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Trying to Decide on Convential Tank vs. Tankless

Hi,

I am new to this forum but was hoping for some thoughts/opinions. We currently have a 50 gallon natural gas AO Smith conventional tank water heater which runs out of hot water during the 3rd shower (shower heads are unmodified presumably no more than 2.5gpm). We are rapidly approaching the point where we will need to have 4 showers back to back (or perhaps 2 concurrently) as the kids get older. We have 3 full baths in a 2 story house in the Portland, OR metro area. I have received quotes/estimates from 3 plumbing companies. My choices at this point are: 1) a 75 gallon 76k BTU conventional tank, a 2) hybird Rinnai 180, or a 3) Navien NPE-240A. Our gas line is apparently adequate for tankless per the reps from these plumbing companies. We will sometimes have visitors and could have up to 6 showers in one morning. The order I have the choices also happens to correspond price, cheapest to highest price. We'd like to make an investment and are not put off by the higher price of tankless. I have done what research I can and still feel like it is difficult to know whether if I do tankless there is more chance of a pressure drop with a couple showers going at once or whether tankless is more prone to breakdowns due to the increased complexity of the units. I am leaning toward the Navien. I'd appreciate anyone's thoughts, thank you in advance.

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 15,851
    If it were mine to do, I'd go with the bigger conventional tank. Simpler, cheaper, and more reliable. But that's just me.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    shafer97201Gordy
  • Abracadabra
    Abracadabra Member Posts: 1,948
    The navien only does 5.5gpm at a 67 degree rise. If you are going to be having 3 showers going at once you will not be happy with a tankless unless you are looking to install 2 in parallel. I'd go with the tank. 2 decent 50g tanks in parallel or a 75g.
  • gschallert
    gschallert Member Posts: 170
    If it were mine to do I'd go with the Navien. No flow problems at all for most of the year with your inlet temps and even down around 40 degrees you should be able to run 2 showers concurrently without issue and none of the recovery rate issues that are inherent with tanks so back to back to back showers no problem at all. I just got rid of a direct fired tank and will never have one again. If a large indirect isn't an option for your home, I think tankless is the next best option for your scenario.
  • Jack
    Jack Member Posts: 1,045
    Kids! Amazing how much hot water they require;) My 2.5 gpm shower with the Rinnai tankless (I have to get that in there, but I am biased) with a 50* supply temp and 120* set point uses 2.1 gal of hot mixed with .4 gal cold to arrive at my comfortable shower temp. You can bring the load of 3 showers back to the capacity of the tankless by changing the shower heads. You don't need much. There are good 1.5, 2 & your current 2.5 gal shower heads. You can go to 2's and be in good shape, or 1.5's in the kids baths and keep a 2.5 for the master. You will not run out of hot water with the tankless regardless off how your loads stack and you can fill and use a bathtub better with the tankless. The RH180 will not match a 199kbtu tankless, but it will outproduce the 50 gal tank.
  • Abracadabra
    Abracadabra Member Posts: 1,948
    Sorry... I had to...

    HatterasguyGordySolid_Fuel_ManDan Foley
  • gschallert
    gschallert Member Posts: 170
    Here's a couple examples of what the Navien can do based on your location:

    P= hot water ratio
    Th= Supply hot water temp
    Tc= Cold water inlet temp
    Tm= Desired mix temp at fixture

    P=(Tm-Tc)/(Th-Tc)

    Scenario #1 dead of winter Portland OR

    P=(110-40)/(140-40)
    P=70/100
    P=.7

    .7 x 2.5 gpm = 1.75 gpm of 140 degree water

    In the scenario above in order to take (2)concurrent showers where you need 100 degree rise you need 3.5 gpm and the Navien delivers 3.9 gpm flow @ 100 degree rise - also bear in mind I used 110 as the desired shower temp and the national avg is actually lower than that.

    Using the same scenario and lowering the supply water temp from 140 to 125 you get:

    P=(110-40)/(125-40)
    P=70/85
    P=.824

    .824 x 2.5 gpm = 2.06 gpm of 125 degree water

    Now that you've lowered the SWT you need more at each shower to achieve desired temp so in this scenario it wouldn't keep up with two concurrent showers. Fwiw I have my in-laws DHW supply temp
    set higher to offset the lower inlet temps of winter and will drop it back down to 120~125 for spring/summer/fall.

    Your avg inlet temps are around 50 so when you plug that # in you now get:

    P=(110-50)/(125-50)
    P=60/75
    P=.8

    .8 x 2.5 gpm = 2 gpm of 125 degree water per showerhead, the Navien delivers 5.2 gpm @ 75 degree rise - So two concurrent showers with capacity to spare. Switch to some low flow showerheads and get that down to ≤ 2gpm and you've got even more capacity to spare and you can run 3 concurrent showers no problem.
  • gschallert
    gschallert Member Posts: 170
    @shafer97201, my last post didn't go through, something about needing to be approved so I sent you a direct message with the equations for you to see what kind of capacity you'd have with the Navien.
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,148
    Hello, You might want to look into combined big burner and small tank like HTP makes: http://htproducts.com/crossover-water-heating.html Adding storage really makes things behave. :)

    Yours, Larry
  • lchmb
    lchmb Member Posts: 2,997
    go with a tank..toss in a mixing valve and keep the tank at 150 degree's...should handle all your needs...
  • Brewbeer
    Brewbeer Member Posts: 616
    @shafer97201 do you heat with a warm air furnace or a boiler? If a boiler, consider an indirect tank.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
    Solid_Fuel_Man
  • hot_rod
    hot_rod Member Posts: 14,786

    Hello, You might want to look into combined big burner and small tank like HTP makes: http://htproducts.com/crossover-water-heating.html Adding storage really makes things behave. :)

    Yours, Larry

    Clever product.

    How does it provide a tank at 167F water and run as a condensing heater?

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Gordy
    Gordy Member Posts: 9,514
    Can't beat a tank. Higher storage temps, and mix down.
  • kcopp
    kcopp Member Posts: 3,721
    Keep your 50 gallon tank for now. Add a mix valve now to it and boost the storage temp. Then you can make a switch down the road.
  • Zman
    Zman Member Posts: 6,565
    @shafer97201
    Are you still here?
    What is the age and btu rating on your existing tank?
    Has it been serviced or flushed?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • shafer97201
    shafer97201 Member Posts: 2
    It is about 5 years old, BTU rating is 40,000, yes it has been flushed and serviced last year.
  • BornForDying
    BornForDying Member Posts: 40
    > @Abracadabra said:
    > Sorry... I had to...

    Ive been seriously considering installing a tankless in my own house. This video has changed my mind. :)
  • gschallert
    gschallert Member Posts: 170
    Here's a couple examples of what the Navien can do based on your location:

    P= hot water ratio
    Th= Supply hot water temp
    Tc= Cold water inlet temp
    Tm= Desired mix temp at fixture

    P=(Tm-Tc)/(Th-Tc)

    Scenario #1 dead of winter Portland OR

    P=(110-40)/(140-40)
    P=70/100
    P=.7

    .7 x 2.5 gpm = 1.75 gpm of 140 degree water

    In the scenario above in order to take (2)concurrent showers where you need 100 degree rise you need 3.5 gpm and the Navien delivers 3.9 gpm flow @ 100 degree rise - also bear in mind I used 110 as the desired shower temp and the national avg is actually lower than that.

    Using the same scenario and lowering the supply water temp from 140 to 125 you get:

    P=(110-40)/(125-40)
    P=70/85
    P=.824

    .824 x 2.5 gpm = 2.06 gpm of 125 degree water

    Now that you've lowered the SWT you need more at each shower to achieve desired temp so in this scenario it wouldn't keep up with two concurrent showers. FWIW I have my inlaws DHW supply temp
    set higher to offset the lower inlet temps of winter and will drop it back down to 120~125 for spring/summer/fall.

    Your avg inlet temps are around 50 so when you plug that # in you now get:

    P=(110-50)/(125-50)
    P=60/75
    P=.8

    .8 x 2.5 gpm = 2 gpm of 125 degree water per showerhead, the Navien delivers 5.2 gpm @ 75 degree rise So two concurrent showers with capacity to spare. Switch to some low flow showerheads and get that down to ≤2gpm and you've got even more capacity to spare and you can run 3 concurrent showers no problem.

  • gschallert
    gschallert Member Posts: 170
    edited March 2017
    dup post deleted
  • rocky_mtn
    rocky_mtn Member Posts: 13
    Just go with an AOSmith 75 gallon vertex 100k btu. Something like 129 gallons per hour recovery plus one heck of a dump load. It's basically a commercial water heater in a residential jacket. Money well spent.
  • Robert_25
    Robert_25 Member Posts: 299
    The existing water heater would probably work quite well with a mixing valve and a higher temperature setting.

    I would also swap the showerheads, - at least for the kid's bathroom. I have used the High Sierra brand 1.8 gpm shower heads with good results.
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