Sizing of tankless water heater. Few questions.

Discussion in 'Tankless Water Heater Forum' started by C Wolford, Sep 4, 2019.

  1. C Wolford

    C Wolford New Member

    Joined:
    Jan 23, 2019
    Location:
    IL
    I am on city water. treatment plant is 7 miles away.

    1st when I was looking in to this last year, I found at winter my TAP water was coming in at 36-46 can't remember. Not sure if there water treatment company would have a log. The thing is on all the ground water maps I am close to the line of 57/52. Is this normal? All I know is I recall a 10-20 lower temp from the ground water chart.

    2nd. I have a 1 bath, 4 person home. Wife like to get the Washing Machine, and dishwasher going at the same time maybe a bath/shower. So I should account for the max flow of all this for hot water demand?

    3rd. Bath tub filling. I know the valves mix cold with hot, but from my understanding all the charts they give have these water heater set to 105 F. That would require straight hot to fill the tub right? So to get a true flow rate need for the tub I need to find the mixing % with cold at a give delta T right? The short of #3 is I do need to have a higher water temp than the 105 f most manufacturers use to rate there flow rate with. correct?

    4th Am I crazy and over thinking here or am I on the right track?

    Thanks for your time.
     
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    You're overthinking this, but do the math if you want. For incoming water temps of 40F you'll definitely want a 199,000 BTU/hr condensing tankless (if tankless).

    No matter how high you raise it and mix it down, the limitations are the max burner output and the temperature difference between what's coming in the cold mains and what's going out the tap. For a tub fill 110F is more realistic, for a shower 105F is fair. At 40F incoming that's a 70F or 65F temperature rise.

    Every gallon per minute (gpm) is very close to 500 lbs per hour. It takes 1 BTU to raise a pound of water 1 degree F. The max output of a 199K tankless is about 190,000 BTU/hr (in your dreams- in practice it's slightly less.)

    So for a 70F bathtub fill your max fill rate is 190,000/70F=~2700 lbs/hr. Translating that to gpm would be 2700/500= 5.4 gpm

    That's not crazy slow, but it's not super-fast either, and if sharing the burner with other hot water draws it'll take longer.

    A 2.5 gpm shower with a 65F temperture rise takes 2.5 x 500lbs/hr x 65F= 81,250 BTU/hr, which is nearly half the max output of a 199K tankless, and that's OK- it's enough to allow somebody to run the washing machine or run a tap at the kitchen sink without hearing screaming coming from the shower, or even have a second shower running, but not enough to fill a tub at a reasonable rate while someone is showering in the other bathroom (that you don't have).

    With a single bathroom you could cut back on that burner sizing a bit if you don't mind slower tub fills A typical bathtub takes ~35 gallons, so with a 199K tankless it'll take 6 minutes, but it starts to stretch out if dropping back to 150K burner, and takes a LOT longer with a 120K burner.
     
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  4. C Wolford

    C Wolford New Member

    Joined:
    Jan 23, 2019
    Location:
    IL
    Thats what I was afraid of. Seems I got some more reading to do pro vs cons of tank to tankless. I wanted the nice endless hot, on demand, savings etc... but it seems it needs a huge one to do a small 4 person house at least in the winter when the temps are down.

    Thanks for the time and for responding.
     
  5. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    If it's showering performance you're after and have another grand to spend, a drainwater heat recovery heat exchanger that's 50% efficiency or higher under NRCan testing protocols can allow down-sizing the tankless. In showering mode it's effectively using half the amount of burner capacity that it would take to run the shower without it. The current state of the art is EcoDrain's V1000 series, but I'm still pretty happy with the 4" x 48" Renewability PowerPipe I installed about a decade ago.

    It does nothing for tub filling performance, or batch draws from washers, etc, but it doubles the showering capacity without using more fuel by just keeping a large fraction of the heat from going literally down the drain.

    With expensive fuels such as LP or plain old electric tank the "payback" is pretty short on fuel savings if four people are showering daily.

    [​IMG]
     
  6. C Wolford

    C Wolford New Member

    Joined:
    Jan 23, 2019
    Location:
    IL
    I was looking at energy saving devices like this, but still have to research them more. The water heater is on the top of the todo list, its from 1984 XD. was not working well when I moved in cold shower in 2-3 mins. I had to rewire its insides. It had melted black wires, also needed a lower element, and removed about 5 inches of sediment. I seen a few rust spots on the tank so it has to go sooner rather than latter. I might not have my engineering bachelors yet but my alternative energy / industrial electrician associates sure has came in handy for something as its about worthless for jobs.

    I don't have gas lines. I don't want gas inside the home for many reasons, mostly budget building remodeling that would keep back draft issues at bay until the need for a HRV or similar. I atm its add an extra drop electrical service drop for tankless freeing up space, adding gas line tankless outside (have to read up on this), or keeping big tank.

    Thanks for the info Dana.
     
  7. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    An electric tankless is almost useless in northern Illinois. With 40F incoming water and 105F at the shower head a 2gpm shower draws 19,000 watts. At 2.5 gpm it's getting on to 24,000 watts, (100 amps @ 240VAC) just to cover one shower, with little to no margin. It takes power wiring fat enough to haul the F150 out of the ditch with too.

    A heat pump water heater or a plain old electric tank would be a better option.
     
    Terry likes this.
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