8 Responses to “Multiple Solar Hot Water Tanks – Does It Make Sense?”


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  1. Marty

    Dr. Ben,
    I am new to building solar systems. I’m very glad to be here! I was hoping I might get some good suggestions on my specific situation. I’ve been considering building solar collectors and investigated self-built systems found here and on other sites. My modest objective is to preheat water for a whole house, on-demand electric water heater and to pump hot air into my insulated and covered crawl space for simple radiant heating. I’m trying to keep it low tech, with minimum investment and complexity, making the most of what I already have. I’m not trying to hit a home run, but to assist the active heating systems I’ve already got that currently serve me, my wife and my daughter. After analyzing my 55 year old, 2100 sq. ft. house, materials and orientation, I’m thinking that I could turn the whole south facing red brick wall of
    the house into a modified trombe wall solar collector. When I say the whole wall, I don’t mean every square inch, but to put up as many 4×8 collectors as possible or as needed.

    Here are two photos of my house.

    As you can see, I have very little in the way of south facing windows but lots of brick, including a very tall and wide brick chimney. The exposure is almost perfectly southern.

    My idea is to paint large areas of the brick wall black with copper or PEX tubing connected directly to the brick. Next, enclose the painted brick and tubing with a simple insulated glazed box, with proper sealing between the box and the brick.

    Penetration of the building with forced air and water pipes would be at the bottom of the foundation into the crawl space, where the original crawl space vents are still located (but now sealed off by insulation). I don’t want to penetrate the wall or the roof if I can avoid it.

    My biggest concerns and issues include the following:

    1. Forcing cold air from the crawl space up and then back down again into the crawl space after being heated. It would be a long way for the air to travel, particularly on the chimney. Maybe I should only use the chimney for the hot water heating function? Would it be better to use the heated air as a preheater to the heat pump system already in place and avoid the crawl space idea?

    2. Will I need a drainback system? I’m thinking that the insulated collectors will allow the wall to radiate heat after the sun goes down and keep the pipes from freezing. Of course, better safe than sorry, and I’d probably best include this feature no matter what.

    3. During the summer, the hot water might not need any additional heating, and I’m thinking about a solar powered hot tub if possible. I certainly wouldn’t want hot air forced into the crawl space then. I was thinking that the collectors could actually remove heat from the wall mass and possibly reduce my cooling needs. Would I need to cover the collectors during the summer?

    4. Could large water containers placed in the crawl space provide a diffused water wall approach that could benefit heating and cooling?

    I’m sure there are many other issues, but thought this initial post might start me off in the right direction with comments from more experienced souls.

    Many thanks in advance for any comments or suggestions.

    Marty Tennant
    Georgetown, SC

    • Dr. Ben

      I am afraid you are in for some real trials trying to get the things you mentioned to work.

      1. Covering your south wall with glass and painting it black will definitely capture heat – in the summer as well as the winter. The excess summer time heat could cause real problems. You will have to cover or shade it in the summer. Just heating the wall in the winter will cut your energy bill without any elaborate distribution system.

      2. I never recommend using plastic in a solar collector, particularly plastic pipes. I have seen 100% failure with any plastic used in solar collectors – except plastic pool heaters which don’t get very hot.

      3. The first thing I tell anyone with an older home is to upgrade the insulation in the ceiling and floor, and install double pane windows. This is the smartest money you can spend. Nothing to oil, or tighten, and you energy bill goes down dramatically. Once this is done, a little solar will go a long way.

      4. I don’t recommend putting heat into your crawl space. I recommend insulating the floor from below.

      Good Luck.


  2. Marty

    You have been very kind with your time and expertise. I’ll carefully review your suggestions and do what makes sense. So far, you’ve helped me avoid going down blind alleys that don’t make economic or scientific sense!

  3. Nice post, thank you for sharing. It is no doubt that the solar power has been developed for several years, and it is increasingly being used in daily life such as solar hot water system, solar car and so on. In the future, it will be essential for us decreasing the cost and developing it in more industries, as it is the green energy for the planet.

  4. Alex Kelley

    Dr. Ben,

    What’s your objection to using multiple solar storage tanks in a parallel piping configuration to accomplish larger storage goals? 120 gallon tanks are readily available at a good price and are easy for installation crews to work with. Larger tanks are many times not feasible due to door opening restrictions. Plus, I argue that a pre-engineered, OG-300 system gives a customer confidence that his/her system will perform as promised so a solar contractor could parallel multiple OG-300 systems together to meet a design goal.

    I appreciate your feedback,


    • Dr. Ben

      Hi Alex,

      Thanks for writing. Another article on this blog titled “Commercial Solar Hot Water Systems – Increased Value via Optimal Tank Performance” covers most of the reasons multiple tanks fall short of a single integrated system.

      But you do raise a good point about problems getting a tank into an equipment room. I have found that even in new construction, intended to have solar thermal on it, architects and planners have no idea what the space requirements are for solar components, and no idea about the relationship between the collectors and tank, not to mention a simple piping run. These people are not mentally deficient, they just don’t know how solar engineering and equipment works.

      Frequently, there is neither adequate space nor good access to the equipment room. We have worked out several solutions for commercial jobs.

      1) Widen the door. On a $200,000 job, taking out a single steel door and installing a double steel door is a small percentage of the cost. Even residential door exchange is possible. It all depends on the individual location.
      2) Build a lean-to outside the equipment room wall. We don’t ever want a solar tank in the rain, so a simple lean-to building adjacent to the equipment room wall is sometime a straightforward solution to size and access.

      One of the things I did when designing a line of Fluid Handling Systems was to try to envision where they are going. For residential systems up to 500 gallons of storage, I created a line of tank systems that were 30 inches wide. They would go through almost any 3′-0 door in a house. The attached picture shows a 400 gallon tank sitting in a customer’s basement, circa 1984. The picture is poor quality, but I hope you can see the details. The dealer had to get the system down a full flight of outside concrete steps and through the door at the far end of the basement.

      Above 500 gallons, we build 52 inch wide rectangular tanks that will go through a 60 inch wide door. Two recent tanks are an 1800 gallon system and an 800 gallon system. Our cylindrical tanks are 64″ in diameter, so they go through a double 3 foot door. Cylinders are cheaper than rectangles for the same gallons. Rectangular systems are chosen when there are narrower doors, or we need the maximum gallons per ft2 of floor space.

      When you talk about paralleling OG300 systems, I cringe. Think about it. Multiple tanks, multiple pumps, multiple controls, extra heat loss from higher surface area, excessive piping between tanks to make them appear identical to the piping scheme. What’s to love about a system like that?

      It is true that systems like I have described are not generally available. I haven’t seen any integrated systems on the market other than my own, and right now we are not making the residential sizes (80-500 gallons), but we will in the near future.

      So, without proper equipment available, what can you do? You need 250 gallons of storage which you can’t find. I can’t fault you for using multiple 120s, but I would strongly suggest you use one pumping system and one control system and one heat exchanger system. Don’t do multiple systems, do one system using multiple tanks.

      If you come across any commercial solar hot water jobs that require 500 gallons of storage or more, let me price out a Fluid Handling System for you. You will find that the final cost including labor is extremely competitive to multiple tanks not mention thermodynamically superior. I’d be more than happy to discuss any such projects with you by phone.

      I hope this helps.

      Dr. Ben

  5. I think that the solar power system is becoming popular now, as it is not too expensive, it is quite effective in saving energy and protecting the environment. Thanks for sharing.

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