Dual Water Heaters

Home Forums Public Forums General Plumbing Dual Water Heaters

Viewing 17 reply threads
  • Author
    • #273720
      Avatar photoAnonymous

        My large home has two gas hot water heaters piped in series, with a hot water recirculation line. Question #1: Should the recirc line return to the bottom of the first (input) heater or the second (output) one? Question #2: How do you select the best (most comfortable, most efficient) temperature settings for the two heaters?

      • #288387
        Avatar photoJerry Peck

          This sounds like your water heaters are set up for maximum gallonage capacity for hot water storage, with the recirculation pump to keep hot water only just a few moments away from every hot water point of use.

          With this being the case, I would recommend piping the recirculating system to the second water heater in the series to provide the maximum stored gallonage of consistent hot water. This should also be more efficient for fuel use. The first storage tank will not come on as a result of heat loss due to the recirculating system piping, however, it is always available to provide back up hot water for peak use periods.

        • #288388
          Avatar photoTommy Tyler

            Thanks, Jerry. That’s the way it is piped now. I assume you set the output heater to the temperature of hot water you prefer, then set the input heater lower, but how much lower? A lot, or just a little bit? Clue: There are only two occupants in the house, so normal hot water demand is not excessive.

          • #288389
            Avatar photoSylvanLMP

              I happen to like piping my hot water heaters (2 or more) in parallel with all piping equal for proper draw with a heat sink of 30″ (28″ according to the book) to prevent stratification of hot water into the colder line (Supply).

              Remember heat goes to cold so during non usage time the hotter water will transfer
              (natural cavatition) to the colder tank and you will have transfer between these two tanks VERY inefficient.

              Here your preheating water to go from one tank to the next with lots of stand by losses and ILL bet you a donut you didn’t put in the loop for a heat sink ( NEVER A CHECK VALVE)

              If one tank fails in Series your system is DOWN END OF GAME , in a properly piped parallel situation you can isolate one tank via valves.

              No preheating is needed both tanks are set for the desired temperature as they draw equally.

              I happen to like my return piping piped into the cold water supply to temper the water entering the tank, less possibility of thermo shock, BUT Hey that’s my hang up from being a certified boiler and pressure vessel inspector.

              To fine tune your system you can add (3) a few gauges to check the outlet temperature of both heaters then the actual temperature going into the twined in piping (header)

              Have fun.

            • #288390
              Avatar photoDONSPLACE

                This question is sort of like my own, except that my system is still on the drawing board.
                I want to preheat the incoming cold water with a solar tank, and have the electric heater downline from that.
                I will build a ‘coffin’ at ground level, insulated and with reflective interior. In that I’ll put a recycled waterheater from a scrap pile and plumb it in a straight line, input through the tank, to output, to the heater.
                Any ideas as to the problems and/or wisdom of doing that?

              • #288391
                Avatar photoJMattero

                  I have often thought about preheating water with solar heat, especially in the summer. I have a Southe facing brick wall on the side of my garage that, I think, would be a great thermal mass to draw from. Where did you get information on this type of setup? I am in suburban Philadelphia, so I am not sure if this would work in the winter, since I am simply thinking about putting something similar to a car radiator against this brick wall, and running the incoming water line to it and then to my hot water heater. I guess I wopuld have to create a bypass for the winter months. Anyway, good luck!

                • #288392
                  Avatar photoSylvanLMP

                    Using solar energy to preheat the water is a great idea in certain parts of the country/world

                    Using a 2nd hand heater even for a storage tank is a BAD THING.

                    Most if not all residential tanks are not made to an ASME rating unless specified and they do cost more (lots more stringent testing and welding procedures)

                    A hot water tank is cheap and you can order one just for solar heating applications with all those cute bells and whistles like a circulator (Bronze or other non ferrous type of course)

                    Plus even the storage tank will require some type of pressure/temperature protection.

                    In this kind of application you would want to pipe in series as you want the preheated water to temper the incoming cold water when conditions are right YET you have the option of electric or gas back up.

                    The other piping arrangement was installed improperly BUT a home inspector is not really trained to know why or how to pipe in gas appliances.

                    There are several proven cost effective piping applications that solar power is designed for, just take your time and do your homework.

                    If you have any questions feel free to E mail me or contact a licensed Master plumber in your area NOT a handyman.

                  • #288393
                    Avatar photoHarold Kestenholz

                      On average, one square foot of area on a good quality solar panel is able to raise one pound of water from ground water temperature , about 50F, to 120F in one hour. Thus in an 8-hour solar day, the one square foot of solar panel will raise the temperature of one gallon of water.

                      This is the least understood feature of solar water heating. Wishful thinking or magical thoughts lead to the belief that the sun is able to heat a lot of water with only the surface of one side of a tank or a few square feet of radiator. The tank-in-a-box was one idea. The radiator in a glass-covered wall was one of the first sold.

                      Experimentation is not to be discouraged; but it is possible to research solar design. This was well-documented folowing the energy crisis in 1974. There were all sorts of schemes to put one roll of copper or plastic tubing on the roof and save a fortune. An internet search for solar influx and design will bring a wealth of information gleaned from 25 years of research.

                      See: http://www.solaraccess.com/samessageboard/default.asp http://energy.sourceguides.com/businesses/byP/solar/solar.shtml http://www.ases.org/

                      They can give a quantum leap in the opinion of how much work is already done in this research.

                    • #288394
                      Avatar photodaveroconn

                        Slyvan is right on the money about these two heaters being plumbed in series with same length piping.

                        This solar system sounds like a heat exchanger set up that we use to capture tempered reverse osmosis reject water. We use reject water to preheat incoming city water before the heaters.

                        Use thermometers in separable brass wells placed pre and post of your solar tank this will enable you to calculate your temperature differential. The higher the differential the better the efficiency.

                        Respectfully David F. Walling

                      • #288395
                        Avatar photoJerry Peck

                          Being as this setup is for maximum gallonage of hot water, the two tanks should be set to the same temperature.

                          However, being as there are only two of you and maximum gallonage is not needed, just crank the first water heater down to around 100° f. If you ever need the extra gallonage capacity, at least this will not be putting ‘cold’ water into the second tank, it will already be ‘warm’ and will not take as much time to heat up.

                        • #288396
                          Avatar photoSylvanLMP

                            Excuse me BUT I know for a fact that this has to be a misprint NO way anyone would GIVE type of advice.

                            No REALLY QUALIFIED professional “plumber” would ever advise a temperature of LESS then 140 degree’s if they bothered to read the ASSE or ASPE standards.

                            Even the plumbing codes state 140-160 wash water and 180 for sterilization of in lieu of chemicals. 140 degrees will kill most pathogens and other great stuff like Legionnaire disease.

                            The AGA is concerned about BURN lawsuits and they cant get the public to understand to take responsibility for their own actions so they lowered the water BUT increased this other very real health hazard. Hey for ever action there is a reaction HUH?

                            Of course not every one is a Qualified “plumber” and therefore they wouldn’t KNOW of the other codes and health department requirements.

                            As the temperature goes down the health risk goes up in these pathogenic incubators But hey they are saving you a few pennies HUH?

                            By the way Thank you Dave. SEE WE do agree on the important stuff LOL

                          • #288397
                            Avatar photoTommy Tyler

                              I think you misunderstood, probably because you’re hung up on piping the two heaters in parallel instead of series. Jerry wasn’t suggesting my hot water final temperature setting, he was suggesting the output temperature of the pre-heat water heater, which is what I asked for in the first place. I would still set the second heater to 140 degrees or whatever temperature hot water desired. Thanks again, Jerry. As it turns out, that’s about where my plumber had set it originally. He also told me the output heater would do most of the work (true) and not to be surprised if it needed replacement long before the pre-heater.

                            • #288398
                              Avatar photobungie

                                Sylvan is right. You cannot set the first tank below 140f (65c)The first tank in the line can then brew legionella. You might be lucky for a while and the second tank might kill it as it flow through. But as soon as the second tank blows a fuse or you lose power, you have the bugs in your hot water supply right up to the taps.

                                LEGIONELLA KILLS

                              • #288399
                                Avatar photoTommy Tyler

                                  bungie, that doesn’t make sense! If the first heater could grow bugs because it’s set below 140F, then why wouldn’t I have bugs in my cold water supply, right up to the taps? Can you quote an authoritative reference on this?

                                • #288400
                                  Avatar photoHarold Kestenholz

                                    Tommy Tyler, a web search for the word Legionella brings untold numbers of such references, for instance: http://www.dhmh.state.md.us/publ-rel/html/0800legion.htm

                                    Like most pathogens, they need to be in sufficient numbers to overcome immune systems.

                                    Most gas hot water heater manufacturers do not provide controls on new products that are able to be set below 120F.

                                  • #288401
                                    Avatar photodaveroconn

                                      THIS IS IMPORTANT!!!!

                                      The organism does not pose a threat to society until it becomes an opportunistic respiratory pathogen. A healthy person or animal that either drinks water containing the bacteria or breaths in the air containing the bacteria cannot become ill from L.D. The bacteria which is known to be present in water supplies is harmless until the environment is changed to an acid condition with a pH level between 6.6 and 6.9, combined with a temperature at or above 100°F. Hot water temperatures at or above 140°F will kill the organism. In cold water temperatures the organism is present but does not reproduce or become a pathogen.

                                      Once you get this bug in your system it’s just looking for the right conditions then it kills everyone it comes in contact with. Tommy, listen to Sylvan don’t create an environment for this bug to live in. The bug is in your cold water supply and everywhere it is just waiting for the right conditions to become a killer.

                                      Respectfully David F. Walling

                                    • #288402
                                      Avatar photoTommy Tyler

                                        Thanks bungie, Harold, and David. I’m indebted to you guys for making me aware of this problem. As we speak I’m raising my temperature settings, and plan to install some temperature gages.

                                      • #288403
                                        Avatar photoJerry Peck

                                          NEVER set the water heater temperature to OVER 120° f when using the water directly (no mixing valve) from the hot water outlet of the water heater. This is not a commercial installation, this is a residence, you do not want to cause 2nd and 3rd degree burns do you?

                                          Here is why:

                                          Just thought I would share this with those that do not have it. It’s been useful for me in the past.


                                          It takes over 5 minutes to get 2nd and 3rd degree burns from water which is 120 degrees.

                                          Water which is 130 degrees will produce 2nd and 3rd degree burns in about 30 seconds.

                                          Water which is 140 degrees will produce 2nd and 3rd degree burns in less than 5 seconds.

                                          Water which is 150 degrees will produce 2nd and 3rd degree burns in about 1 1/2 seconds.

                                          Water which is 160 degrees will produce 2nd and 3rd degree burns in about 1/2 second.


                                          The above is from A. O. Smith literature and warning labels.

                                          I was not worried about stagnate water because the two tanks are in series. The water is constantly being changed.

                                          In SylvanLMP’s parallel plumbing setup, the water in one tank could end up never being used, however, that is not the case with the series plumbed system.
                                          [Edited by Jerry Peck on 08 November 2000]

                                      Viewing 17 reply threads
                                      • You must be logged in to reply to this topic.

                                      Pin It on Pinterest

                                      Share This