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Thread: Rinnai RH 180 Hybrid

  1. #1
    DIY Junior Member Cynicalguy's Avatar
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    Default Rinnai RH 180 Hybrid

    Hello,

    Has anyone installed or know about the new Rinnai Tank/Tankless hybrid. I claims 180 gallons in the first hour. I know Rinnai is a great brand but this is a brand new product. Worth a shot or wait?


    http://www.rinnai.us/hybrid-tank-tan.../product/rh180

    Thanks for your help.

  2. #2
    In the trades Dana's Avatar
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    It probably works just fine, but whether it makes sense for you depends on how you use hot water (and how much hot water you use.) It's standby losses are probably much lower than an atmospheric drafted tank, and it'll never short-cycle like a tankless, so it's probably comparable in net efficiency to a non-condensing tankless, but would support more rapid tub fills than a tankless with twice the burner, with none of the quirks of a tankless. It's probably a dumb 2-stage burner rather than the more sophisticated modulating controls of a tankless too.

    By having a non-modulating burner greater than 75K they don't have to run an EF test on it, and can present it's steady-state combustion efficiency as it's thermal efficiency. At 80% steady-state it's comparable to an atmospheric drafted standalone's combustion efficiency, but since it's using a tankless type heat exchanger and a pump rather than a center-flue heat exchanger, it doesn't have nearly the parasitic loss. It would likely score in the mid-70s in an EF test and hit near that over a wide range of daily use volume, which isn't that different from the typical as-used efficiency of an 82-86% efficiency tankless (which never quite meet their EF numbers in the real world due to short-cycling losses on short draws.)

  3. #3
    DIY Junior Member Cynicalguy's Avatar
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    Thanks Dana, and wow, what a detailed answer. It will work for my family perfectly. I just wonder if buy a brand new type of product is safe.

    And anyone know the price of this unit?

    Thanks

  4. #4
    DIY Senior Member zl700's Avatar
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    Default It's actually not new

    It was developed about 3-years ago and it's acceptance by the industry was dismal at best. The new Rinnai Japanese US president decided to reintroduce it partly because of the larger interest lately in hybrid units and the impending 2015 new tank efficiency standards may now draw some interest.

    Not cheap for a 80% water heater (it's a tankless mounted onto a tank), over $1,800 wholesale to a plumber.
    If Payback is so important to you, why are you not driving a Toyota Corolla?

  5. #5
    DIY Junior Member Cynicalguy's Avatar
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    Thanks for the help. Here is my situation.

    We bought a house with a 40 Gallon water tank. I have 2 small children.

    The big problem is that I like to take baths after work. And the soak tub takes up all the hot water. After the kids baths and my bath, my wife can't do laundry, wash dishes or shower herself. I don't want her not to shower! :-)

    The reason I am not looking at a straight tankless is that I don't have a drain near my utility room so I would need to pump the water through the ceiling to the sump pump.

    What do you guys think?

  6. #6
    In the trades Dana's Avatar
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    A non-condensing tankless would work at a similar efficiency as the Rinnai, but a condensing unit is measureably more efficient.

    A 50 gallon condensing Vertex (with a 76KBTU/hr burner) recovers in half the time of a standard 50 gallon HW heater, the street-price is cheaper and it has much higher thermal efficiency than the Rinnai. And it will deliver substantially faster fill times for the soaker tub than any tankless, and (probably) wouldn't require a significant gas-plumbing upgrade that bigger burners would.


    It's pretty common to use the little condensate pumps used for air-conditioning units to deal with tiny trickle of condensation you get out of a condensing hot water heater.

    For about $600 you could install a drainwater heat recovery heat exchanger and keep the existing HW heater, as long as you promise to shower before filling tubs. With any decently sized DWHR unit, during showering flows it only uses half the heat of a shower, extracting the other half from the drain stream. This shortens the recovery time (after showers only) and boosts the "apparent capacity" of the HW heater. But since it only works during simultaneous HW & drain flow, it does NOTHING for tub-filling capacity/efficiency.

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    DIY Junior Member Cynicalguy's Avatar
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    Thanks for all the information. I checked and I currently have a 50 Gallon which I drain to fill my tub. If I didn't bathe this wouldn't be an issue. So do you guys think the AO Smith GPHE-50 Vertex Water Heater Residential Power Vent Nat Gas 50 Gal. 76,000 BTU would be better for me than the Rinnai RH-180? It is mainly for the nighttime after the bath when showers and laundry and dishes need to be done.

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  8. #8
    In the trades Dana's Avatar
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    The output of an 80% combustion efficiency 91,300 BTU/hr burner like the Rinnai is (0.80 x 91,300=) 73,040 BTU/hr

    The output of a 96% combustion efficiency 76,000 BTU/hr burner like the Vertex is (0.96 x 76,000=) 72,960 BTU/hr

    That's about 99.9% the thermal output of the Rinnai, and more than twice the thermal output of your existing hot water heater.

    The Vertex uses 15% less fuel, and costs less out of pocket up front. (There may be utility/state/local rebate or tax subsidy money for it too.)

    Which one do YOU think would be better?

  9. #9
    DIY Junior Member Cynicalguy's Avatar
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    Well the Vertex seems like a much better choice.

    Finally, I keep the temperature setting low on my water heater because of scalding concerns because of the kids. If I turned the heat up and installed a mixing valve would that possibly give me the extra hot water I need?

    Thanks again Dana and all!

  10. #10
    DIY Junior Member Cynicalguy's Avatar
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    Also, am I correct in reading that the Vertex needs a drain and the Rinnai does not?

  11. #11
    In the trades Dana's Avatar
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    The Vertex is a condensing burner, and yes, you'll have to figure out a condensate disposal strategy. The tiny float pumps used for air conditioners use thin flexible tubing easily routed to a sump or sink, etc.

    http://www.winhomeinspectionelizabet...ate%20pump.JPG

    A condensate pump plus 50' of tubing about a $50 cost-adder. The condensate trap on the exhaust ell drains into the pump, when it's deep enough to trip the float switch the pump sends it to where ever you terminate it's output tubing. It takes a LOT of hot water use to get the daily volumes of condensate up to anything substantial, it may be fine to just pump it into a 5 gallon bucket, which you can dump whenever it starts collecting any volume, if you prefer not to drill through the ceiling/wall/slab to get rid of it. Much of it would evaporate on it's own, but you don't want to just let it drip on to concrete, since natural gas exhaust condensate is mildly acidic (similar to the acidity of red wine), and over time would make for a punky patch of concrete next to your hot water heater as it degrades the limestone. (It's not perfectly kosher, but I've seen people drill through the slab and let gas exhaust condensate drain into the drainage gravel under most slabs, which isn't very harmful- way better than letting it drip on concrete.)

    If there is enough hot water to fill the soaker tub with the tank set to 125F, there is little point to cranking it higher. Above 130F it will cut into the condensing efficiency of the Vertex a bit, and above 140F it will cut into it a LOT. If that question was regarding turning up your current hot water heater, cranking the storage temp might help you fill the tub if it was marginal and not quite getting there, but it won't affect the recovery time much. You are basically burner-limited on recovery times after a big tub-fill, and a 40K /80% burner only delivers 32,000BTU/hr. Whether you set the storage temp at 120F or 140F, after a big tub fill the average temp in the tank will be below 90F (actual average temp depends on incoming water temp), and it needs to rise at least another ~30F before it's shower-worthy. You have about 400lbs of water in the tank, thus it takes 400 BTU to raise it one degree, and a 30 degree rise then takes ~12,000 BTU. Delivering 12 K takes 22 minutes with the existing 40K/80% burner, so it's probably not quite recovered yet even after you've already toweled off the kids. But a 12,000 BTU input only takes 10 minutes with either the 76K/96% burner or the 93K/80% burner.

  12. #12
    DIY Junior Member Cynicalguy's Avatar
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    So which would be better for me, the Vertex or the or the non-hybrid Rinnai RU98i tankless?

  13. #13
    In the trades Dana's Avatar
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    The -98i will never run out of hot water, but it'll be slower in filling the tubs, and the installation costs will likely run considerably higher. You may even need a new gas meter/regulator to handle the burner size, which is ~5x the size of your current HW heater, whereas the Vertex burner is less than 2x bigger for input, more than 2x bigger for output.

    The Vertex is probably the simplest replacement, and I'm guessing it'll meet your needs, being 2x faster on recovery than what you've been suffering through.

    As a point of comparison, I have a 48 gallon indirect that I keep at 130F, and a modulating boiler that delivers about ~50K of output to the tank once the tank has dropped under 110F, and falling to about 25-30K when it's finishing a burn when the tank is near 130F. With this setup 5-10 minutes after full depletion on tub fill it's possible to take an "endless shower", endless only due to the ~25 KBTU/hr being returned by a drainwater heat exchanger. My hot water performance on showering after tub-fills is about the same as what you'd get out of the Vertex, but in my case, only due to the drainwater heat exchanger covering the difference in burner output. With the Vertex, once it has recovered to about 105F (a typical shower-head output temp), the burner alone is sufficient to keep up with a 2 gpm shower literally forever- you don't need to wait until the burner kicks off with the tank at the full setpoint temp. It won't keep up with a 3 gpm gusher shower forever though (but with drainwater heat recovery it could), nor will it support a six side-spray luxury shower for very long.

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