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Seems like an easy question but I can't get an answer... How long can a shower be?

We have a Rheem 42VR50FV / PRO50-40F natural gas water heater (50 gal) that's coming up on 7 years old. we have the extended warranty kit for total of 12 years.

We got our bathroom redone and have a 2.5g rain head showerhead in there.

With that flow rate, is there a rule of thumb how long we can take a shower / sequential showers before it gets cold? It seems after 10 - 15 minutes, we're turning the temp valve up / finishing up. Which I guess is right? that's 37Gallons so that's most of the tank / the make up water is nice and cold...

From this page,

It talks of 1st hour gallons of 83. Calling rheem, they can't explain what that means.... certainly doesn't seem that you can take a 83 gal/2.5GPM = 33 minute long shower expecting it to be warm.

it's just wife and me now, but when kids come home, we do have to plan who / when takes showers so they are all warm. That's the way it is? A good argument for an instant hot unit? They say 50 gal is a nice size unit for 4 people... as long as you aren't all getting ready near the same time?! : )

Oh, it's a Toto anti-scald valve. That just keeps you from getting scalded right? It doesn't keep the temp the same as hot line drops in temp? Do those exist? Set it for temp you want and then it does what's needed (as best it can) to keep it that temp as hot water drops in temp?)


  • Jamie Hall
    Jamie Hall Member Posts: 23,322
    That first hour rating assumes that you use up the 40 gallons -- and then it can heat another 43 gallons over the next hour. So... you take one shower for 33 gallons or so (teenagers?) and then you are going to have to wait about half an hour for the next one. And so on.

    And the anti-scald valve is just that -- it limits the maximum, but won't maintain temperature. For that you need a thermostatic mixing valve -- and that won't do it either, once the water heater is run out.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Gordy
    Gordy Member Posts: 9,546
    What is the current storage temp of the water heater?

    You can extend usage by raising the storage temp. However a thermostatic mixing valve should be used.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    They do make temperature/pressure valves built into the fixture, but the pressure will drop to maintain the hot water until you're out of hot water.

    There was an error rendering this rich post.

  • GW
    GW Member Posts: 4,693
    The first hour rating has little to do with how fast you can deplete the hot water. It’s a bit of a useless rating.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
  • Tim McElwain
    Tim McElwain Member Posts: 4,625
    What is the listed recovery rate on the heater?
  • GroundUp
    GroundUp Member Posts: 1,908
    Pretty subjective to tank temp and tap temp. If you keep the tank at 120 degrees and take 120 at the showerhead, with 2.5 GPM you've got 20 minutes stored. In that same 20 minutes, your 40,000 BTU/HR recovery has created 13,333 BTU which is enough to raise 22.87 more gallons 70 degrees (50 to 120) and effectively extends your shower 9 more minutes. All theoretical of course, but should give an idea. If you were to turn up the tank temp to 160 and utilize a thermostatic mixing valve set to 120, you'd extend the life of your stored heat by 33%, also lengthening the life of your hot showers by the same amount
  • hot_rod
    hot_rod Member Posts: 22,157
    With a tank type water heater, as soon as you open a hot tap, cold water enters the tank and the temperature starts blending down. You never really get the full tank capacity as 120°, or whatever it is set at.

    Elevate the tank temperature as other have mentioned and more cold gets blended extending your drawdown.

    Higher operating temperatures do reduce the tank life and cause more minerals to precipitate out. At some point a larger tank makes better sense if you have a high demand and dump load.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jamie Hall
    Jamie Hall Member Posts: 23,322
    It's late at night, so a flip answer... I knew a Chief Petty Officer once who insisted that one could get soaped, scrubbed, and rinsed clean in 1 minute and 30 seconds by the watch. He was right...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • KC_Jones
    KC_Jones Member Posts: 5,739
    > @Jamie Hall said:
    > It's late at night, so a flip answer... I knew a Chief Petty Officer once who insisted that one could get soaped, scrubbed, and rinsed clean in 1 minute and 30 seconds by the watch. He was right...

    My father was in the Marines, add shaving and getting dressed to that list and change the time to 5 minutes. He’s 72 and has slowed down, but yeah the military gets it done. In boot camp they timed the head calls.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
  • Gordy
    Gordy Member Posts: 9,546
    With a family of four three of which were ladies I never had issues with running out of hot water with a conventional gas fired 40 gal. All though it was turned up.
  • KC_Jones
    KC_Jones Member Posts: 5,739
    Family of 5, 2 female 3 male. We routinely take 5 back to back showers with only a few minutes in between, dad (me) goes last we don't run out with a standard 50 gallon atmospheric.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,304
    edited February 2019
    Hello, I wonder if someone makes a 1.5 gpm shower head you could like? If so, it would help with length of showers. ;)

    Yours, Larry
  • Zman
    Zman Member Posts: 7,569
    You have a 40,000 BTU/HR hot water heater. It is only 62% efficient, so your output is 24,800 BTU Hr.
    If your water is coming into the building at 45 degrees and you need your shower water to be 105 degrees, that would mean your temp rise delta would be 60 degrees.
    24,800 / 60 / 500 (constant) gives you .83 GPM continuous.

    It gets a little more complicated with the storage side of things. Depending on the tank temp you maintain, the storage in the tank gives you a bit of a buffer. With a 2.5 GPM flow and only .82 GPM production, you are always running at a 1.68 GPM deficit. If you could get the full 40 gallons of storage out of the tank, (you will only get about 1/2 to 2/3) you would get a 23 (40 / 1.68) minute shower before things get cold.
    Your 10-15 minute showers sound about right.

    Your solution would be:
    Live with the shorter showers
    Install low flow shower heads
    Install heater with higher BTU output
    Install heater with more storage

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    Have taken 3 showers almost back to back awith a 50 gallon 38k BTU atmospheric in michigan with 40F entering water. Tank set ot about 125F.

    Heck, our 50 gallon electric does fairly well, 2 back to back showers, but 3rd one better be short. but I have a recirculation loop now so instant hot water and about 2 gallons extra storage essentially.
  • ch4man
    ch4man Member Posts: 296
    all of the above plus, all the calculations assume the shower head is using 100% hot water.... not happening.

    there are plumbing issues that appear to be water heater issues.
    take the hot water pipe temp right at the outlet of the water heater and the temp elsewhere ant the same time. if the point of use is cooler the there is some cold water bypassing into the hot side diluting the hot water.

    most water heater mfgs state a tank can provide 60-70% of the tank with no more than a 30 degree drop and be in specs.
    a 50 gallon WH should give 30-35 gallons of water thats within 30 degrees of starting temp. i have never got them to state "at what inlet cold water temp" those ratings are based.

    this time of year with the deep frost WH performance suffers
  • SuperTech
    SuperTech Member Posts: 2,166
    Agreed about WH performance suffering. I've seen more than usual this year, tank failure, mixing valve failure, fouled up tankless coil coils and plugged flat plate heat exchangers. I even had a nice one where a plumber ran a hot water supply line inside an exterior wall and it froze and burst. Why it never happened in prior years I can't explain.
  • GBart
    GBart Member Posts: 746
    Forget most of what you just read.

    The tan ratings are based on a temp rise as well, say, 70F, so from 50F water you go to 120 and then you have x for the first hour.

    In winter with city water you will have colder cold water, maybe even with a well. Take a 5 gallon bucket, measure the height and mark the half point, that's 2.5 gal. Run a shower into it for 1 minute and measure, that's your per minute gal, X 60 = per hour.

    By the time your kids grow you'll be needing a new one anyway, if you have a boiler go with an indirect water heater which can put out in excess of 200GPH, nothing touches them for output and efficiency. But if you run the test you'll know what you need per hour.
  • drooplug
    drooplug Member Posts: 26
    edited February 2019
    I've been using this 1.5 GPM shower head for years. Pressure is good, but the volume of water is noticeably less. I've been happy with it and my wife has never complained about it. The kids don't know anything else. :wink:

    JUGHNE Member Posts: 11,062
    For many years here it was thought that a 40 Gal NG WH with maybe the 39,000 btu input or whatever it was, before energy savings mandates came into effect, would provide all the hot water anyone needed. And it was so.
    The first 50 gallon tank I ever installed was in a dairy farm house for the family. They needed a new WH and wanted a upgrade.
    They had 5 boys and 2 girls, stairstep children including a set of twins. This means all the children were at home and covered just about every other grade in school. All sorts of activities in the evenings and for sure Sunday mornings. They had two 3/4 baths plus a full bath.
    They milked cows twice a day (this implies showering twice a day for everyone.)
    The 50 gallon tank was enough for all this activity. All the children are still alive today despite the short showers, thru strict training, they had to endure.
    The parents of those 7 children are younger than me. (we had only 4 kids).
    Wife and I survive now with a 120 gallon tank fed by a heat exchanger from an 80,000 mod con. Though her 50 gallon tub being constantly topped off can tax the system a little.

    Our world has certainly changed.
    My parents generation.....don't get me started!
  • Leonard
    Leonard Member Posts: 903
    edited February 2019
    Other issue is have to say where you live.
    City water here in southern NH comes in at 42 deg in February.
    In Florida it's ~ 70 degs

    In a heat exchanger to heat water the energy you need to add is:
    (GPM) x ( delta T) x (500.67) = BTU/hr

    So using my tanklless coil in boiler : raising 42 deg F water to 101 and flowing it out thru my 3 GPM shower head from the 70's needs ~ 87.1k BTU/hr