Effective Output In Winter vs Summer

Discussion in 'Tankless Water Heater Forum' started by Haselsmasher, Apr 16, 2014.

  1. Haselsmasher

    Haselsmasher New Member

    Messages:
    4
    Location:
    CO
    I'm very close to signing a contract on a tankless install. (It could happen tomorrow - which is why I hope I can get input ASAP here.)

    I live in northern CO. If I assume a winter cold water temp of 40 degrees F I need to have a 75-80 degree raise to get it to 120. The spec sheet for the unit I've decided on (Navien) says it'll produce 5.1 gpm at that temp rise.

    We have three showers in the house - with flow rates of 2.25 gpm per shower. On one level I'd conclude that in this winter scenario the heater couldn't supply three showers simultaneously. However on another level I'm thinking that showers don't get used at full hot; that one can likely assume that 75% of the total flow needs to be hot. So if I take 2.25 gpm/shower *.75 hot/shower *3 showers = A little over 5 gpm of hot needed for 3 showers.

    So - what do folks think? Is it reasonable to conclude that in winter this unit could support 3 showers simultaneously? (It's a 199,000 BTU unit.)

    Thanks!

    Jim
    Last edited: Apr 16, 2014
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
    New England
    Depending on your specific circumstances, your water may be colder than that...it's generally not as cold there as here, but I've seen incoming water temps in my NH condo at 33-degrees and I'm near sea level fairly close to Boston. Depending on the cold water temp, you will use various proportions of hot/cold, and keep in mind that 120-degrees at the outlet may or may not be 120-degrees when it hits the supply valve of your shower.

    Some people love their tankless, some don't. You can do the math yourself...for rough calculations, a gallon of water is 8#, and it takes one BTU to raise one pound one degree. Your tankless is BTU/hour, so divide by 60 to get a one-minute output.
  3. Dana

    Dana In the trades

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    The better-accuracy napkin-math version (as opposed to doing it completely in your head):

    A typical shower head temperature is about 105F. With incoming water at 40F (to both the cold and hot side) that is only a 65F increase from incoming temp to shower head temp.

    At typical shower head delivers about 2 gallons per minute. At 8.34 lbs/gallon and 60 minutes per hour that's 1000.8 lbs per hour, but let's just round it off to 1000 lbs, since nothing else in the calculation has better than a 1% accuracy.

    At 1 BTU per degree F per pound (that's a definition, not a calculation) , a single shower takes 1000 lbs/hr x 65F= 65,000 BTU/hr.

    Assuming a net efficiency of 90% at full-fire full flow, the output of the tankless is 0.9 x 199,000= 179,000 BTU/hr.

    Which is enough to support only (179,000/65,000= ) 2.75 showers @ 2 gpm per shower.

    A low flow shower head is more likely to be only pulling ~1.7 gpm, and would only take 55,250 BTU/hr. x3 that's only 165,750 BTU/hr, so you'd have a little bit of margin to play with there- enough to tolerate 35F incoming water or nearly scalding 108F showers.

    Bucket-test your flow rate volumes against a stopwatch, then do the math. You'll make it just fine at 1.7gpm per shower, but won't even support 2 showers if they're running 3 gpm (old-school gushers.)
  4. Haselsmasher

    Haselsmasher New Member

    Messages:
    4
    Location:
    CO
    Thanks very much for the replies. They're very helpful.

    I've done some calculations using a couple of methodologies - and no matter how I slice it the bottom line is this: Running 3 showers will be REALLY tight. My guess is that when this occurs each showerer might not have quite as hot a shower as they'd like. It's close enough (to being good enough of a solution) it's certainly not pushing me to install a 2nd tankless unit.

    Regarding efficiency: The spec sheet for the unit (Navien) has what is called an *Energy Factor*. I assume that is not efficiency - otherwise I would think they'd call in Efficiency. Can someone elaborate on what Energy Factor is?

    Also, the thought occurred to me: What if I left the current water heater tank in place (physically) but decommissioned. Use it only as a storage tank - and plumb it so the tankless input line is connected to the tank. Therefore, at least for the 40 gal that the current tank can hold, the tankless would be warming room temperature water and not water straight from the main. Obviously once that tank is processed the tankless would be heating water of temp straight from the main. Has anyone ever done something like that?
  5. Dana

    Dana In the trades

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    The labeled energy factor is an average efficiency under a standard set of D.O.E. specified test conditions, and is not directly related to it's steady-state efficiency under other conditions. Firing rate, flow rate, and the incoming water temperature are primary determinants of the steady state efficiency of a tankless.

    The steady state efficiency of most atmospheric drafted gas-fired tanks is about 80%, but their EF test numbers are much lower than 0.8 due to the high standby losses of room air convecting through the center flue during idle. Higher volume use of a tank-type hot water heater raises it's as-used EF, and lower volume use results in longer idle periods, and lower EF.

    The D.O.E. EF test exaggerates the as-used efficiency of tankless units by quite a bit, since it the draws are all high-volume draws, whereas with a tankless there is some loss to flue purges & ignition cycles on every draw, independent of the volume drawn, which can be a substantial fraction of the total fuel burned during short low-volume draws. Those losses during showers are negligible compared to the total fuel use, but not for quick hand rinses.

    The steady state efficiency during a single shower will be close to the EF number, but maxed out with 3 showers it'll be a bit lower due to the fall-off in efficiency due to higher fire-side combustion gas velocity at max-fire resulting in less condensing (= lower heat of vaporization recovery). The "sweet spot" where efficiency maxes out is usually somewhere in the lower third of the firing range.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    Depending on the type and quality of the existing tank, it may take quite awhile for the incoming water to reach room temp. Something like a well storage tank, where it doesn't have insulation may work better, but buying a new one would likely never recoup the costs. If you have enough height and could install a drain water heat recovery system, you'd probably be better off.

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