are piped together would shed some light on the problem. A real model number, from the nameplate, would help as well.

  • bob eck
    bob eck Member Posts: 930
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    Plus photos
  • Jack
    Jack Member Posts: 1,047
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    DO YOU HAVE A CONTROL WIRING THE TWO UNITS TOGETHER? What are the flow rates thru the unit? The 2520 at a 70* temp rise will deliver 4.3 gpm.
  • Trango
    Trango Member Posts: 5
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  • Trango
    Trango Member Posts: 5
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    Update

    The dang thing fired up last night! I had tried to replace the water flow servo on the right heater. Made a mess of it stripping one of four screws out, and had to put everything back with replacing the servo.

    When I out power back on the unit, I hear a whirring sound which I haven’t heard before. Like an old door opening.

    Then a shut the hot water feed of the left unit off and turned on the taps, and voila, the right unit huffed and puffed and finally came to life!

    The flame was a little yellow at first, then slowly burned blue but seems to cut in and out with water flow cutting in/out.

    I shut the unit off to flush the exchanger with vinegar for an hour, turned it back on again. Water runs consistently now and flame is burning solid blue as well.

    I think the problem may be due to stuck servo fins. Somehow, my messing about with it may have agitated or kickstarted it. That allows cold water to flow and the unit to finally wake up.

    Thank you for your responses! Have a great week guys!
  • Jack
    Jack Member Posts: 1,047
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    I forgot that the 2520 required the MSA control to twin units. Your units must be '05 or earlier. Are they running together as you wish? With the MSA or the current EZ-Connect they balance the flow themselves so the factory says no reverse return necessary, but to me, basics are basics and RT makes sense. I would call Rinnai. The number is on the side of the unit and request operating instructions for the 2520's and the MSA controls.
    Trango
  • Trango
    Trango Member Posts: 5
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    As soon as I have both units re-activated, the original working unit becomes the master again. I wonder if the default is always that unit, only activating the 2nd one only when more hot water is called. Problem is we are just a family of 2 with 4.5 baths, will never get to use that much hot water. At least I know how to easily switch between the two now, and will do that periodically to make sure both are working. Thank you again for your responses!
  • ratio
    ratio Member Posts: 3,626
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    Check the manual of the programmable controller. Rotating the lead unit is something that controls have been doing for decades, I have a hard time imagining that option isn't present.
  • Jack
    Jack Member Posts: 1,047
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    The MSA control will rotate the firing order. You should not have to change the system over if the MSA is installed correctly and functioning.

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