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Thread: Tankless the only option? - MultiHead Shower

  1. #1

    Question Tankless the only option? - MultiHead Shower

    Hi,
    I'm about to install 2 Rinnai R94LSi tankless water heaters (with a re-circulating system) to handle a new bathroom remodel I'm working on.
    The new shower will have 4 Kohler water tiles (2.5GPM), a hand shower (2.5GPM) and an overhead shower (4GPM). The heaters will be used for the rest of house as well (1.5 other bathrooms, std stuff, 1 sink in kitchen, 1 washer, 1 dishwasher). The new shower will use the Kohler 3/4" Thermastatic valves (though considering the DTV system as well).

    This forum has me paranoid that I"m making a huge mistake, though I don't know of any tank option to get the water flow to support such a shower. My current water heater (10yrs old) supplies enough water for my wife and I with our pre-remodel shower. It takes 2min for water to get from turn on in our kitchen currently, hence the want for the recirc system. We're having the units installed by a Rinnai authorized installer to prevent installation hassles.
    I should note that we're in Texas. So things don't get overly cold.

    Are there tank systems that support such a setup?
    Spending all this money and ending up with a cold or fluctuating shower temp would drive me nuts.

  2. #2
    Master Plumber master plumber mark's Avatar
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    Talking try this new hybrid...

    this came out and it seems to be pretty sweet...

    i would like to try one out to see how well it works


    I know it has to be cheaper than two Rhanni units...

    at about 2300...





    .

    http://www.eternalwaterheater.com/


  3. #3

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    One of these units (GU32) can do 6GPM at 75F. Measuring my water this morning, it was around 60F. To get to 120F, (60F rise), it could 7GPM. So I'm still likley going to have to look at 2 units.

    Has anyone here had experience or problems with a recirculating system on a tankless?

    The eternal site says they don't have issues with reciruculating systems.

    I'm considering a DTV setup or a Kohler Thermostatic valve. A lot of the tankless problems seem to be centered around thermostatic valve issues. I'm struggling to understand how the hot water from a tankless is any different from the hot water form tank system when it gets mixed...

    Thanks for the help.

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    Illinois Licensed Plumber SewerRatz's Avatar
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    Last continued education class I was in I heard the instructor talking about a new standard in pressure and temperature balancing valves. It uses a wax that reacts to temperature and pressure fluctuations and it reacts instantly. Unlike the old bi-metal temperature balancing, and the old slug type pressure balancing valves. I was wondering if these new valves are the ones they are having issues with and if anyone here has come across this new style valve.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The Grohe valve I have uses that technology.

    With a tankless system, to ensure they get a temperature rise, they use a restrictor. This may or may not slow your flow until you reach the limit. The better ones also modulate the burner to accomodate both volume and temperature variations. If you exceed the available heat requirements, the output will both hit a wall with the pressure and the temperature will drop. Many valves have some troubles with both of those asymettrically changing.

    Each brand has different requirements for what is necessary (or even recommended) for recirculation with a tankless system. If you don't go with a demand system that you turn on only before you want hot water, and need it instantly all the time, then you need something to buffer some hot water so the tankless systems aren't running constantly, so that adds to the cost with a tank and the extra plumbing.

    Not sure what I'd do (other than I probably wouldn't have one in the first place!).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Geologist sjsmithjr's Avatar
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    I may be looking at this wrong, but if I understand your criteria correctly, you may want to consider keeping the hot water system to the master shower independent from the rest of the house. The rest of the house sounds pretty straight forward and you could benefit from a simple, inexpensive, easy to maintain and service, straight forward solution to that part of the house.

    That would free you and your plumbing contractor up to focus on how to provide a steady supply of hot water at a constant temperature to the equivalent of 4 simultaneous showers for the anticipated time of use.
    -Sam Smith
    Licensed Professional Geologist - AL, TN, KY

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    DIY Senior Member chris8796's Avatar
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    Quote Originally Posted by Nicholas_ii View Post
    I'm struggling to understand how the hot water from a tankless is any different from the hot water form tank system when it gets mixed...

    Thanks for the help.
    The difference is the water pressure of the hot and cold side. In a tank system, the hot and cold have the same pressure, since a tank causes very little resistance flow. In a tankless system, the heater must maximize surface contact area for maximium heat exchange, which cause alot of friction and loss of dynamic pressure. This has the hot water at the shower having less dynamic pressure than the cold side. If you understand how a "pressure-balanced" faucet works, you'll understand the problem.

    Buried back on page 38 of the RS94 manual you'll see the gpm vs pressure loss chart. You'll see once you get past the low flow rates (single shower), pressure loss becomes a real issue.

  8. #8

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    Would you expect that I would still have hot water pressure issues with 2 9.4GPM units? Practically, I don't think I would ever use more than 10, maybe 12.5.

    There's no data for how 2 units in tandem work together.

    It's a also a bit unclear how the recirculating system will play into affect here.

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    DIY Senior Member chris8796's Avatar
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    I'm not sure what Rinnai recommends here. If they are plumbed in series the problem would be twice as bad. If the are plumbed in parallel the pressure problem would be half, but your minimum flowrate would be double. The other option would be to raise the temperature so you would use less gpm, but that would create a scald hazard.

  10. #10
    Geologist sjsmithjr's Avatar
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    They would be plumbed in parallel to meet the demand and set up for staged firing to keep the minimum flow rate the same. I have no idea how this would work in practice with the variables at hand.

    Also, at a 60F rise wouldn't two R94s max out at around 12 GPM at which point the output temp would start to drop?
    Last edited by sjsmithjr; 02-23-2009 at 07:24 AM.
    -Sam Smith
    Licensed Professional Geologist - AL, TN, KY

  11. #11
    Retired Defense Industry Engineer jadnashua's Avatar
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    I'm sure the manufacturer must have considered this situation, but balancing multiple units in parallel would be difficult.

    My guess is that they run them in series and use a restrictor sized for the combined output capability...using the second unit (closest to the destination) with a flow sensor, and only triggering the first one when the second one's output cannot keep up with the demand.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Geologist sjsmithjr's Avatar
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    Default You'd think that.

    I should have been more explicit on at least two counts. Two R94LSi's plumbed in series will not work properly; they should be plumbed in parallel and linked using the "EZ Connect" feature. At least that's what Rinnai will tell you.

    Tankless is not the only option in a heavy residential installation. I would think a commercial tank model or two could also meet his demands. Perhaps the OP could pose this question to over in the tank forum.
    Last edited by sjsmithjr; 02-23-2009 at 12:54 PM. Reason: Added tank info
    -Sam Smith
    Licensed Professional Geologist - AL, TN, KY

  13. #13

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    I'm now seriously looking at the heaters from
    http://www.eternalwaterheater.com/ Thanks to the suggestion from this forum.

    I'm having a plumber come in to give a quote tomorrow. Unfortunately, it still looks like I might have to have 2 systems to handle a 60F rise for 10-12GPM.
    From what I can tell, it solves the solves the cold sandwhich and startup temp problems and prioritizes flow over temp, which I can live with more than having my thermostatic valve freeze me out.

    Unfortunately, there aren't many reviews out there as far as I can tell. That means I'm eating the manufacture's cake and letting him tell me how good it is.

    Anyone have experience with this kind of Hybrid?
    Is Eternal the only hybrid like this? I saw a blurb about an AO Smith hybrid, but it looks completely different.

  14. #14
    Geologist sjsmithjr's Avatar
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    Can you tell the forum more about your balancing criteria for system selection? Are tank type systems out of the question, for whatever reason? Your hot water demand can be met by any number of means; it would help to know a little bit more what's important to you.
    -Sam Smith
    Licensed Professional Geologist - AL, TN, KY

  15. #15
    DIY Member dimprov's Avatar
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    One thing different about the Eternal is that it's minimum BTU consumption is 47,500. That's more than 2x a Rinnai or Bosch.

    I happen to have the actual BTU figures documented in another thread:
    http://www.terrylove.com/forums/showthread.php?t=26964

    David

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