Considering two dual-tankless options - feedback requested

Discussion in 'Tankless Water Heater Forum' started by mcdavis, Dec 31, 2016.

  1. mcdavis

    mcdavis New Member

    Joined:
    Dec 31, 2016
    Location:
    Oregon
    I'm in the market for a new hot water system for a large home (5000 sq ft).
    - 6 bathrooms, 4 full
    - spa jacuzzi
    - 2 dishwashers
    - 1 shower with two lines, 3 heads + 2 body wash
    - dual laundry rooms
    - total of 13 sinks in the house

    Water heaters are quite close to the gas source and exterior wall (4-6' vents). Thinking we will upgrade to 2lb gas. Considered running a dedicated line to the tankless, but there are a lot of BTUs in this house - close to 1,000,000 if everything is going at the same time...

    I'm going to be here a while and am looking for the best possible design.

    I looked at stainless tank vs. tankless and decided on tankless. I need ~150gal. Current have 1x75gal and 1x50gal. Both power vented. Pretty small space - maybe 5x5.

    I'd like one control system for both tankless heaters, so I want them to communicate. If they can load shed/alternate to maximize longevity that would be a great bonus.

    Considering two options:
    1. 2xNavien NPE240A + external re-circ.
    2. 2xRinnai RUC98i's + HTP everlast stainless 10gal buffer tank (1500w electric so it will work with 120V 20A) + external re-circ. Cannot use two RURs or RUC+RUR if you want just one control system for both units - silly limitation.

    It is a difficult decision. Option 2 is about $1k more expensive. The Navien has two SS burners and a built-in buffer tank. I'm leaning toward option 2 - the Rinnai design - because:
    a.) they appear to have the best reliability
    b.) they have the longest track record
    c.) they are the most commonly used design and as a result could be easier to have serviced
    d.) best venting (UBBink concentric)
    e.) I like the remote 195T control.

    Location is the Pacific NW - water hardness = 1 - it is very good water.

    Thoughts?? Thanks, -mark
     
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    What is your wintertime incoming water temperature? That will help dictate what and how many tankless systems you may need. You will need substantial gas lines, and LOTS of BTU's to raise water high enough in the potentially large simultaneous hot water demands.
     
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  4. mcdavis

    mcdavis New Member

    Joined:
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    Location:
    Oregon
    Water temp varies between 38 and 75. Typical winter avg. may be around 50. I like my water at 130, so the temp rise in winter is likely around 80.

    Anyone else hear about new Rinnai tankless options coming in the spring??
     
  5. mcdavis

    mcdavis New Member

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    Dec 31, 2016
    Location:
    Oregon
    I should add that I am ok with the sizing of the units. If I get 9.6gpm (2x4.8 per 80 degree rise), that is fine with me...don't often have a lot of simultaneous use.

    I'm more interested in feedback on the design, components, etc. Thanks! -mark
     
  6. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Why do you "like" your water at 130F ? That is a DANGEROUSLY high temperature, with a significant & rapid scald potential if sent to the distribution plumbing at that temp. Typical tub-fill temperatures (after the tub faucet's mixer) are ~110F or cooler, typical showering temperatures are ~105F.

    While 130F is a reasonable storage temperature to limit pathogen growth an a tank type water heater, the output is typically tempered to ~115F prior to the distribution plumbing. With a truly tankless or tiny-tank tankless water heater there is no potential for stagnating water growing pathogens, since the water temperature is no maintained at that temp, and the water is 100% purged with nearly every hot water draw.

    If the house is heated with a hydronic boiler (pumped hot water) you may be better off supplying a least half the hot water with an indirect tank off the boiler, depending on the boiler, and the distribution plumbing topology.

    Independtly of your hot water heating solution, if you have at least 5' of vertical drain downstream of the multi-head shower, you can get quite a lot of additional showering performance out of a drainwater heat exchanger without adding more burner, and it will pay for itself in fairly short years on fuel savings alone if you actually USE that shower regularly.

    [​IMG] [​IMG]

    The fatter & taller they are, the higher the return efficiency, and it's generally true that the fattest & tallest that fits will pay for itself quicker due to the higher performance.

    While drainwater heat recovery may not always be economic for single low-flow shower heads & parsimonious showering times, for gusher showers like yours it always pays off. In your case it if saves you the cost of another tankless it pays for itself up front.
     
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    If you really want 130-degree water in the winter, you will not get the flow rate you described! It would be closer to 3.50gpm. ANd, unless you cascaded them, that would mean a really slow tub fill. I run the numbers in one of the tutorial posts - you can run them yourself to double-check. They'd work fine in the summer, giving you lots of simultaneous hot water use, but could be disappointing in the winter. That assumes you periodically clean out the mineral deposits - even with your relatively soft water, it will still be necessary, just probably not as often as those living where it is harder. My other major point to people is finding someone who both stocks parts and understands how to actually repair them. If you're lucky, that may not be an issue, but you may not have a lot of choices. Pretty much anyone can change out a tank, not everyone can reliably fix a tankless system, nor will they likely have parts available readily to them in the middle of the night or on a weekend.
     
  8. Dana

    Dana In the trades

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    BTW: Unlike most states, Oregon even offers a tax credit subsidy for drainwater heat recovery (called "Waste water heat recovery" on that document).
     
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    The calculations aren't all that hard with some basic math. 1BTU will raise one # of water 1-degree F. That's the output of the burner, not the input as they're never 100% efficient (well, an electric one could be nearly 100%). Say you had a 200K BTU burner that was 75% efficient, you'd use 150K BTU as the output. The spec sheet should give you an idea of the efficiency. A gallon of water weighs ~ 8.35#. So, then the #gallons desired/min*60minutes will give you per hour. Then, multiply that by 8.35#. Multiply that by the number of degrees you want to raise that water and you'll have your BTU output requirement. Tankless systems look wonderful if you have mild water temps, but can be really problematic if you have cold (mine nears freezing after a cold spell - I've actually measured it at 32.5 after running for awhile like you'd get taking a shower in my home) or you need lots of volume. A tank's output will be nearly constant until you've used about 75-80% of it's capacity, depending on the burner if it starts out at set temperature. A tankless' output will start to drop (some will throttle the volume) as you exceed its burner capacity.
     
  10. mcdavis

    mcdavis New Member

    Joined:
    Dec 31, 2016
    Location:
    Oregon
    Thanks for all the responses folks!

    A few things:
    1. I like 130 in the tank for pathogen control...the 105-115 range is fine at the faucets...I assume that significant loss will occur in the pipes, even with re-circ. It is literally 20 degrees outside right now - we see 2-3 weeks of this a year because we live at 1000'.
    2. The drainwater heat exchanger is something I will look into, thanks for the recommendation.
    3. Yes, I will do a regular cleaning of mineral deposits. Thanks Jim. I also have an installer who is very knowledgeable about service...he gave me a list of all the problems he sees & fixes with Navien vs. Rinnai...he installs and services both.
    4. Thanks for the tax credit info Dana!
    5. Jim: calc's - 2 x 199k BTU burner x .95 = 378k BTU output. 378,000lbs or water by 1 degree. My rise is ~80 degrees. So, 4725lbs of water by 80 degrees or 565gal of water....which seems like a lot?

    I'm really interested in the show later this month to see if Rinnai releases anything new...rumors are that they will.

    Any general feedback on the idea to use 2xRinnai + HTP 10-gal stainless? Seem like a reasonable system? Know of anything better??

    -mark
     
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    A couple of things to recalculate...you're using the INPUT value of the system, you need to find out the outlet which will be somewhat lower - they are not 100% efficient! Plus, you're assuming both, and if you have each one supplying things on its own, while yes, you might get that amount, if one of them needed more than 1/2, you wouldn't have it. That's a bit under 5gpm based on your inlet gas use on each branch. A tub could use all of that on one branch. A multi-head shower could easily use all of that as well. The saving grace would be that any water use (except maybe for say a dishwasher or washing machine if you wash anything on hot), you won't be using full hot - you'll be mixing it with some cold, which improves your numbers. What most people do, though, is lower the tankless outlet to just above the max they want for their big users (showers/tubs), which means you may be nearly all hot going in and not mixing it with cold. There is really no big deal with using a lower temperature when you have a tankless system, since the water fairly quickly returns to below the critical temperature range unless you're running recirculation or have a buffer tank in the device. If you insulate your pipes, with recirc, the water temp will be quite close to that at the heater way at the other end.
     
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