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  • Q's About Adding Rooftopox Water Heater

    I'd like to supplement my existing electric hot water heating system with a rooftop solar system in order to reduce electrical use/cost. I've been searching the net for "How-To's", but haven't found much of anything useful in terms of understanding design. Everything I've found are advertisements for systems and they don't explain whether they are independent or not. What are some good sources for learning about these things. I need to understand if I can connect a rooftop box-type heater into the existing electric heater and how that is done mechanically. Also, should these systems be drained in wintertime? I live in Virginia Beach and have a porch roof attached to the garage where the heater is located. The porch faces southeast and gets no shade. It has a new flat roof with rubber membrane. Like most, I'd like to keep installation costs down, but I will hire a plumber and electrician if needed.

  • #2
    if your roof can take the weight, and your climate is not too frosty, a roof-top batch system is about as simple as it gets.
    http://www.rheem.com/products/water_...water_heaters/
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    • #3
      Check out a site called "builditsolar.com". Lots of info there.

      Batch or "breadbox" heaters are relatively inexpensive and a decent 1st project for those with plumbing skills. Designs are many and pretty freeform.

      The basic idea is that something/anything under glass in the sun will get hot. If in an insulated enclosure with a combination of fixed and moveable insulation to reduce losses when the sun is not shining/at nite, a lot of the mass of water will retain more heat for a longer period of time. Example: An old water heater stripped of insulation, painted flat black and placed in an insulated wood box with a couple old single or double glazed storm doors for a top & equator facing front , with removable/hinged and insulated top and equator facing front, all to prevent night and cold weather loss, will put a big dent in an electric bill. They do work. I've built a couple back in the day.

      Before that however, lower the thermostat setting on the water heater, get low flow shower heads and fixtures, insulate the living crap out of the existing tank and insulate the hot water lines. Before all that, if you have one of those hot water recirc. systems that gives you instant hot water at any tap, tear it out. Biggest and most flagrant waste of energy around, made doubly worse if heating H2O with electricity. Basically, lowering the load a solar device needs to meet will make it easier and less expensive to build and maintain the solar device.

      As mike suggests, be careful about putting heavy devices on any roof, particularly a flat roof that probably wasn't designed for much of a dead weight.. Also keep freezing conditions in mind. They will happen. Expect it. Virginia beach is not a terribly severe winter climate, but freezing conditions will only need to happen once.

      In any and all cases, check with local building authorities to ensue safety and conformance to local building codes.

      Good luck. With your skills, some information and some common sense, you'll be fine and probably learn a lot along the way. Most of this is a long way from rocket science. Just think safety before all else.

      Almost forgot: Welcome to the neighborhood.
      Last edited by J.P.M.; 03-18-2017, 11:42 PM. Reason: Added welcome.

      Comment


      • #4
        http://www.builditsolar.com will give you some ideas.
        I suggest you avoid rooftop batch heaters if there is any chance of freezing temperatures.

        Comment


        • #5
          Thanks for the replies. After receiving email notification of the first response, I followed the provided link to rheen and didn't see any system identified as "batch" solar heater, so I researched batch soloar water heating online. That's the type of system I was envisioning. Simple in concept and cost. After reviewing a few videos and documents, it dawned on me that these batch systems would be nothing more than extra storage unless the cold water supply entered the solar tank first. I went to bed contemplating this. The videos don't discuss plumbing, but I was able to find and confirm my suspision on page 13 here: http://ir.library.oregonstate.edu/xm...95F?sequence=1 . That design makes sense. I haven't gone through the Oregon State design in detail, but it looks like a great source, coupled with the videos like this; https://www.youtube.com/watch?v=F6BvsxFPN2o .

          Regarding freezing, the Oregon document states that insulated polybutylene piping creates a "freeze tolerant" system. In a test, their pipes froze, but did not burst. I'm more concerned with the loss of heat in the solar tank on winter nights. Sure it's insulated, but won't the water cool downcausing it to flow back into the electric tank where it will be reheated by electricity. I wonder if there's a temperature sensitive valve to prevent such things. Anyway, good info.

          I rebuilt my porch roof last summer using treated 2x6 joists on 16" centers. They're on hangers where they butt up against a ledger board on the house. I don't know what rating that would calculate to, but I suspect it would be sufficient. Will double-check before moving forward. More concerned with damaging the rubber membrane. Will check with roofing company.

          Lastly, don't know why I didn't receive email notifications after receiving notice to the first reply. I'm definately subscribed to the thread.

          Comment


          • #6
            Originally posted by LarryJ View Post
            Thanks for the replies. After receiving email notification of the first response, I followed the provided link to rheen and didn't see any system identified as "batch" solar heater, so I researched batch soloar water heating online. That's the type of system I was envisioning. Simple in concept and cost. After reviewing a few videos and documents, it dawned on me that these batch systems would be nothing more than extra storage unless the cold water supply entered the solar tank first. I went to bed contemplating this. The videos don't discuss plumbing, but I was able to find and confirm my suspision on page 13 here: http://ir.library.oregonstate.edu/xm...95F?sequence=1 . That design makes sense. I haven't gone through the Oregon State design in detail, but it looks like a great source, coupled with the videos like this; https://www.youtube.com/watch?v=F6BvsxFPN2o .

            Regarding freezing, the Oregon document states that insulated polybutylene piping creates a "freeze tolerant" system. In a test, their pipes froze, but did not burst. I'm more concerned with the loss of heat in the solar tank on winter nights. Sure it's insulated, but won't the water cool downcausing it to flow back into the electric tank where it will be reheated by electricity. I wonder if there's a temperature sensitive valve to prevent such things. Anyway, good info.

            I rebuilt my porch roof last summer using treated 2x6 joists on 16" centers. They're on hangers where they butt up against a ledger board on the house. I don't know what rating that would calculate to, but I suspect it would be sufficient. Will double-check before moving forward. More concerned with damaging the rubber membrane. Will check with roofing company.

            Lastly, don't know why I didn't receive email notifications after receiving notice to the first reply. I'm definately subscribed to the thread.
            Your post had a couple of website links which automatically gets "unapproved" and went for Mod review. I approved it and everyone can now see the post.

            Comment


            • #7
              Originally posted by LucMan View Post
              http://www.builditsolar.com will give you some ideas.
              I suggest you avoid rooftop batch heaters if there is any chance of freezing temperatures.
              What type of system would you recommend instead.

              Comment


              • #8
                Originally posted by LarryJ View Post
                Thanks for the replies. After receiving email notification of the first response, I followed the provided link to rheen and didn't see any system identified as "batch" solar heater, so I researched batch soloar water heating online. That's the type of system I was envisioning. Simple in concept and cost. After reviewing a few videos and documents, it dawned on me that these batch systems would be nothing more than extra storage unless the cold water supply entered the solar tank first. I went to bed contemplating this. The videos don't discuss plumbing, but I was able to find and confirm my suspision on page 13 here: http://ir.library.oregonstate.edu/xm...95F?sequence=1 . That design makes sense. I haven't gone through the Oregon State design in detail, but it looks like a great source, coupled with the videos like this; https://www.youtube.com/watch?v=F6BvsxFPN2o .

                Regarding freezing, the Oregon document states that insulated polybutylene piping creates a "freeze tolerant" system. In a test, their pipes froze, but did not burst. I'm more concerned with the loss of heat in the solar tank on winter nights. Sure it's insulated, but won't the water cool downcausing it to flow back into the electric tank where it will be reheated by electricity. I wonder if there's a temperature sensitive valve to prevent such things. Anyway, good info.

                I rebuilt my porch roof last summer using treated 2x6 joists on 16" centers. They're on hangers where they butt up against a ledger board on the house. I don't know what rating that would calculate to, but I suspect it would be sufficient. Will double-check before moving forward. More concerned with damaging the rubber membrane. Will check with roofing company.

                Lastly, don't know why I didn't receive email notifications after receiving notice to the first reply. I'm definately subscribed to the thread.
                The Rheem design Mike linked is one type of batch heater that uses a flat plate design with storage above the collector and operates via thermosiphon. Most all batch heaters are storage devices that, in one way or another, combine storage and heating into one device. With a batch heater, you will usually have a 2d storage device in the basement or garage, being fed by the batch heater which becomes part of the existing plumbing system. BTW, nothing mandates that a batch heater or any other solar device be placed on a roof. Access to unobstructed solar irradiance is the biggest performance criterion.

                The designs shown in your links are similar to what I suggested. Lots of variations are available, limited only by imagination and local building codes. For batch heaters, wind loading is usually less than the combination of dead, concentrated load of ~ 60 gal. of H2O plus seismic loading.

                On the UO plans, forget the selective surface coatings. Their effective emissivity fades after a few years, thus losing any thermal advantage. The adhesive also inhibits heat transfer and tends to come unstuck, making matters worse. That was unknown in 2001, and not probably not conceived of in 1986. I used hi temp. flat black spray paint like that used on ICE headers and let it outgas for a few weeks before sealing the box. I also made the glazing removable for serviceability.

                I'd disagree with logic or system design methods that use polybutylene as a part of a pressure boundary when used for some type of freeze protection. I'd check with building codes on that one as well.

                As for what you did to a roof, that may sound and be sufficient, but without strength calcs, it means nothing with respect to whether or not your roof is sufficiently strong for the proposed loads. Don't shoot me, I only know how the game is run. Check with your local building dept. They have the final say.
                Last edited by J.P.M.; 03-20-2017, 12:39 AM. Reason: Spelling.

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                • #9
                  It's not the danger of the poly tubing freezing, its the copper tubing in the flat plates that will burst.
                  I recommend drain back systems and that's all that I install. The drain back tank adds a couple of hundred dollars to the initial cost, but reduces maintenance required on the system. Stagnation and freezing are easily eliminated, along with glycol breakdown from overheating.

                  Comment


                  • #10
                    Originally posted by LucMan View Post
                    It's not the danger of the poly tubing freezing, its the copper tubing in the flat plates that will burst.
                    I recommend drain back systems and that's all that I install. The drain back tank adds a couple of hundred dollars to the initial cost, but reduces maintenance required on the system. Stagnation and freezing are easily eliminated, along with glycol breakdown from overheating.
                    I understand the logic and how it works. What I was alluding to was that, for a lot of reasons, polybutylene is a poor choice as a pressure boundary.

                    At the risk of Solar Pete accusing me of condescension: Besides, some parts of a system can freeze (elbows, nooks/crannies in the piping, valves, low spots, nite sky exposed portions of a system and thus colder high spots, etc.) before others, exposing them to possible icing/ice possible buildup. Ice expands in all directions including radially. It's more than just an increase in total system volume due to ice volumetric increase/density decrease. Variable volume or flexible pressure boundaries can work, but for anything like reliable freeze protection, usually require careful consideration and a bit more certain knowledge of possible/logical freeze points than is known with any certainty a priori (or post priori for that matter). Most users and few others have the knowledge/experience. Even with those attributes, it's still a bit of a crap shoot. There are better ways to get more reliable freeze protection.

                    With that said, otherwise, I'd agree with you that a drainback system is superior, not only as a freeze protection scenario but also for other reasons you cite. Sometimes insurance against failure costs a bit. Convince the OP. First cost seems to be the driving force here.

                    Besides, if the OP is staying with a batch heater, it'll be kinda hard to make a batch heater drain back. I mean, if it's a true batch heater/ a rooftop tank, how do you turn the pump on/off ?

                    Comment


                    • #11
                      Great discussion, guys. The OP is still learning the different systems. I've done some reading on BuildItSolar website, but still have lots to learn and evaluate before selecting a design.
                      Cost is a driving factor, but only as it relates to return. I'm the sole occupant of this house. I use hot water primarily in the morning. I only use it for showers and washing dishes. There's a statement on BuiltItSolar indicating that, with a batch system, you'd better use the hot water in the evening because it will be gone by morning (paraphrased). This leads me to believe that I won't save much in electrical cost. Heck, I don't really know how much it's costing me independently. I have electric everything.

                      With the heater currently situated in an unheated garage, I wonder if I'd make more headway at reducing my electric bill by insulating it to the hilt. Regardless, I'm gonna do a solar project of some kind. A solar oven looks like a good start.

                      Comment


                      • #12
                        Here in NY our local code would not allow a batch heater on your roof. Remember a gallon of water is approximately 8#+
                        A single 4 x 8 flat plate panel weight is around 80# and would be able to supply you with 40 gallons of 140 -150 degree water per day for 5-6 months per year. Using a 5-10 gallon drain back tank with a heatexchanger would be easily retrofitted to your existing electric water heater. Unfortunately the insulation on standard electric water heaters lose about 2-3 degrees per hr vs .5 degrees per hr loss on a good solar water heater. You can always upgrade at a later date. I have tried over insulating standard water heaters but can't get the losses down to where I'm happy with them.
                        Check out the AET solar website there is lots of good info.

                        Comment


                        • #13
                          Originally posted by LarryJ View Post
                          Great discussion, guys. The OP is still learning the different systems. I've done some reading on BuildItSolar website, but still have lots to learn and evaluate before selecting a design.
                          Cost is a driving factor, but only as it relates to return. I'm the sole occupant of this house. I use hot water primarily in the morning. I only use it for showers and washing dishes. There's a statement on BuiltItSolar indicating that, with a batch system, you'd better use the hot water in the evening because it will be gone by morning (paraphrased). This leads me to believe that I won't save much in electrical cost. Heck, I don't really know how much it's costing me independently. I have electric everything.

                          With the heater currently situated in an unheated garage, I wonder if I'd make more headway at reducing my electric bill by insulating it to the hilt. Regardless, I'm gonna do a solar project of some kind. A solar oven looks like a good start.
                          Slightly tongue in cheek: Depending on location, join a health club and shower there. ~ $25/month. Treat the eye candy and conversation as a bonus.

                          Standby losses alone from my 80 gal tank only amount to ~ 110-120 kWh/mo. (~ 7-8 or so BTU/deg. F*hr., measured.). Batch tank losses sitting on a roof will be quite a bit higher. I've not measured them in past iterations of life, but depending on insulation level of the box, and as a real rough 1st approx. only, losses from, say, an R11 insulated box on a roof, containing, say, an 80 gal tank, might be ~ 18,000 BTU/day, or more, with most of that occurring after dark. That might mean a tank temp. drop of 25-30 deg. F or more, less in summer, more in winter. Depending on conditions/temps/time of year/etc., I kind of doubt ALL the heat will be lost.

                          Keep in mind that a batch heater, or any solar device, will only act as an adjunct to an existing heater, not a complete replacement. The solar device will usually feed the main heat providing device, with that heater boosting the water temp. the rest of the way to the required service temp.

                          Still, and still mostly agreeing with Lucman, IMO, you're better off with a solar flat plate if you're hidebound and determined to get a solar H2O heater, but I doubt it'll be cost effective. Even if you get a DIY batch heater past the building inspector, your usage and situation sound like you could get your already low hot H2O usage and losses lower yet with useage reduction and energy conservation measures such that no solar assistance could be made cost effective.

                          If the goal is to lower your electric bill by paying less for hot water through either making more hot water with solar or using less hot water through conservation and insulation, solar will be more $$ and more hassle, not to mention more maintenance. Do this first: Insulate the living crap out of the existing water tank. Get low flow shower heads. Take shorter showers. If you have a hot water circulating system, get rid of it.

                          I've got 2 people in my house. My daily hot water use requires ~ 25,000 BTU/day (~ 7.3 kWh/day), including tank and line losses. Depending on what you pay for power and how low you can get your usage, @ , say $0.14/kWh, that's ~ a buck a day. I've got a solar flat plate system for hot water and I like it and I'm keeping it. But, if I had it to do again, today, knowing what I think I might know, I'd give VERY serious consideration to a heat pump water heater.

                          After you insulate the living bejesus out of everything, with your use as low as it is, I suspect you'll have a hard time justifying the cost of a heat pump water heater any more than you'll be able to justify the cost of most any solar water heater, DIY or otherwise. I suspect your water heating bills just ain't that much.

                          Good luck.

                          Comment


                          • #14
                            Originally posted by LucMan View Post
                            Unfortunately the insulation on standard electric water heaters lose about 2-3 degrees per hr vs .5 degrees per hr loss on a good solar water heater. You can always upgrade at a later date. I have tried over insulating standard water heaters but can't get the losses down to where I'm happy with them.
                            Check out the AET solar website there is lots of good info.
                            FWIW:

                            My single 80 gal. tank loss is ~ 5 BTU/hr. deg. F. delta T. It has 6 " of fiberglass added to the sides and 2" of polyurethane top/bottom. With a base 70 deg. F. delta T. and a constant basement temp. of 70 F. I estimate the 80 gal. tank filled with 140 deg. F. H2O plus piping and contained piping H2O has an effective thermal mass of ~ 686 BTU/deg. F.

                            The immediate line loss, say the first 5-8 ft. of 3/4" insulated line (All 1" Armaflex) lines to the house and collector equipped with check valves and heat loops, valves, etc adds about 2 - 3 BTU/hr. deg. F. delta T to the loss. Line loss for small diameters is and has always been harder to control. More FWIW, I found mostly the same with industrial systems when I designed them.

                            The buttoned up temp. decay over 24 hours done and measured several times about 7 years ago after a system redesign, with tank contents turned over 1X/hr. via the pump to destroy any stratification, and also measure tank temps. has given a fairly constant tank/local associated line loss of 7.42 BTU/hr./deg. F delta T., with, as mentioned in a prior post to this thread, an approx. and estimated split of ~ 5 BTU for the tank and ~ 2.5 BTU or so for the piping/valves/pump/instruments/etc. in ~ 18 ft. of pipe split between 4 lines. The tank is an 80 gal Lochinvar, plumbed for solar w/ 2" of foam between the tank and the jacket.

                            T, h2o, start = 140 F.
                            T, h2o finish = 124 F.
                            T, b = Basement temp. = 70 F.
                            t = total test time time = 24 hrs.

                            U*A = tank/local line combined loss
                            M*cp = tank/lines/contents mass* specific heat product == effective thermal mass = 686 BTU/deg. F.

                            (T, h2o finish -T,b)/(T, h2o start - T, b) = EXP[-( U*A*t)/(m*cp)]t

                            = (124-70)/(140-70) = EXP[-(U*A*24)/(686)]


                            --->>> 0.2595 = (U*A*24)/686

                            --->>> U*A = 7.42 BTU/(hr. deg. F. delta T.)

                            --->>> 1st hour heat loss ~ (7.42 BTU/(hr.* deg F.) * (140-70 deg. F.)/686 BTU/deg. F. ~ = 0.76 F./hr.
                            Last edited by J.P.M.; 03-20-2017, 11:55 PM. Reason: Added missing parens.

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                            • #15
                              Those losses are about as good as it gets. Do you have a heat trap (8-10" inverted trap) on the HW outlet?

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