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Tankless water heater help

Rac Member Posts: 4
Hello- can someone help me determine what proper size tankless water heater I need? Our hvac company is recommending two 199,000 BTU by Navien. One would be for the main floor and basement which would be a total of 1 full bath, 1 half bath and a kitchen. The other unit would be for the second floor and would have 3 full baths plus washing machine. When I called an online tankless water heater company to purchase our own, they said 199,000 is way too much and we are overspending and one of these is enough for the whole house or buy two smaller ones for the separate levels. Can someone help? Thanks


  • EzzyT
    EzzyT Member Posts: 1,270
    I would recommend not going with any tankless’s and go with a tank style high efficient unit.
  • Rac
    Rac Member Posts: 4
    Why would you suggest that? We’ve had tankless in another house and liked it and this time it was recommended to save space in our basement.
    HVACNUT Member Posts: 5,480
    What are you using for space heating? IMO there's better options. Like @EzzyT , a high efficiency direct fired, power vent water heater, or if you have a water or steam boiler, an indirect water heater.
    There's a lot of maintenance with tankless water heaters, and repairs aren't cheap.

    With a tankless, get yourself a Pony Pump, some washing machine hoses, and stock up on white vinegar.
  • EzzyT
    EzzyT Member Posts: 1,270
    You can go with a single tank style unit that can deliver much higher water demand than 2 or 3 tankless’s while using a lot less of a gas load. Typical tankless’s have a high pressure drop and a tank style unit virtually has a very low pressure drop.
  • Rac
    Rac Member Posts: 4
    @HVACNUT we are using gas forced hot air heating
  • Rac
    Rac Member Posts: 4
    @EzzyT ok thank you for explaining that makes sense. We were looking at it more from a space saving perspective but it’s much less money as well to go with a tank style
  • EzzyT
    EzzyT Member Posts: 1,270
    And as @HVACNUT has mentioned the maintenance would be a lot more too with tankless’s verses a tank type unit.
  • motoguy128
    motoguy128 Member Posts: 394
    I don’t understand installing two independent units. More efficient and cheaper cost to have a single installation.

    We only install tankless. But I agree the tank type high efficiency units are better.

    If it a a large home I strongly suggest a recirculating loop. It’s an awesome luxury to have instant hot water throughout the house. I even ran a full temp loop to my kitchen sink so can get 135-140f water to really get grease off pots and pans. But you need a mixing type kitchen faucet for safety.
  • Leon82
    Leon82 Member Posts: 684
    I personally wouldn't go below 199k if it was a tankless

    It will run two shower heads, but I'd someone opens a faucet you will get a pressure drop.
  • atrufant73
    atrufant73 Member Posts: 5
    I would probably go tank, but even if you do multiple Navian’s why not casecade them??
  • Jamie Hall
    Jamie Hall Member Posts: 22,187
    They really have to be in parallel -- cascading them would get you hotter water for a given flow, but not more hot water at a sane temperature. That said, if they both fire at once -- which is quite likely -- you'd be looking at 400,000 BTUh demand on the gas line. Which is a lot; I'd be surprised if your existing gas service could handle that. I agree with most of the guys up above -- a nice gas fired tank type. And don't skimp on the size. You have the potential for a pretty high short term demand there, and the cost differential is in the trimmings, not much in the size of the unit.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • HarryBarry
    HarryBarry Member Posts: 1
    edited January 2020
    From what I can tell, an online tankless water heater company is right. You don't need two big 199,00 BTU tankless units for your house. One is absolutely enough. You might also need a couple of the smaller ones closer to the showers. Or install a recirculation pump.

    upd. I don't know why but the link I posted disappeared (maybe mods thought I was spamming?). Anyway, here it is. It was very useful to me. Hope it helps.
  • mattmia2
    mattmia2 Member Posts: 8,893
    Without knowing the minimum incoming water temp we can't say anything. In someplace like AZ where the incoming temp is 70 degrees F 200,000 btu/hr might be enough. In someplace like Michigan where the incoming water temp is 35 degrees F it is barely enough for 1 bath and a couple sinks.
    rick in Alaska
  • hot_rod
    hot_rod Member Posts: 20,827
    How much HW you want, or need, and the temperature lift.

    I think most are rated at a 77° lift, so for example their output number (gpm) would be flow rate at 55°-127°, or whatever you incoming water is plus 77°.

    105°F is a common shower temperature, really no need to run higher, although plumbing codes suggest 120F supply. That would need to be mixed obviously for a shower :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • rick in Alaska
    rick in Alaska Member Posts: 1,441
    I have mixed feelings about tankless heaters.When you need to fill a big tub, they will fill it all day long without running out. But the biggest problem I see is if you have small demands, such as just turning on a sink to rinse your hands real fast, the heater will come on, do its prepurge , and then shut off because you have turned the faucet off before it ever has a chance to fire and get moving any hot water to the fixture. So it has a tendency to do a lot of short cycling in its life.
    But the biggest problem with them is if you have a high efficiency dish washer or washing machine. They have a tendency to pulse small shots of water in, and the water heater never has a chance to fire, so you never get hot water in the unit at all.
    As far as maintenance goes, I don't have any problems with them except for cleaning bugs out of the fan and cleaning the electrode once in a while. It will depend on your water though, as to how much you would have to do to keep the heat exchanger clean. We do not have really hard water here, so I have never had to de-scale a heater.
  • plumbbob
    plumbbob Member Posts: 19
    rinnai tankless 7.5 gpm