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  • My tankless tank.

    Just an experiment I did with my own tank heater. I've always found first hour recoveries between mfgs. to seemingly have inaccurate math to them.

    Any input is welcome. Remember though, I will not lie for any reason. Just report results. The test is not completely accurate either in my opinion. But here goes:

    I have a 40 gallon, 40,000 btu, A.O. Smith NG heater in a hut outside. The pipe distance from the top of the heater to the shower head is about 15'. No pipes are insulated.

    I wanted to time the amount out of the showerhead to point of failure to see how many hot gallons it produced. The max temps. on the thermometer measures about 128 degrees at the spout sitting in a cup. I DO NOT have the ground temp today. Thermometer starts at 100 degrees. (1st inaccuracy)

    So I took a shower at comfortable hot temp,(2nd inaccuracy-subjective) then the next person took theirs. I told her not to touch the temperature and leave it running when done.

    What I found. The temperature held for approxiamately 58 minutes before adjustment was needed to keep the temperature "feel". (Didn't measure with thermometer. 3rd inaccuracy-subjective again.)

    Then at around 78 minutes the temp. leveled off at warm and would not go cold for the next 10 minutes. Didn't get warmer either. Just stayed at the steady temp.

    I live on a well with a 30-50 switch control. Also there are two filters the main passes through as the water needs cleaning up. They currently need changing. The showerhead in no way has knockdown pressure but does have adequate pressure. Might take a pic. of it running sometime and add to the thread as proof.

    The numbers (78 minutes x 2.5 gpm=195gallons)seemed high to me so I checked the gpm on the showerhead. Got about 1.5 gpm. Some will completely disagree but it honestly looks/feels about the same as many 1st floor showerheads on municipal water I've worked on.

    So just to recap:

    40 gallon/40,00btu NG heater in an outside hut.
    30-50 psi well control.
    2 clogging filters.
    128 degree max. showerhead temperature.
    15' piping with whatever radiant heat loss.
    2.5 gpm Delta Showerhead.
    78 minutes of tempered usable water.
    1.5 gpm produced at the showerhead.

    After the 78 minute mark I could not or did not get the water to change temperature feel for 10 minutes and ended the experiment. It WAS still equally warm for the full 10 minutes.

    Conclusions for today: The heater will produce 1.5 gpm x 78 min. for a total of 117 gallons of useable temperatured water.

    Remember. I have no reason to lie. I've read first hour recovery's, btu's, flow charts, blah blah blah. I'm just reporting the results for anyone to learn from, dispute, discuss, & point out problems.

    One of the reasons I did this is I believe first hour recovery charts mean nothing. There are too many factors left out of them to give any degree of accuracy and every single installation is different & must be approached as such.

    J.C.

  • #2
    Re: My tankless tank.

    I think that you need a better thermometer to do this accurately. Don't leave it to your sense of touch.

    Comment


    • #3
      Re: My tankless tank.

      I think the next test I'll rig one up with the showerhead spraying on it with checks every 10 minutes.

      But remember too, in this first run it's what "felt" hot to the end consumers. So that's satisfactory gallons of tempered water.

      J.C.
      Last edited by BobsPlumbing; 02-23-2009, 07:07 PM.

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      • #4
        Re: My tankless tank.

        Yes I understand that... but my wife thinks I use cold water I think she is trying to peel her skin off.

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        • #5
          Re: My tankless tank.

          I know someone the same way. Will perform second wasteful test soon.

          J.C.

          Comment


          • #6
            Re: My tankless tank.

            Interesting ... if you are in the mood for wasting water ...

            I would be interested in running 2 showers at once. With one source running a tank may be able to keep up to a tankless, with more, the tankless "I would think" would start pulling away ...

            Back when I had a tank ... it was okay for one source draws. But as soon as multiple sources hit it, it simply could not keep up. There could be other factors impacting that (age of the tank, incoming water temp, etc) too.

            It would also be interesting to know what the "showering" water temperature was.

            Some thermometers can take a while before they will register a change.

            Interesting!

            Comment


            • #7
              Re: My tankless tank.

              Originally posted by theoak View Post
              Interesting ... if you are in the mood for wasting water ...

              I would be interested in running 2 showers at once. With one source running a tank may be able to keep up to a tankless, with more, the tankless "I would think" would start pulling away ...

              Back when I had a tank ... it was okay for one source draws. But as soon as multiple sources hit it, it simply could not keep up. There could be other factors impacting that (age of the tank, incoming water temp, etc) too.

              It would also be interesting to know what the "showering" water temperature was.

              Some thermometers can take a while before they will register a change.

              Interesting!
              I think you'll be right on two or more things operating. 40,000 btu's vs. around 200,000 btu's at x gpm will make the difference. Will try to get the test more accurate soon.

              I'm always trying to get the "real" answer for customers as opposed to just put in A-B-or C.

              I also have kept check on your tankless updates and have referenced a time or two. Thanks.

              J.C.

              Comment


              • #8
                Re: My tankless tank.

                jc here is the reason you got a lot of water.

                1.5gpm is not a realistic draw of hot water. 40,000 btu should be able to keep up for a long time as the cold coming into the tank gets mixed with the already hot water.

                do the same test at a bathtub running full blast. you will get approx 28 gallons of hot water above 100 degrees.

                or shut off the main burner and do the test.

                1.5 gpm with a 40,000 btu burner is the reason for the high numbers.

                rick.
                phoebe it is

                Comment


                • #9
                  Re: My tankless tank.

                  I have a 40 gallon super stor. I do not believe it is possible to run it out of hot water given the input vs the max draw.
                  sigpic

                  Comment


                  • #10
                    Re: My tankless tank.

                    Originally posted by NHMaster3015 View Post
                    I have a 40 gallon super stor. I do not believe it is possible to run it out of hot water given the input vs the max draw.
                    What is your boiler's input to it? I had a 120,000 BTU boiler with a 40 gallon indirect tank, and when I filled my large tub it would run out in less than 10 minutes. With my 80 gallon tank I've never run out.

                    Comment


                    • #11
                      Re: My tankless tank.

                      Originally posted by cpw View Post
                      What is your boiler's input to it? I had a 120,000 BTU boiler with a 40 gallon indirect tank, and when I filled my large tub it would run out in less than 10 minutes. With my 80 gallon tank I've never run out.
                      A 120,000 btu boiler coupled with a 40 gallon indirect tank will produce AT LEAST 200 gallons of 115 degree water the first hour. That sounds like plenty of hot water.

                      ...but

                      For immediate fill of a large tub, the usable capacity of the water heater must exceed the tub capacity.

                      Example:

                      Let's say your big tub holds 75 gallons.

                      Multiply that times .6 to get your tank capacity which is 45

                      A 40 gallon indirect fired heater will give you only 36 gallons of usable hot water. Not enough to fill tub.

                      A 60 gallon tank will give you 46 gallons of hot water. Cuts it pretty close if someone else wants hot water while you're filling tub.

                      An 80 gallon tank produces 56. Enough to fill tub a with plenty to spare.

                      Comment


                      • #12
                        Re: My tankless tank.

                        Originally posted by PLUMBER RICK View Post
                        jc here is the reason you got a lot of water.

                        1.5gpm is not a realistic draw of hot water. 40,000 btu should be able to keep up for a long time as the cold coming into the tank gets mixed with the already hot water.

                        do the same test at a bathtub running full blast. you will get approx 28 gallons of hot water above 100 degrees.

                        or shut off the main burner and do the test.

                        1.5 gpm with a 40,000 btu burner is the reason for the high numbers.

                        rick.
                        I'm not one of the smarter folks on the forum, but I did figure out that little mystery from the get go.

                        Don't know if I agree to say it's not a realistic draw of hot water at 1.5gpm. Just running a very usable shower. Gonna' change the filters on the next run though.

                        One point I'm drawing attention too is the first hour tables in water heater manuals & how they are inaccurate in real world use. I was able to get 117 gallons of tempered water out of a 40 gallon water heater. I am very aware that the gallon number will decrease as fixture volume & pressure increases. Which brings me to my original thought....

                        ....The tables in the manual should have temperature degree rise at x volume & y pressure for each water heater to give a real idea of what one could expect in each application.

                        Unless I missed it, my Rheem book does not have a table including this. If someone knows of one, please link it as I could use it.

                        Thanks,

                        J.C.

                        Comment


                        • #13
                          Re: My tankless tank.

                          Page 2 has a chart showing temperature rise for gas and electric heaters based on GPH and btu ratings

                          http://www.hotwater.com/lit/sizing/B2010.pdf

                          Just about all gas heaters are based on a 75% recovery rating

                          Comment


                          • #14
                            Re: My tankless tank.

                            Originally posted by plumberscrack View Post
                            Page 2 has a chart showing temperature rise for gas and electric heaters based on GPH and btu ratings

                            http://www.hotwater.com/lit/sizing/B2010.pdf

                            Just about all gas heaters are based on a 75% recovery rating



                            Thank you Mr. PC! Now I just have to do a little conversion into real world fixture use with pressure. (I think. Duhhhhh....)

                            J.C.

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