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Thread: Which Tankless To Install

  1. #1
    Licensed Building Contractor dtherrien's Avatar
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    Default Which Tankless To Install

    I am just about ready to replace my 20 year old State 40gal hot water heater.
    I have decided on one of the two: Rannia Rc 80HPI or the Navien NPE 210A...both would be LP gas.
    My house is set up for propane. In the last 10-11 years i install a Rannai room heater in the downstairs (Finished basement) and also one in the upstairs. They have run flawlessly.
    I have two full baths and 5 people in the house.
    Cook Stove
    2 Rannai heaters
    And then the new water heater.
    I know with the Rannai i will have to run a new 3/4" line from the tank.
    The Navien states that i can use the existing 1/2" line....but i think i will go with the larger if i decide on the Navien.
    I am partial to Rannai....but i dont know much about the tankless water heaters.
    Which of the two brands would you people recomend?

    David

  2. #2
    Jack of all trades DonL's Avatar
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    I think you may regret running it on propane in the long run. Electricity is getting cheaper.

    I could be wrong.


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    Retired Defense Industry Engineer jadnashua's Avatar
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    The big thing about tankless systems is their temperature rise ability. In MA, you ground temperature can get quite low in the middle of the winter...I've measured 33-degrees at my upstairs tap (that's after running through a string of basements in our condo complex, then up a couple of stories!). This can stress a tankless, and you must size it according to the actual temperature rise you require and the volume you need at any one point.

    In pretty much anyplace in the USA, propane will end up cheaper per BTU than using electrical, and to use electric on a whole house tankless would likely require upgrading your main panel and line back to the street! There may be a few exceptions where you have subsidized hydro-power available or in a co-op.

    If you size the thing properly, they can work in any climate, but that doesn't mean they are the most economical way to do it. In some cases, you may need more than one in series to get the temperature rise and volume you want...just depends on your typical max draws, OR, if you can accommodate the quirks of the system.

    You need to read the fine print on the volume available with YOUR temperature rise requirements and compare that with your actual volume will be available.
    Last edited by jadnashua; 10-07-2013 at 04:43 PM.
    Jim DeBruycker
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  4. #4
    Licensed Building Contractor dtherrien's Avatar
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    Propane or electric are my only options. I am not set up for fuel oil.
    I was also thinking about a hybrid electric tank water heater...but not sure how much it will save me in the long run. In the summer with the a/c and the pool filter running, my electric for 3-4 months are up.
    I dont think my water in the winter gets below 40. I have well water.
    I also question what the temp rise will be with the tankless.
    I like the compactness of the tankless.
    I am trying to go with something more efficient. My hot water heater now is using the greatest share of the propane.
    The cook stove doesnt use much and for most of the winter i burn wood.
    A year i go through about 550 gallons of propane....thats only turning the room heaters on if we are gone for the weekend.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    FWIW, most of the advertised GPM on tankless are based on 50-degree input water, so you have to read the fine print to adjust it for your bigger delta.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    DIY Senior Member zl700's Avatar
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    Both heaters you noted are high-efficiency condensing and actually the colder the inlet water the more efficient they are. Cold temps don't hurt the units but can reduce output because of desired temperature rise. All quality tankless such as these have water adjustment valves to meter the flow once the unit hits full fire to maintain desired output temp set point. That being said the energy cost to raise the water is what's looked at.
    According to AHRI, the there is no higher efficient gas-fired water heater in the world, at the Navien .97 EF.

    How do they make water, it's about the BTUs, and the rise required. The NPE is fired at 180,000 BTus, and the is a bigger one, the NPE240 at 199,900 BTUs.

    The NPE you mention is both more efficient and higher fired than the a Rinnai mentioned so let's use that.

    180,000 / 8.34 / 60 / 80 (water heated from 40 to 120) x .97 = 4.36 GPM of continuous hot water
    Mixing it down to 110 for bathing, basically you have 4.75 GPM.
    The bigger 240 models would give you about 5.25 GPM at 110 degrees.

    One thing you never do if you need a constant flow higher than that is never never never run two or more tankless units in series, unless this is a commercial application for example and the second one is a high temp booster feeding a dishwasher for example.

    Tankless water heaters have serpentine piping, elbows, valving, screens flow sensors and more. Installing a condensing tankless inline with existing piping is like adding 50' of 1/2" water line in the middle. Imagine the pressure drop and reduced flow of two units in series, with a equivalent of 100' or more of piping. Check the manufactures, they are always installed in parallel, and commonly cascaded based on load with cables or panels that the manufacture requires.

    The nicest thing about the Navien is multiple units are cascaded with a inexpensive cable between controls, and then the venting can be done in a common pipe.

    The Navien A model you mentioned is the premium tankless with a on-board circulator included that allows external circulation or internal circulation to solve cold water sandwiches and hot water delays.

    You just need to decide is 5GPM or so max enough, to meet your needs? Bear in mind that's 2 standard showers, how hard is it to start the wash after bathing? Remember its endless hot water but at a controlled flow rate depending on rise, BTU and number of tankless.
    Last edited by zl700; 10-07-2013 at 05:50 PM.
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    DIY Senior Member zl700's Avatar
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    One more thing
    Navien states it allows 1/2" gas line because it allows inlet pressures as low as 3.5" WC. This allows for higher distribution piping pressure drops but does not allow the installer to undersize and undercut all the other appliances that need higher inlet pressures. On LP this would include proper tank size to allow needed BTU LP evaporation, regulators to carry full loads and piping of course
    If Payback is so important to you, why are you not driving a Toyota Corolla?

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    Licensed Building Contractor dtherrien's Avatar
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    zl700...yes both models are condensing units. I dont think i would be drawing more than the 4.5-5 gpm.
    I am going to replace the gas line with 3/4" from the tank into the house. I have a 250 gallon tank (@ 80% full) a regulator at the tank and one at the house.
    With the current models available, is the Navien a better choice?

  9. #9
    DIY Senior Member guy48065's Avatar
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    Does venting cost & convenience matter? Navien is PVC, Rinnai is concentric steel.
    Romeo and Atlanta, MI

  10. #10
    In the trades Dana's Avatar
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    At the recent 5 year propane pricing in MA a bigger Stiebel Eltron or AirTap heat pump water heater would make more financial sense. (A GE Geospring might cut it if you're a showering rather than a tub-bathing family, and install a drainwater heat exchanger.) During the summer it would be lowering your cooling load and dehumidifying the basement, and during the winter it would cost no more than (and probably less than) running a condensing propane tankless. The only issue is capacity, and space. The installation is much easier and cheaper than a tankless, even if the hardware cost for an 80 gallon unit is more expensive.

    If you're going with tankless despite my brilliant advice ( ;-) ) Rinnai is the worlds largest manufacturer of gas-burning appliances with a very good rep. Navien (Kyong Dong Boiler Company) makes some good stuff at attractive price/performance points, but the quality control and field support in the US isn't nearly that of Rinnai. If Navien, make sure to only use factory certified techs who is willing to support both the product & installation.

    With expensive fuels like propane the payback on ~50% efficiency or higher drainwater heat exchangers is pretty quick for showering families,(that would be a 4" x 48" or a 3" x 60" or bigger.) It roughly doubles the apparent-capacity of tank HW heaters, and doubles the efficiency during simultateous drain + HW flows.


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    Jack of all trades DonL's Avatar
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    Nice Post Dana.

    Propane is a great backup fuel. If you like to waste energy, and can afford to use it as a primary fuel that is great.

    I would try to use a electrical means of heating. Not a tankless water heater tho. And a propane unit will not work in a power outage.

    Using a propane electric generator as a backup, to power essentials makes a bit of sense.


    That is just me, in this day and age.
    Last edited by DonL; 10-08-2013 at 10:06 AM.
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  12. #12
    In the trades Dana's Avatar
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    DonL: The residential retail rates for electricity in MA can range as high as 22 cents/kwh, nearly 2x the TX average. Other locations on municipal utilites can sometimes be as low as 12 cents. Exactly which utility he is on makes a difference. The delivered propane price/gallon history isn't too promising either, averaging over $3/gallon over the past 5 years.

    A condensing propane tankless that scored in the high 90s on an EF test is only about 0.85 in real-world use (short-cycling losses eat up much of the condensing efficiency.) At 91,600 BTU/gallon at 0.85 effciency you're getting 77,860 BTU gallon into the water. tThat's 12.84 gallons/MMBU, so at a cost of $3/gallon you're paying ~$38.50/MMBTU for heating water.

    A cheap EF 0.90 electric tank delivers 0.9 x 3412= 3071 BTU/ kwh into the water, which is 325 kwh/MMBTU. If he's paying the state average of 15 cents/kwh that's ($0.15 x 325=) $48.84/MMBTU for heating water- a tad-bit more expensive than condensing propane.

    But with an EF 2.2 heat pump water heater the cost during non-heating season periods would be (0.9/2.2 x $48,84=) $20/MMBTU, or about half what it costs with condensing propane.

    During the heating season the ~60% of the non-electric portion of the heat going into the water comes mostly from the heating system, at whatever efficiency & cost that is. Cooling off the basement a degree or two with the water heater also lowers the heat loss from the building slightly, so there are significant secondary factors making the model not-so-simple, but the annualized net cost will be much lower than heating the water with either electricity OR a propane tankless. It'll likely average between 65-75% of the cost of doing it with a condensing tankless.

    If you then add a decent sized drainwater heat recovery heat exchanger the water heating costs come down by another 20-35%- depending a on what fraction of your total hot water use is from simultaneous flows like showers.

    At recent average MA propane & electricity prices the Rinnai space heaters are substantially more expensive to heat with than using mini-split heat pumps too. Over the course of a heating season a pretty good mini-split delivers ~10,000 BTU/kwh in this climate (better than heat pump water heater efficiency). At 15 cents/kwh that's $15/MMBTU.

    The Rinnai space heaters run in the low-80s for efficiency- figure about 76,000 BTU/gallon, or 13.16 gallons /MMBTU, which at $3/gallon costs $39.50.

    Even heating with the world's crummiest third-world-imitation mini-split would cost less than half what it costs to heat with propane here. If electricity & propane were my primary options, I'd be heating with mini-splits (and I know people in central MA who do.) A decent mini-split typically pays for itself in about 3 heating seasons or less for most MA locations when displacing oil or propane.

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    Licensed Building Contractor dtherrien's Avatar
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    Dana...Where can i find more info on the mini-splits that you are talking about?
    Yes i am in central Ma. and electric and propane are high. I am paying about $4.50 gallon for propane....company says i am not using enough to get the heat rate. I am thinking about getting my own tank and get delivery from who ever has the best price.What is that setup in the picture above? I understand the drain water part...but what brand is that heater and does it heat both the domestic hot water and water for home heating?

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    DIY Senior Member guy48065's Avatar
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    Anybody,
    Having just researched tankless condensing HWH myself I've been wondering why Rinnai stands alone in not offering any models that use PVC venting. Have I missed something bad about plastic vent vs. concentric steel?
    Romeo and Atlanta, MI

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    DIY Senior Member Reach4's Avatar
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    Quote Originally Posted by guy48065 View Post
    Anybody,
    Having just researched tankless condensing HWH myself I've been wondering why Rinnai stands alone in not offering any models that use PVC venting. Have I missed something bad about plastic vent vs. concentric steel?
    I would think that concentric steel would have the advantage of pre-heating the incoming air from the exhaust heat. It would also leave one hole in the house instead of two.

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