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  • Designing a large solar hot water system for a dorm.

    I am researching to build a hot water system for a dorm (housing 30-35 people) for a school located in Tijuana, MX. This is mostly hot water for showers, and some staff laundry. The dorm roof has a south facing slope and plenty of roof space.

    Where can I find information on ideal manifold size vs riser size? Ideal number of risers per linear/square ft of collector? And then ideal gpm flow for a large array of panels (250 sq ft)? The builditsolar site has most of the risers at 1/2" diameter with 6' spacing. I am looking at 3/8" diameter risers with 2" spacing. I'd assume more risers with a smaller diameter will give me a better heat transfer.

    Has anyone tried stranded uninsulated copper wire for use as heat sinks? I was thinking about trying to "weave" it across my collector every 1/2" or so. I can wrap it around a riser and spot solder it for good contact and then on to the next riser.

    If I use double layer reflective insulation behind my tubing, should I paint it or leave it silver? http://www.homedepot.com/p/Reach-Bar...3054/203536788 My idea was to build my copper pipe manifold with wire for heat sink and paint it black. I would place corrugated tin (leave it silver) behind it about 1"-2" (with a riser above each "valley" to act kind of like a parabolic mirror). Behind the tin would be the reflective insulation - followed by foam insulation. Maybe I don't need both the reflective insulation and the corrugated tin? I figured the tin would protect the insulation from UV damage and the angles may help direct reflected light.

    I have access to sheets of single pane tempered glass from patio doors. Which means my panels will be 45" x 75". A little smaller than most collectors but you work with what you got. Based on builditsolar I figure I need to build about 10 of these to generate enough hot water.

    I hope to use the existing LP waterheaters as backup heatsources. I still have to figure out my large storage tank. I am figuring on needing 275-350 gallons.

    I have not been able to view the draws and pictures attached to posts here because I do not have any posts yet. I did a intro post but I still cannot view them. Hopefully this post will allow for viewing as well as glean info from you guys. Please give input into my design plans and/or point me to more information.
    Last edited by Mike90250; 09-01-2014, 02:22 PM. Reason: added hot water to title

  • #2
    Originally posted by IowaBoy View Post
    I am researching to build a hot water system for a dorm (housing 30-35 people) for a school located in Tijuana, MX. This is mostly hot water for showers, and some staff laundry. The dorm roof has a south facing slope and plenty of roof space.

    Where can I find information on ideal manifold size vs riser size? Ideal number of risers per linear/square ft of collector? And then ideal gpm flow for a large array of panels (250 sq ft)? The builditsolar site has most of the risers at 1/2" diameter with 6' spacing. I am looking at 3/8" diameter risers with 2" spacing. I'd assume more risers with a smaller diameter will give me a better heat transfer.

    Has anyone tried stranded uninsulated copper wire for use as heat sinks? I was thinking about trying to "weave" it across my collector every 1/2" or so. I can wrap it around a riser and spot solder it for good contact and then on to the next riser.

    If I use double layer reflective insulation behind my tubing, should I paint it or leave it silver? http://www.homedepot.com/p/Reach-Bar...3054/203536788 My idea was to build my copper pipe manifold with wire for heat sink and paint it black. I would place corrugated tin (leave it silver) behind it about 1"-2" (with a riser above each "valley" to act kind of like a parabolic mirror). Behind the tin would be the reflective insulation - followed by foam insulation. Maybe I don't need both the reflective insulation and the corrugated tin? I figured the tin would protect the insulation from UV damage and the angles may help direct reflected light.

    I have access to sheets of single pane tempered glass from patio doors. Which means my panels will be 45" x 75". A little smaller than most collectors but you work with what you got. Based on builditsolar I figure I need to build about 10 of these to generate enough hot water.

    I hope to use the existing LP waterheaters as backup heatsources. I still have to figure out my large storage tank. I am figuring on needing 275-350 gallons.

    I have not been able to view the draws and pictures attached to posts here because I do not have any posts yet. I did a intro post but I still cannot view them. Hopefully this post will allow for viewing as well as glean info from you guys. Please give input into my design plans and/or point me to more information.
    Since the reflected light from the back is not guaranteed to hit a collector tube on the way back out, I would have a black surface on the upper side and an air gap between that that the foil insulation. Just painting the top layer of the foil black may not be the best use of the foil.

    I am not sure why you are looking at "heat sink" material in the collector. The ideal is to have the light heat the surface of the tubes and transfer that heat to the water or other fluid. Air to tube heat transfer is another story and for it you want as much area as possible with decent heat transfer. Aluminum gives you more bang for the buck than copper.

    You do not want your risers to be too small, both for the area they intercept and the resistance to fluid flow. As an example, a screen with 50% hole area will block air flow much worse than just covering the half of the area.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

    Comment


    • #3
      Originally posted by IowaBoy View Post
      I am researching to build a hot water system for a dorm (housing 30-35 people) for a school located in Tijuana, MX. This is mostly hot water for showers, and some staff laundry. The dorm roof has a south facing slope and plenty of roof space.

      Where can I find information on ideal manifold size vs riser size? Ideal number of risers per linear/square ft of collector? And then ideal gpm flow for a large array of panels (250 sq ft)? The builditsolar site has most of the risers at 1/2" diameter with 6' spacing. I am looking at 3/8" diameter risers with 2" spacing. I'd assume more risers with a smaller diameter will give me a better heat transfer.

      Has anyone tried stranded uninsulated copper wire for use as heat sinks? I was thinking about trying to "weave" it across my collector every 1/2" or so. I can wrap it around a riser and spot solder it for good contact and then on to the next riser.

      If I use double layer reflective insulation behind my tubing, should I paint it or leave it silver? http://www.homedepot.com/p/Reach-Bar...3054/203536788 My idea was to build my copper pipe manifold with wire for heat sink and paint it black. I would place corrugated tin (leave it silver) behind it about 1"-2" (with a riser above each "valley" to act kind of like a parabolic mirror). Behind the tin would be the reflective insulation - followed by foam insulation. Maybe I don't need both the reflective insulation and the corrugated tin? I figured the tin would protect the insulation from UV damage and the angles may help direct reflected light.

      I have access to sheets of single pane tempered glass from patio doors. Which means my panels will be 45" x 75". A little smaller than most collectors but you work with what you got. Based on builditsolar I figure I need to build about 10 of these to generate enough hot water.

      I hope to use the existing LP waterheaters as backup heatsources. I still have to figure out my large storage tank. I am figuring on needing 275-350 gallons.

      I have not been able to view the draws and pictures attached to posts here because I do not have any posts yet. I did a intro post but I still cannot view them. Hopefully this post will allow for viewing as well as glean info from you guys. Please give input into my design plans and/or point me to more information.

      Is the system being used every day? Will it ever be subject to freeze up (doubtful in MX, I know).

      The rough rule of thumb (sorry for the metric but it works well here) is 1m2 of panel for each person to give, in your area, probably 80% of the hot water needs or one standard 4x8' panel for 3 people. 10 panels is as many as you should do in a row and leave the last output free to move because you could get up to perhaps 2" of movement overall. The sizes are not hard and fast rules, just guidelines.

      If you want to build it yourself, stick to 1/2" risers and 1" headers, risers spaced 150mm (6") o/c. You will not get any more heat out of a closer spacing. If you did, all the big makers would be doing it too, especially the Germans who go for every last watt. You won't need to do the fancy stuff to get good performance out of it where you are so i would just get the aluminum wrapped around the riser (pipe in a groove). Get the risers brazed to the header, not soldered or you will be back fixing leaks for years.

      Your fins don't need to be thicker than .040 aluminum or copper (if you want to spend the bucks) and get a gallon of Solkote II which is a very good efficient selective surface.

      As for the tank, anywhere between 50 to 100L/m2 of collector area is needed. If you are doing a drainback, it can be less as there is no need to remove the heat as there is with a glycol system.

      For the flow rate, another rough rule of thumb is between .3 and .6L/min per m2 of collector area. Most people go on the higher side of this but it means either bigger piping to the panels or a bigger pump (or both). Higher flow rate means lower output temps but similar amount of heat transferred.

      Comment


      • #4
        Originally posted by MikeSolar View Post
        Is the system being used every day? Will it ever be subject to freeze up (doubtful in MX, I know).

        The rough rule of thumb (sorry for the metric but it works well here) is 1m2 of panel for each person to give, in your area, probably 80% of the hot water needs or one standard 4x8' panel for 3 people. 10 panels is as many as you should do in a row and leave the last output free to move because you could get up to perhaps 2" of movement overall. The sizes are not hard and fast rules, just guidelines.

        If you want to build it yourself, stick to 1/2" risers and 1" headers, risers spaced 150mm (6") o/c. You will not get any more heat out of a closer spacing. If you did, all the big makers would be doing it too, especially the Germans who go for every last watt. You won't need to do the fancy stuff to get good performance out of it where you are so i would just get the aluminum wrapped around the riser (pipe in a groove). Get the risers brazed to the header, not soldered or you will be back fixing leaks for years.

        Your fins don't need to be thicker than .040 aluminum or copper (if you want to spend the bucks) and get a gallon of Solkote II which is a very good efficient selective surface.

        As for the tank, anywhere between 50 to 100L/m2 of collector area is needed. If you are doing a drainback, it can be less as there is no need to remove the heat as there is with a glycol system.

        For the flow rate, another rough rule of thumb is between .3 and .6L/min per m2 of collector area. Most people go on the higher side of this but it means either bigger piping to the panels or a bigger pump (or both). Higher flow rate means lower output temps but similar amount of heat transferred.
        A couple of items:

        I'd make the risers 1/2' as well. One thing might be worth clearing up unless I'm missing something: For flat plate collectors, fins are essential. Those fins need the following characteristics:
        - They need to be a dark color. As suggested, Solkote is probably as good or better than some.
        - They need to be METALLURGICALLY bonded to the risers - soldered, brazed, ultrasonic etc. Laying or simply clamping and or gluing the risers to the fins is next to useless. Bonding paste or heat transfer cement is little better. There is a LOT of sound engineering behind that statement and something I've spent many years studying, experimenting, evaluating and specifying materials and situations for industrial heat exchanger applications. Solar thermal is almost identical.
        - They are best if the same material as the risers. This usually means copper.

        Make sure the insulation, coatings and other non metallic materials do not outgas. If they do, they will quickly coat the interior of the glazing. Not good.

        Don't use wood for anything, especially collector frames. The heat, regardless of insulation, will dry it out, lower its kindling temp. and start a fire. Trust me, I did it many years ago. Also, make sure any and all other materials are temp. suitable.

        Consider the flow arrangement carefully. Piping design is a balance of many variables and competing priorities. Compromises, choices and balancing of needs is the name of the game. FWIW, if you can arrange the design and flow layout such that the riser flow is turbulent and uniformly distributed rather than laminar, you'll increase thermal performance by something like ~ 10%. I'd like to tell you how long it took to measure, calc and figure that one out, but I did, and have #'s and data to prove it. There are other advantages as well, and a few drawbacks.

        Watch out for freeze ups. Frosts do occur around here 1 or 2 times/yr. It only takes one and you're S.O.L. CA effectively mandates closed loop systems with heat exchangers to effectively kill the freeze problem. You, IMO anyway, have an advantage - you're in MX. Depending on your control logic, you may be able to avoid the HX with it's added complexity and cost and use a drainback system or instead, have a system that circulates potable water through the collectors, using controls that recirculate heated water through the collectors when panel temps. drop close to freezing. Sounds like a waste, and it tends to be more wasteful in northern climates. Around here, mine operates about 20-30 min./yr. A lot cheaper/easier than a H2O/glycol HX with all the hassle. Just have a backup scheme. (BTW, my system's been around since before HX closed loop became defacto mandated.

        Get yourself a copy of Duffie & Beckman - the solar thermal bible. All of what I speak of above and much more is in there.

        Remember, the devil is in the details. What you don't know not only can, but will get you.

        Good luck.

        Comment


        • #5
          Even better for the layman is:

          http://books.google.ca/books?id=uTKb...page&q&f=false

          A more current bible for solar that has a LOT of do's and don'ts from 40 years of German panel making. It is well worth the money. Viessmann also sells it under their name. (you will have to google a seller. I bought mine many years ago from the authors)

          I would argue that you can use wood for frames, with provisos: It must be well insulated away from the absorber. The country with one of the highest per capita installations of solar thermal systems, Austria, has a very high degree of wood framed collectors working very well.

          Comment


          • #6
            This manual may help
            Attached Files

            Comment


            • #7
              After rereading the OP's 1st post and other posts to this thread that followed, including mine, some further thoughts on DIY solar thermal collectors and overall system design:

              1.) Unless you have a lot of experience, don't do it for anyone other than yourself. Leave design work for projects that have potential to harm others to professionals - Designers, Tradespeople and Engineers who do that sort of thing for a living.

              2.) A layman may have use for a book with a lot of do's & don't, but I'd suggest and hope someone building a system for use in what sounds like the public sector would be more than a layman in the subject and thus already passed through the basics.

              Perhaps an analogy: Kind of like thinking you can design automobiles by virtue of having read the manual that came with your vehicle.

              Sound engineering needs more than a cook book design mentality.

              3.) How to manuals and sites like builditsolar are fine for DIY, I have quite a few such manuals and particularly recommend the reference LucMan gave. That source is entirely fit for purpose, but is for op. & maint., not DESIGN as stated in that manual. much as a general survey and, as with other manuals of that type, will probably keep you out of trouble. But for serious, safe, fit for purpose solar thermal design and engineering some knowledge of first principles is necessary. That's what sources like Duffie & Beckman, Lunde, Meinel & Meinel, Kreith and others were written for.

              If I was the owner or in responsible charge of a facility, I'd damn sure want the designers of that facility's systems to be capable of not only owning and using how to manuals for reference (not design) in a pinch, but indeed, due to their knowledge and experience, being capable of writing such manuals as well if they so choose. I wouldn't want a layman.

              4.) To the OP:
              Seriously, don't make the collectors, buy them. It will be safer, cheaper, better quality, longer lasting and better fit for purpose than you can do yourself or design. (and BTW, they probably won't have any wood in them any more than today's water tanks are made out of wood by coopers).

              As for the rest of the design, unless you are an experienced designer of fluid systems and solar thermal systems, don't do it. Get education in the basics of science and engineering and then hold the tools and look over the shoulders of professionals. You'll learn a lot and things will be safer and better as a result.

              Comment


              • #8
                J.P.M.,
                How does California "effectively mandate a close loop system"?

                Comment


                • #9
                  Originally posted by MikeSolar View Post
                  J.P.M.,
                  How does California "effectively mandate a close loop system"?
                  Correction: Change that from "closed loop system" to "Direct Forced Circulation systems".

                  My nomenclature is different than used in the CSI thermal program handbook. I apologize for any confusion caused.

                  Para. # 2.4.1 (a), p. 13, and pars #12.2, pp. 86 and 87 of the handbook exclude systems that circulate potable H2O through a collector system without an external heat exchanger or double freeze protection, one of which (freeze protection methods) can be manual, but the other one must, among other requirements have a separate pump that is powered by an uninterruptible power source. Freeze protection is also allowed by drainback which takes very careful piping design and some perhaps unusual and/or additional tankage and other considerations. They are also sometimes rather noisy.

                  While all these methods of dealing with freeze protection are workable from a design standpoint, as a practical and financial reality, they make the simpler methods of freeze protection (some of which are, as a matter of some opinion, viable and reliable), ineligible from any CSI rebate. That ineligibility is what I was referring to by my statement of CA effectively mandating what I incorrectly called "closed loop systems". I would guess that few would choose a system that excludes a rebate over one that allows a rebate for what they view as the same duty.

                  As a practical matter, direct forced circulation systems (to use the CSI definition and terminology) using recirculation schemes for freeze protection with manual dumping as backup can still be installed, but ineligible for any rebate funding.

                  A somewhat ironic twist to all this may be that all the extra equipment (heat exchangers, separate pumps, uninterruptible power supplies, extra piping, tankage, non standard piping arrangements, etc) added to secure a CSI thermal rebate probably increases the system cost at least as much or more than any realized CSI rebate would offset, making direct forced circulation systems perhaps as economically viable without a rebate as the others with a rebate. Most folks have not a clue about this state of affairs and assume big brother knows best. I'd bet the vendors love it as a way to add equipment (and $$'s)to a system and (with some justification IMO) blame the gov.

                  Things may be getting back to a more normal state of affairs as the CSI thermal funding for electrically heated DHW systems has been used up. Nat. gas systems still have funding however.

                  That's the essence of my reasoning.

                  Thank you for catching my mistake with respect to definitions.

                  Comment


                  • #10
                    Originally posted by J.P.M. View Post
                    Correction: Change that from "closed loop system" to "Direct Forced Circulation systems".

                    My nomenclature is different than used in the CSI thermal program handbook. I apologize for any confusion caused.

                    Para. # 2.4.1 (a), p. 13, and pars #12.2, pp. 86 and 87 of the handbook exclude systems that circulate potable H2O through a collector system without an external heat exchanger or double freeze protection, one of which (freeze protection methods) can be manual, but the other one must, among other requirements have a separate pump that is powered by an uninterruptible power source. Freeze protection is also allowed by drainback which takes very careful piping design and some perhaps unusual and/or additional tankage and other considerations. They are also sometimes rather noisy.

                    While all these methods of dealing with freeze protection are workable from a design standpoint, as a practical and financial reality, they make the simpler methods of freeze protection (some of which are, as a matter of some opinion, viable and reliable), ineligible from any CSI rebate. That ineligibility is what I was referring to by my statement of CA effectively mandating what I incorrectly called "closed loop systems". I would guess that few would choose a system that excludes a rebate over one that allows a rebate for what they view as the same duty.

                    As a practical matter, direct forced circulation systems (to use the CSI definition and terminology) using recirculation schemes for freeze protection with manual dumping as backup can still be installed, but ineligible for any rebate funding.

                    A somewhat ironic twist to all this may be that all the extra equipment (heat exchangers, separate pumps, uninterruptible power supplies, extra piping, tankage, non standard piping arrangements, etc) added to secure a CSI thermal rebate probably increases the system cost at least as much or more than any realized CSI rebate would offset, making direct forced circulation systems perhaps as economically viable without a rebate as the others with a rebate. Most folks have not a clue about this state of affairs and assume big brother knows best. I'd bet the vendors love it as a way to add equipment (and $$'s)to a system and (with some justification IMO) blame the gov.

                    Things may be getting back to a more normal state of affairs as the CSI thermal funding for electrically heated DHW systems has been used up. Nat. gas systems still have funding however.

                    That's the essence of my reasoning.

                    Thank you for catching my mistake with respect to definitions.
                    I certainly didn't catch any mistake you may have made. I really didn't know the rules in Cali. From what you have written, it seams it only applies to getting rebates and not for code purposes so if someone didn't care about the rebates, they could put anything they wanted in, I assume.

                    I wonder if one of those old sunspool draindown thingies would be allowed?

                    Comment


                    • #11
                      Originally posted by MikeSolar View Post
                      I certainly didn't catch any mistake you may have made. I really didn't know the rules in Cali. From what you have written, it seams it only applies to getting rebates and not for code purposes so if someone didn't care about the rebates, they could put anything they wanted in, I assume.

                      I wonder if one of those old sunspool draindown thingies would be allowed?
                      My guess would be that if valves of that type are in an existing system, they're probably grandparented in. However, my understanding, perhaps incorrect, is that they are no longer made. If that's the case, the point of their acceptability as part of a CSI rebated system is moot. Other dump valves may be suitable, provided they can be energized with an independent and uninterruptable power source in the event of a power outage.

                      I'm a bit out of my element on drainback type systems as I've not designed any. As to code acceptability, and as I wrote, my understanding is that direct forced circulation systems (single loop, potable H2O through the collectors), as defined by CSI, can still be installed, just ineligible for state rebates. Around here, to my observation and IMO only, that's a deal killer for most of the solar ignorant, which is most everyone, and why I said it was an effective CA mandate for complication.

                      One other point I left out: Many areas in CA are subject to freezing temps. for a significant portion of the year, making direct forced circulation systems circulating potable H2O through the collectors much less practical. In that case and for those areas, if it was me, I'd use H2O/prop. glycol as the collector loop working fluid and a HX interface with the potable loop, or maybe no solar thermal, depending on cost and the PITA factor(s).

                      Comment


                      • #12
                        Originally posted by J.P.M. View Post
                        My guess would be that if valves of that type are in an existing system, they're probably grandparented in. However, my understanding, perhaps incorrect, is that they are no longer made. If that's the case, the point of their acceptability as part of a CSI rebated system is moot. Other dump valves may be suitable, provided they can be energized with an independent and uninterruptable power source in the event of a power outage.

                        I'm a bit out of my element on drainback type systems as I've not designed any. As to code acceptability, and as I wrote, my understanding is that direct forced circulation systems (single loop, potable H2O through the collectors), as defined by CSI, can still be installed, just ineligible for state rebates. Around here, to my observation and IMO only, that's a deal killer for most of the solar ignorant, which is most everyone, and why I said it was an effective CA mandate for complication.

                        One other point I left out: Many areas in CA are subject to freezing temps. for a significant portion of the year, making direct forced circulation systems circulating potable H2O through the collectors much less practical. In that case and for those areas, if it was me, I'd use H2O/prop. glycol as the collector loop working fluid and a HX interface with the potable loop, or maybe no solar thermal, depending on cost and the PITA factor(s).
                        I've been doing solar thermal work for the last 25+ years and, where I am, solar thermal lives and dies by rebates. Feast or famine. We have always looked to Germany and California with envy. We always get the question about payback and with really cheap gas prices, it is a pain to get people to accept a 15+ year payback. So....the effort is to design a system that cuts the cost in half without sacrificing longevity and performance. Not much has been done to accomplish this, IMO so I have developed a "drain down" system that, I hope will drop the cost 40% or better. I mentioned the "sunspool" because it was a good system until it failed and it will fail. But it eliminated the need for HX, non standard tanks, glycol, exp tank, fill valves, etc. and so the price was good.

                        The one I have developed (but not quite ready for prime time) does this too and has a failsafe to drain if there is a problem. I wondered what restrictions I would have marketing it in California other than the obvious NSF approvals and ratings etc. In Canada, a typical 2 panel SDHW system is about $8-10k installed (without rebates) and I was wanting to figure out what the market would be like for GOOD system installed at $5-6K.

                        Comment


                        • #13
                          Originally posted by MikeSolar View Post
                          I've been doing solar thermal work for the last 25+ years and, where I am, solar thermal lives and dies by rebates. Feast or famine. We have always looked to Germany and California with envy. We always get the question about payback and with really cheap gas prices, it is a pain to get people to accept a 15+ year payback. So....the effort is to design a system that cuts the cost in half without sacrificing longevity and performance. Not much has been done to accomplish this, IMO so I have developed a "drain down" system that, I hope will drop the cost 40% or better. I mentioned the "sunspool" because it was a good system until it failed and it will fail. But it eliminated the need for HX, non standard tanks, glycol, exp tank, fill valves, etc. and so the price was good.

                          The one I have developed (but not quite ready for prime time) does this too and has a failsafe to drain if there is a problem. I wondered what restrictions I would have marketing it in California other than the obvious NSF approvals and ratings etc. In Canada, a typical 2 panel SDHW system is about $8-10k installed (without rebates) and I was wanting to figure out what the market would be like for GOOD system installed at $5-6K.
                          I envied people in CA for a long time too. So I moved here. Weather's great. Gov. messes w/your head a bit more here, but maybe not as much as Bubba & John Birch might claim.

                          On progress:
                          A lot has actually been done to improve longevity and performance (and other attributes) of solar heating equipment. The problem, perhaps from your perspective, and mine is that most of that progress happened before we got here. People have been putting water through conduit for several thousand years and heating stuff with the sun for at least as long. Solar thermal is a pretty well developed state of affairs. Plumbing and fluid transport systems even more so. There's not much new under the sun.

                          On cost, payback, prices and subsidies:
                          Until the world of alternate energy collectively improves its mouse trap technology enough so that it can honestly demonstrate the ability to stand on its own - devoid of the need to drag off the government tit of subsidy, rebate and tax shelter (not unlike the oil, coal and nuke business as some, including those knowledgeable about the tax code and other types gov. interference might argue), that largesse will be a way of life if it (the alternate energy world) is to survive and, I'd suggest, not unlike an addiction.

                          Good luck in your endeavors. Come up with the better mousetrap, build one and demonstrate that it's better to support your claims.

                          Don't be too surprised if someone has already thought of your idea. Happens all the time - to me anyway.

                          Comment


                          • #14
                            Suggestion, unless you are a thermal engineer, or have some very good, proven plans, leave the collector design to the pros. Using copper, you have to monitor PH and make sure you are not dissolving the pipes. You should have a large, insulated, stainless steel tank for storage, 100 - 200 gallons worth. And you will need backup heat.
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