Installing 2 Tankless - Did my plumber do it right?

Discussion in 'Tankless Water Heater Forum' started by bbddpp, Sep 5, 2014.

  1. bbddpp

    bbddpp New Member

    Sep 5, 2014
    I'm building a fairly large home with a finished basement where my utilities will be. I purchased 2 Rinaii RU98i units with the intent to run them in tandem. My builder informed me that these would need to be located on an external wall, and as close to the master bathroom as possible, which put them in a finished area in the basement (not the unfinished utility room). My thought was running two units in tandem would afford me a higher GPM here in the northeast.

    Reluctantly, I agreed, however today I arrived to find that instead, he decided to install one unit and some lines at that location, and the other unit across the basement, clear on the other side of the house. So, it looks like he basically split the plumbing in half. One unit for some lines, one for others.

    There really isn't a great scenario I can think of to do this other than cost. Linking together got me more GPM to the bathroom shower or tub once I got it flowing. Now I'll only realize one Rinaii's worth depending on where I am in the house. I don't even see a plus to having one isolated form the other at all.

    Other than a shorter travel time for the hot water to some of the areas on the other side of the house where the 2nd unit was installed, was there a reason for this? Should I be pleased or did my plumber instead take a money saving route?
  2. Leon82

    Leon82 Member

    Mar 13, 2014

    It may be 6 of one half a dozen of another. you can run a fixture on each side of the house without worrying about flow reduction. however if 2 fixtures run on the same heater you may experience a flow rate drop and cold water sandwiches. If your contract states tandem you should be able to get him to redo it. If I had 2 I would do a tandem setup. He may have decided it was less work to do it that way.
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  4. Stuff

    Stuff Well-Known Member

    Mar 7, 2013
    If you have a tandem setup would that not double the minimum activation rate as well? If you then had a low flow faucet you would not be able to get hot water. So then a recirculation system is needed. Maybe that is what your plumber wanted to avoid.
  5. solalo

    solalo New Member

    Aug 14, 2014
    We need to know how many showerheads and faucets you are planning to use and how close the water heaters are going to be to those outlets to tell for sure if it is better or worse but even without that information, I suggest you consider going along with the way he installed them because I am not a big fan of tankless water heaters installed in tandem. My reasons are as follows.
    1-) It is best to put the tankless water heaters as close to the location of hot water use as possible. These things can take a few seconds to fire and then a few seconds more to heat the water in the heat exchanger. All that added to the time for the hot water to run through the pipes and get to your faucet can be a long wait and gallons of water wasted. So, instead of concentrating them in one part, you might want to dedicate then to different parts of a large house so each part of the house gets hot water quickly.
    2-)"Stuff" is right pointing out that tandem installation will double the minimum activation rate. If the flow rate falls below a threshold both units will stop and the water will run cold. Rinnai has the lowest minimum flow but there is still no reason to double it. If you are planning to connect two tankless water heaters to each other, it is best to do it in series.
    3-)Getting more gpm waterflow by tandem installation will not necessarily give you what you expect. Rinaii RU98i has 10 gpm and you will probably not need all that at any time and if you are actually able to use all that water at a time, you might find out that it goes lukewarm instead of hot. A tankless water heater's ability to supply hot water is limited more by its gas input (BTU) than by waterflow. In other words, there is no need to pay that much attention to getting more gpm.
  6. lifespeed

    lifespeed Member

    Apr 27, 2007
    I think the only good advice you have received so far was from the builder: you're in the cold pacific northeast with water temperatures near freezing so install parallel tankless from Rinnai for a fast bathtub fill in the coldest of weather.

    From there it goes downhill with the plumber not following work instructions (was he given any by the builder?) and some of the questionable comments on this forum.

    I assume you want to maintain a high hot water flow even in winter. To do this, hot water heaters should be installed in parallel (who posted "in series?!!"), linked by what Rinnai calls "EZ connect" (see p. 4 of the design manual) which is a control link that operates the water heaters seamlessly in parallel. I am pretty confident a proper parallel installation has a mechanism for ensuring that only enough flow to trigger 1 tankless water heater is required. These water heaters can control flow through their heat exchangers, so the "slave" heater would shut down until triggered by demand from the master.

    You will need about 400K BTU of gas, which requires large piping and gas meter, although a new house might have a 2psi gas system which is more easily distributed through smaller pipes. You should have service valves installed on the heat exchanger water pipes and threaded unions in all pipes to allow removal for service. I would recommend at least 1" copper cold and hot trunk lines (larger if you're building a mansion), reducing only for the tees for various rooms. But the piping is probably already done . . .

    Your concern about hot water latency is best addressed with a recirculating pump system with an Aquastat, timer and pushbutton activation. I use and like the Redytemp. For your application you would want the larger pump.

    Lastly, hard water deposits build up in all water heaters, but it is a bigger issue inside a small tankless heat exchanger. If you have more than 5 grains of hardness in your water like most of the USA the lowest-maintenance thing to do is install a salt-based water softener. If you don't want a softener you can run hard water through a tankless, but you will then need to make use of those service ports your plumber installed (?) to flush it with vinegar, reliably and annually, or it will be ruined over time. Some people have ruined their heater through lack of maintenance.

    Your tankless installation will make the difference between having endless large volumes of hot water with no waiting, or a limited volume of hot water in the winter, with a pipe flush delay for hot water. IMHO, what you described is not a correct installation. Good luck.
  7. lifespeed

    lifespeed Member

    Apr 27, 2007
    And here is a good article on how a proper dual installation works. Yes, the minimum flow rate remains that of one heater.
  8. bbddpp

    bbddpp New Member

    Sep 5, 2014
    Not sure how I missed so many earlier replies but I want to first say thank you so much to all who replied and took the time to take interest in my situation.

    Time has passed since the first post and I had learned a lot so hopefully we did this right. We ended up with 2 Rinaiis connected to each other in parallel on one side of the home, below the kitchen. We then have a cold line running across the house to the other side of the basement to a very small electric water heater tank. At the time, we have not installed a recirc pump to save on costs. Theory is that the smaller tank will be used to run water in the master bathroom above, until the Rinaii's water has reached. There may be a slight sandwich until we get temps adjusted JUST right but the idea is that the Rinaii water will mix with the stuff from the small tank as it arrives in the bathroom.

    Hopefully best of both worlds, instant hot water both in kitchen area and master bath with little cold sandwiching and low power usage.

    I'll post some photos perhaps if you might be kind enough to look and are interested, I would love to hear comments on if this was done properly.
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