18 Responses to “Design Advantages of Drainback Solar Hot Water Systems”


Read below or add a comment...

  1. Lynn Maguire

    Hey Dr. Ben,
    I am having a problem with my hot water panel fogging up (inside, of course.) I had the plumber who installed it here and he pressure tested it and there was no leak. He said the “box” is not water tight and rain gets inside. Of course, I was relieved and we HAVE had a ridiculous amount of rain (Central Ca.) BUT – I saw it again today and it hasn’t rained in 3 weeks. I do tend to get it wet when I’m watering plants, but it still does not seem right. Any ideas? Replacing it is definitely the last resort considering how heavy it is and how hard it is to get to..
    Lynn Maguire

    • Dr. Ben

      In the early days, collector manufacturers tried to seal their collectors from any water penetration. They made really tight boxes, sometimes adding silicone caulking at vulnerable points. Trouble is, it didn’t work.

      Collectors expand and contract with temperature like anything else. It is impossible to hermetically seal them for any length of time. Water can leak in around the glass gasket, through the pipe grommets, mitered corners, etc. Once water got in, it would never come out.

      The best collectors have vents to let the collector breathe. When they are hot, the air inside expands and breathes out. When night comes and they cool down, they breathe in. If the atmosphere is damp, moisture comes in too. This can condense on the glass. The next day when the collector heats up, the vents are supposed to allow the moisture to escape with the expanding air inside. Vents also waste heat by allowing air to flow through the box, so a good balance is necessary to control moisture, but not waste too much heat.

      Now, back to your problem. There are three main causes of condensation, or fogging in a collector.

      1. Leaking manifold inside the collector. You say your dealer has ruled this out.
      2. Leaking pipe coupling going into the collector. Under pipe insulation, this is hard to spot.
      3. Leaking collector box. This is what your dealer suggests.

      If the last fogging occurred after you watered the collector, then we have a correlation between external water (not in the piping) and the fogging. If not, then there has to be an internal leak.

      I would need to know a lot more about your particular collector structure to evaluate where the vents are and how vulnerable it is to rain water. Also, the collector mounting frame can let water into the box, depending on its structure.

      In the absence of that information, I can only give you general advice. Assuming the fogging comes from outside water only, the fix is to make sure all rain water sheds off the collector and doesn’t go in.

      I would get a tube of outdoor caulking, rated for at least 250F, and caulk all around the glass frame. Put a bead from the glass up to the metal frame to cover the joint between the glass and the gasket and the gasket and the metal. Put a big bead at the bottom of the glass where water can stand in a pool.

      Check the collector corners. If your box is made of aluminum extrusion with mitered corners, you might want to caulk the seams. The quality of construction is very important in making tight corners.

      Look on the back side of the collector. There is probably an aluminum sheet on the back with a joint all the way around the frame. Caulk around that joint. All those joints are possible entry points for water. The ones at the bottom where the water runs down and pools are the most likely culprits.

      If you know where the vents are, make sure they are unclogged so the venting can work.

      If you want to play detective, just caulk one section at a time and see what happens. Try caulking the horizontal joints across the bottom at the glass and on the back first.

      If this works, you will need to repeat it as necessary as the caulking ages and peels off. If you still have trouble, I would recommend calling your dealer, or the collector manufacturer and ask for a replacement.

      Good luck.
      Dr. Ben

      • Lynn Maguire

        Thank you so much Dr. Ben!
        I wish I had found your site before I bought my system. I’m glad you mentioned the structure holding it up; it is a 2×4 creation of the building contractor, not the plumber, although they were working together.The fogging seems to be an off and on problem. The first time was the first warm, sunny day after a lot of rain and there was even water coming out of the bottom of the panel..Now it is just fogging up. Sometimes. We are supposed to get a lot of rain today & tomorrow, so I’ll have another chance to see what’s going on. Now my problem is how to “inform” my plumber without coming off as a know it all homeowner telling a pro what to do. I’m afraid you can’t help me with that! I’ll try to get him to look at your site.

  2. Ana

    Any thoughts around a closed loop drain back system? This would avoid the need for a large tank that is used as both the heat exchanger and the drain back reservoir and reduces the overall footprint in the building. I am currently looking at a large residential system (serving 150 folks), and I see their large tank as a liability the day it fails.

    • Dr. Ben

      The amount of storage is determined by the area of the collectors. Generally, we use 1.25 – 1.5 gallons of water per sqft of collector. Pressurized or non pressurized doesn’t make any difference. The systems works the same, except that pressure vessels can be very expensive. That is why I use a non-pressurized design.

      The difference between closed loop and open loop is whether the fluid in the loop stays there (closed loop), or is lost as it goes through the system (open loop). The only open loop solar systems in use today are in locations that don’t ever have freezing weather, such as the caribbean. In those systems, the drinking water is circulated through the collectors and back to the tank. If you open a tap, the water in the collectors flushes out the spigot and into the sink, tub or washing machine.

      Open loop systems are cheap because they don’t have any heat exchangers, etc. They do bring fresh corrosion and scale from the water line into the collectors, though.

      So, if you want to supply domestic hot water for 150 people (you didn’t say where, the temperature and radiation data are critical), you might need 1800 ft2 of collector and 2500 gallons of storage. (See the WTU posts on this blog for a 2500 gallon Fluid Handling System).

      There is no simple way around the storage tank. It is like the gorilla that sleeps where he wants. It is always a problem on an existing building because there is never enough space for one. One solution is to build an enclosed “lean to” against the outside wall of the mechanical room to house the tank.

      This is one reason we like to get in on jobs early in the design process, so we can get appropriate space in the mechanical room up front. On retrofits, we do the best we can.

      Good luck!
      Dr. Ben

      • Ana

        Thanks so much for the quick reply. What is the basis for the 1.5 to 2 gallons/sf? Assuming the demand is constant, could the storage volume in a drainback with a heat exchanger solely be based on the amount of water that is typically held in the solar collector plus the piping?

        Just to make sure we are on the same page, the system I was thinking about is depicted here:

        Thanks again!

        • Dr. Ben


          You raise a very good question. How much storage is enough? The website you refer to shows a drainback system consisting of a water heater with a heat exchanger in the bottom for the solar loop. The solar loop has a small drainback reservoir to empty the collectors into. Thus, the storage seems to be the small amount of water in the collector loop and reservoir.

          However, the storage is actually the collector loop AND the water in the water heater. All this volume of water is where the energy goes. The fact that the collectors are separated from the water heater by a heat exchanger does not change the fact that the energy is actually stored in the water heater, so that makes it the “storage”.

          To see the bigger picture, I am going to indulge in a little thermodynamics.

          The efficiency of the solar system is based on its average temperature. Solar systems gain heat by only one means – radiation from the sun that heats the absorber plate in the collector. Solar systems loose heat by three means – radiation, conduction, and convection.

          Any body above absolute zero in temperature radiates energy. To test this, hold your hand about 1/2 from your face. Notice the rise in temperature of your cheek. Your hand is radiating energy back to your face, and your face heats up. Every warm part of the whole collector system loses head by radiation.

          Convection is the transfer of heat by moving a hot molecule from point A to point B. A forced air furnace does this all the time. The water going through a solar collector is doing the same thing: move a hot water molecule from A to B.

          Conduction is heat transfer when the molecule doesn’t move, but bangs into its neighbor making the neighbor hot. This is like dominos. One hot molecule bangs into the next, giving up some of its energy, which warms the second molecule. After a while, all the molecules in the medium are banging around and the energy has been transferred from one end of the material to the other. If you hold a copper pipe on one end and I put a blow torch on the other end, it won’t be long before you feel it. This is conduction. The heat exchanger in a solar water heater uses conduction to transfer heat from the hot solar fluid through the copper or stainless steel wall into the house water. Actually, radiation, conduction and convection all come into play in the heat exchanger.

          The point of this explanation is that the amount of energy transferred by these three means goes up and down depending on how hot the material is relative to its surroundings. If a copper pipe is the same temperature as the air around it, no heat transfer takes place. But if the copper pipe is 200ºF and the air around it is 40ºF, then a lot of heat will be transferred from the pipe to the air, buy all three methods.

          Remember, a collector gains heat by only one method, radiation, and loses heat by all three methods, radiation, conduction, and convection. Therefore, the hotter a solar system is, the more energy it is loosing. The efficiency goes down as the temperature of the solar system goes higher and higher relative to its surroundings. This is principal number one.

          Principal number two is straight forward, but it just doesn’t seem right. Most people equate temperature with energy. High temperature, high energy, low temperature, low energy. Actually, temperature is only half the energy equation. The other half is mass, or volume. Let me illustrate. A table match burns at around 3000ºF. Very hot. But consider that the energy in the combustion of a table match is about 1 Btu. You couldn’t boil water or heat your house with a table match, but it is very hot. You might need 200,000 table matches to heat your house in the winter. Of course, they would only last so long, so you would need another 200,000 matches. The point here is that it takes both temperature and mass to determine energy.

          Back to the solar system. If we don’t have much mass (storage) volume in a solar system, then what ever energy collected will heat up the system very quickly. We have one hot match. Likewise, the system will cool down very fast, because it isn’t holding much heat. So the solar system will be swinging wildly around like a yo yo from hot to cold with every passing cloud. This makes the control system go nuts. The system will turn on and off and on and off. We call this short cycling. The net result of short cycling is that very little solar energy is actually transferred into the load, and the controls and pump wear out quickly. We have a saying: “it ain’t broke, but it don’t work”.

          So, we need enough storage to act like a flywheel and calm the system down so it will run steadily and actually produce usable energy.

          Principle number three. Run the system at the lowest possible temperature that is usable. The average temperature of shower water in the military is 107.5ºF. If I could produce exactly 107.5ºF water temperature, then I would have the most efficient solar system possible. I wouldn’t go one degree higher than I needed, and my losses would be limited to what happens at 107.5ºF. In the home, different uses of hot water have different temperature thresholds. Some clothes washers claim they can do a good job with “cold” water. Not sure about that in the winter when the water may be 50ºF or lower. So, maybe warm water at about 100ºF is good. Dishwashing should be in the 120ºF range for hand washing, and up to 140ºF for automatic dishwashers. Most automatic dishwashers I know of have a booster heating element in them to either raise the water temperature or dry the dishes with hot air. So, lets say we need to supply 120ºF water for the dishwasher.

          We can also use solar hot water for space heating. I have hundreds of systems out there doing just that. We can deliver space heating in all the conventional manners – forced air, radiant baseboard, and radiant slab. Each has a different delivery temperature. Forced air typically runs from ~ 120-130 output temperature down to just above the room air temperature, say 70ºF. A solar system can assist in space heating all the way down to the return air temperature in the ducts.

          Radiant baseboard is typically 160-180ºF when using a boiler, but if the baseboard area is expanded, you can get away with 100-120 ºF water.

          Radiant floors are the most comfortable. They can run at 90-100ºF and do a good job. Solar can assist all the way down to just above the coldest return temperature in the loop.

          So, back to the original question: how much storage is enough? In the early ’80s, this question was being studied by government labs and universities. After running experimental systems and recording the data, the answer they came up with is to use 1.25 to 1.5 gallons of storage water for every square foot of flat plate collector. If you go below that amount, the system gets artificially hot and the efficiency goes down. If you use too big a storage tank, the efficiency actually goes up, but the cost goes up rapidly and the benefit is minimal. Optimization is the point.

          If you haven’t gone to sleep on me yet, I hope this answer has been some help.

          Dr. Ben

          PS – If you have any specific projects you’d like to discuss, I’d be more than happy to help.

          • Ana

            Very nice. Thanks so much. I had supposed the heat demand (domestic hot water in this case) would draw off the energy as it was produced – or nearly so, without consideration of the dynamics of the solar system. In other words, the usage or demand profile is not identical to the production profile so there’s gotta be a buffer in between. The rule of thumb “buffer” is the 1.25 to 1.5 gallons per sf.

  3. Ben, for clarity, when you have the hot water demand from flow meters as XX/usg/day, and have inputed the F-Chart software parameters to calculate the number of solar collectors, to reach the optimal solar storage volume – can you specify the ratios when two tanks are used a little more please?

    What is the preferred ratio of the drainback tank to the domestic hot water tank size? Would they be equal, or do you have some research that shows a 40:60 ratio works better? Have you found graphs or reports that give guidance to the sizing? And is the 1.25 to 1.5 multiple include the sum of both drainback and DHW tanks, or just the drainback?

    – Bruce

    • Dr. Ben

      Good questions. In one of my blogs I wrote that solar engineering and design is a combination of science and art. I include experience under the art part.

      The tank volume to collector area ratio was first discussed by Duffie and Beckman in the 1970s in their defining work on the F-Chart calculation method. As I recall, they specified 75 liters/m2 of collector, which is 19.8 gal/10.76 ft2, or 1.83 gal/ft2. They then talk about a range of .92 gal/ft2 to 7.32 gal/ft2. In fact, the performance of the system changes little as the storage ratio goes above
      1.22 gal/ft2.

      Subsequent tests have shown that a range of 1.25 to 1.5 works fine.
      Above that you are wasting money for little benefit. Below that you are
      loosing performance rapidly. The tanks I make come in sizes of 80gal,
      130, 250, 400, 500, 1000, etc. So, I choose a collector number and size, then do a reference calculation of both 1.25X and 1.5X and pick a tank in that range.

      Your second question was about how to differentiate the solar storage part from the conventional hot water storage in a single tank solar system. The easy answer is 50:50, but I have seen no definitive data on the subject. The tank manufacturers have various baffles, ports, and tubes to prevent mixing of the colder bottom (the “solar” portion) with the electrically heated top portion. I am sure all these methods work to some degree, but basically, you are fighting an uphill battle trying to separate the solar water from the conventional water. When someone draws tap water from the tank, that causes mixing. Increase the flow rate with a bath, a washing machine, and a dishwasher, and you have a lot of mixing going on. I just don’t like the idea of electric elements heating the water going to the collectors. When solar competes with putting heat into the same water as a conventional heater, the solar always loses. I would check to see what the single tank manufacturer says about the solar and conventional parts of the volume.

      While one tank systems have their place, I presonally don’t like the compromises they require. I like a two tank system. The solar water is heated only by the collectors, and the regular water heater heats only its water, they don’t mix. This preserves the theory and the operation of the solar system. It is true there is no room for a separate solar tank in many apartments and houses. For this, we designed a cubical 80 gallon tank that is about the size of a washing machine. A “low boy” 40 or 60 gallon water heater can sit on top of it easily and maintain complete isolation of the solar heated water and the electrically heated water.

      In summary, then, if I am calculating a single tank system, I either take the manufacturers number, or assume 50% of the volume as solar storage. For conventional water heaters, you should cut the dip tube in half to minimize mixing. Some people disconnect the lower element. Gas water heaters are not recommended for single tank systems because they heat from the bottom.

      For two tank systems, I only consider the solar tank volume in the calculations. A conventional electric water heater will have high and low heating elements, and a dip tube that goes to the bottom. It really doesn’t matter what the conventional tank volume is for the solar calculation, but it needs to have enough power and volume to carry the load when the solar is gone. In other words, it should be normal size for the application unless you are willing to have cold showers occasionally.

      I hope this helps!
      Dr. Ben

  4. DoctorDoctor

    I am convinced…sort of. The one down side of the drain back system is the pump requirement. For many houses, the only practical location of components is the storage tank in the basement and the collectors on the roof. This necessitates a pump with a very high head pressure. In contrast, pressurized systems actually push the fluid around in a loop, so the water column behind the pump provides the head pressure.

    My solution would be a combination. Have a small drainback system in upper floor or attic with, say, a 10-20 tank which is non-pressurized. The H20 leaving the collectors would enter the top of this tank and the fairly low-head pump would take it from the bottom. Then there would be a large tank in the basement with a circulating pump.


    * If catastrophe happens in the upper floor or attic, it is “only” 10-20 gallons.

    * The low-head pump from the small drain back system can easily be powered by a photovoltaic. You would probably only need a controller to turn OFF the pump if the water temp got to hot for the upper plumbing.

    * The “no-head” circulating pump can be run by PV, too.

    • Dr. Ben

      You cite the pump power required as being a major difference between a drainback system and a glycol system. You are right. Circulating fluid through a filled pipe takes less power than lifting it up to a height.

      I have tried exactly what you suggested. I had a three story home with 14 collectors on the roof and a 500 gallon tank in the basement. I figured if I put a reservoir in the attic on the return side I would cut the head significantly and save power. The drainback reservoir in the attic was non-pressurized and had a vent in it.

      Problem was, I now created a high static pressure in the basement tank. Water pressure is 0.43 psi per vertical foot, so 30 feet = 13 psi. The definition of “non-pressurized” according to ASME is anything under 15 psi, if I remember correctly. This means I was pushing the limit of a “non-pressurized” system. The non-ASME tanks we use are leak tested at 3-5 psi, so they are not intended for more than that pressure, although cylindrical tanks could withstand about 15 psi max. In addition, a rectangular tank pressurized to 5 psi will “oil can” loudly, so we couldn’t have used a rectangular tank in that scenario.

      Back to the small vented reservoir tank in the attic. In the summer the system will get hot and shut down at 160F high limit. When the temperature falls to 145F, it turns back on. The collectors are now at 300F, so the water entering them bursts into steam until the collectors are cooled down. That steam goes into the little reservoir and hits the top of the water in that tank. The water quenches some of the steam (condenses it) back into water, but some blows out the vent pipe. The amount coming out the vent and lost to the system is related to the area of the collectors and the area of the water surface in the reservoir tank. I call it the collector/quench ratio. I discussed this in some length in a previous blog. The bottom line is that using a small vented reservoir tank in the attic causes a lot of water loss in the form of steam in the summer, or any time the system is left idle and the heat in the tank builds up. This can happen on vacations, for example. I always told my dealers & customers to turn the solar water heater off when it was going to be idle for any time.

      So, putting a small reservoir tank in the attic to cut the head on the main solar tank below brings with it several potential problems.

      In the end I made the decision to skip the reservoir tank in the attic and drain all the way down to the basement. The original pump was a TACO 009, which has a power of 1/8 HP, or 93Watts. I added a second 009 pump and everything worked fine. I eliminated the cost and heat loss from a second tank, and minimized the steam escaping the system with the larger collector/quench ratio of the bigger tank in the basement.

      I also cheated. On my systems, the pumps are placed inside an insulated cavity (think of a refrigerator door) next to the metal wall of the tank. All the heat generated by the pumps was inside the tank insulation and used to add heat to the tank. On marginal days, if the system ran at all, I got a boost from the pump heat. This works fine for smaller “wet rotator” pumps that are not air cooled. Air cooled pumps must go outside.

      I have looked at the power saving between a glycol system and a drainback system many times and always come to the same conclusion. The extra power needed to pump the full height is completely offset by the problems of a glycol system, including the 15% drop in efficiency, and the recommendation by most training programs that glycol system pumps should be one model higher in power anyway. This is is because the glycol solution requires more power to push the same flow rate as plain water. When you add these two things together, the extra power for a drainback system is a much smaller consideration than all the advantages.

      So, in summary, the advantages of a drainback system offset the slight difference in pump power with a glycol system, and the choice is confirmed by all the disadvantages of glycol systems. You pay a little more to run the pump, you save a lot over the lower efficiency and much higher maintenance costs of glycol.

      Dr. Ben

  5. rvetter

    Dr. Ben,

    I am in search of some impartial advice for my existing glycol solar system. If you are of a mind to take a look at the following post and let me know your thoughts they would be appreciated.

    My system was originally installed in 1979 (Colorado Springs). It came with the house I purchased and after getting some work done on in about 6 or 7 years ago it has operated without any problem.

    Specs: 3 4×8 Flat Panel collectors, a external novan heat exchanger, 119 gallon vessel + standard home gas fired water heater. Also 2 convection heating units piped upstairs and downstairs. The system preheats the water before it goes to the hot water tank, and in addition once it hits 110 degrees it heat dumps into the Myson and McQuay convection fan units.

    The 119 gallon tank is failing about 20+ years of use and I would like to replace it. After talking with a few installers about some repiping the heat exchanger to an internal one in a tank, and repositioning the tank in a different area the cost is upwards of 4k. In addition I have also looked at an option to go to 1 119 gallon tank with 2 heat exchangers that would replace both the heat storage tank, and the water heater. The 2nd heat exchager would be piped into my existing boiler as my backup DHW heater when solar isnt enough to heat it properly.

    Lastly, I have also looked into an option of purchasing a alternative heat bank, a square box manufactured by American Solartechnics, that is 200 gallons with a heat exchanger that would be piped in with my existing system in a similar manner to what I have now, but with more thermal storage. However it is not pressurized.

    I have a few concerns and I am not sure I am getting clear answers from the companies giving me quotes.

    -First is it worth the cost to continue using my existing panels. They have no defects that I can see after 32 years, but they have been in operation for a long time.
    -Next after reviewing your drainback lesson I am wondering if my system can/or should be converted with minimum effort using my existing piping.
    -Does my glycol system REQUIRE and pressurized tank?
    -I am concerned that I make the right upgrades to take advantage of my current piping and solar set up. It seems to me that the idea of a dual exchanger tank piped in with my boiler would take away from my ability to run the space heaters in the manner I do now.

    I apologize for the length of my post, but as you can see I am a bit lost on what my next step should be.

  6. Kirk

    Dr. Ben,

    Thank you for your web site. About 30 years ago I had a contractor install domestic solar hot water and was talked into a recirculation system. I had to use a contractor to get the tax credits. I quickly experienced your top two mistakes. The collectors were installed on top of my garage, and during a Fresno, CA hot Summer day I found melted insulation on the garage floor. The collectors were not tilted properly, and the second winter they were damaged by water freezing in the collectors even though I had drained them. I lost a second set of collectors when I was out of town and an unexpected and very cold weather front arrived early in the season and froze the collectors even though the recirculation pump was running. At least the sensor was in the right place, but it didn’t help. I might add a fourth mistake, the roofing jacks must be properly installed to prevent water damage. I experienced this also. Maybe you have some suggestions for installing the jacks around the insulation so that they shed water but don’t conduct heat away from the pipes.
    I’m preparing for a major home remodeling which will include redoing the domestic hot water system. I want to install a drain down solar system. Two 4 by 8 foot collectors with an 80 gallon thermal storage tank seems about right for my 3 bedroom, 2 bath house.
    I think that when discussing solar hot water, you could include were the water goes, not just the collection system. (Maybe this is on the web sight and I haven’t found it yet.) My old system fed into a 40 gallon electric water heater. I have a time of use meter and so put the water heater on a timer. The water heater comes on about 30 minutes before we shower in the morning, and at 6 pm so we have water to cook, wash dishes and clothes. Since no one has been using hot water between these times, the electric tank is cold and I pay to heat it. When the solar panels froze the last time, I noticed very little change in my electric bill, the solar hot water wasn’t making much of a contribution because of the way it was paired with the electric hot water tank.
    My solution is to make sure I use the solar hot water when available. I can think of two ways to do this. One is to put an “On Demand” water heater after the solar tank. If the solar water is hot or even warm, my on demand unit works less or not at all, I’m not keeping a water heater tank hot even when no solar heated water is flowing through it. A second possible solution would be to place a water heater above the solar tank so that convection from the solar tank , through the connecting pipe, to the water heater would help to offset the heat loss of the heater so that it wouldn’t need to turn on, or would turn on less often between hot water use when hot water is drawn into the tank.
    Thanks for your helpful information.

  7. archimuse

    Hello Dr. Ben,

    I am an architect in VA, and I was looking online for answers about my drain back solar hot water system in my house. Imagine my surprise to see a picture of my house on your website! It is in Blacksburg, VA in the Murphy subdivision, built in about 1986.

    We have loved our solar water heater system–it is fun to look at, low maintenance, and keeps our bills very low. Our original heat pump broke recently, and I am having a new 3 ton Carrier 16 SEER furnace and heat pump installed, probably beginning work this Friday. Although the solar system works great (we had it rebuilt before we moved in–new water tank and pumps, but plumbing and panels were still great), an A/C contractor remarked that we have 2 return air ducts–one coming from the basement into one side, one from the main floor above coming into the other side, but there is only a solar water heat exchanger in the return air duct from the main level. This creates a lot of resistance in that return air duct, causing much more air to be sucked in through the basement return air duct, which has no heat exchanger in it. This creates a negative pressure in the basement, an imbalance in the return air ducts, and also allows only half or less of our air to be preheated by solar.

    Two A/C contractors now have suggested removing the solar heat exchanger from the return air duct and relocate it into the main supply duct located just above the new furnace fan unit. This way we can filter once, and the heat exchanger will benefit from the filtering, and all the air will be, I guess, post-heated by the solar heat exchanger.

    In your opinion, is this a good idea, or do you have a better one? My A/C contractor has worked on these systems before, but not since the 1980’s, and he is not an engineer. My solar heater guy, Brian Walsh of Solar Connexions in Blacksburg, says he has never seen a solar water heater set up any other way than with the solar heat exchanger on the return air duct, and moreover he has great respect for the the original designer of the system, who’s name escapes me now, but I think her first name is Mary (I am currently in Montana on vacation and do not have access to all my papers.). But Brian also mostly does photovoltaic now, and only maintains the old residential water systems because no one else can or will. He also reminds me he is not an A/C contractor.

    So, given that works starts Friday, I would love your advice on whether to move the heat exchanger to the supply, and if not, where to move it. Also, if you have any advice on wiring changes that would need to go along with moving the exchanger, if any are needed, to be sure the heater knows to heat with solar water first, then heat pump second if necessary, then auxiliary electric last. We will be getting a nice new programmable thermostat.

    I have mentioned how well the solar system works now to the A/C contractor, and he has said we can move the heat exchanger back to the original return air configuration if this does not work.

    I am also curious about how the heat exchanger stays clean, other than filtering. Is there an advantage to providing a maintenance access panel to check or clean the exchanger occasionally (by a repairman)? it is providing so much resistance lately, I wonder if it isn’t clogged–we will know for sure when they remove the old furnace this week.

    Thanks so much for your advice if you have a minute this week–we would be happy to compensate you if you would like if you feel you have helpful information.

    • Dr. Ben

      Glad to hear that you like your solar system. We have a number of them in the Blacksburg area. Many have one or more space heating circuits (up to four) along with the DHW. On some larger homes, we might still supply a single zone of space heating. It would be tied into the most used zone of the house, such as the kitchen/den area. The solar exchanger coil is always placed in the duct work on the return (cold) side of the furnace, to act as a preheater. We had special coils made with extra fins to give more heat at low temperatures. Putting the coil on the return air side allows the system to add heat the air all the way down to the return air temperature. This allows three states of control logic: 1) solar heat only, 2) solar preheat to regular furnace, and 3) regular furnace only.

      We supplied two different coil sizes: 15 x 18 inches and 18 x 30 inches. The smaller coil is rated to 1000 CFM max and the larger can go to 2000 CFM. The airflow imbalance you describe is supposed to be balanced by the contractor. All air flow systems are naturally unbalanced and need adjustment of the dampers.

      We don’t recommend placing the exchanger coil on the hot side of the furnace. That eliminates control state 2) above and raises the temperature at which the solar system must operate. Let’s look at an example. With the coil on the return air side it can heat the house alone when the solar tank is hot, say 120F or above. Below 120F, the furnace turns on with the solar as preheater. The solar can continue to contribute to the heat all the way down to the return air temp, say 65F.

      Now consider the solar coil on the output (hot) side of the furnace. The solar tank can heat the house alone down to 120F. Then it must be turned off and cannot contribute any heat when the furnace is running. That represents a loss of all the energy in the solar tank from 120F down to 65F.

      The question of whether to move the solar coil is one of logic. First, if it is the smaller coil it might choke the airflow of the whole system. Second, you would be paying money but not getting any more energy out of the solar system. If the solar system was sized for a single zone, it won’t do any better supplying two zones.

      So, I recommend leaving the coil in the single zone and balancing the air flow between zones to eliminate the pressure differential.

      Now to your question about filters and cleaning. The solar coil is identical in construction to a heat pump coil, so filtering, access, and cleaning are the same for the solar coil as for the heat pump coil. If you don’t have a common filter for both zones, then I recommend installing a filter rack and access panel at the solar coil location.

      Let me know how it turns out.
      Dr. Ben

      • archimuse

        Hi Dr. Ben,

        Actually, the A/C folks came a day early–they are at my house now, and I’ve asked them to hold off on relocating the heat exchanger if possible until I hear from you. I am in Montana at a solar powered cabin on vacation! They have advised me that the variable nature of the 16 SEER Infinity heat pump thermostat is going to be too complicated to wire for a three stage heat system (solar, heat pump, auxiliary), so they have recommended we go down to a 15 SEER model. I’ve told them this is fine, but I am a bit disappointed. Anyway, I’m sorry they started work a day early–I hope you still have time to give us some advice–thanks!


  8. michealclark55

    Glad to hear that you like your solar system. We have a number of them in the Blacksburg area.The solar exchanger coil is always placed in the duct work on the return (cold) side of the furnace, to act as a preheater. For more information

Leave A Comment

You must be logged in to post a comment.

Custom Widget 4

Use these areas to feature your content. Each featured area is widget ready so you can easily add any sort of widget capable content. Images may also be added to these featured areas to help maximize the spotlight you put on your home page content.