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$1000 Solar Water Heater -- Storage Tank, Heat Exchanger, Pump, and Controls

Pros and Cons of this Tank Design

It can provide large capacity inexpensively.  The larger capacity, well insulated  tank improves system performance.

 

EPDM lined tanks have proven to have a long life, and at the end of the liner life (15 years?), the EPDM liner can be replaced at a cost of about $50, and you are good to go for another 15 years.

 

The EPDM liner can handle temperatures up to 180F+, although life is reported to be longer if the tank is kept to 170F or below -- it should be really long at the 140F maximum that this system operates at.

 

The tank can be made to fit in, and make efficient use of limited spaces (in my case a limited height crawl space). 
AND, it can be built in pieces that fit into areas with limited access (such as my crawl space). 

 

The tank is unpressurized and vented to atmosphere -- it can (and does) serve as both the hot water storage tank and the drainback tank for the collector.  This combined functionality eliminates one more expensive item from the system.

 

The design allows you to use as much insulation as you like to reduce heat loss.

 

The tank is large enough to allow the use of a large coil of plastic pipe as the heat exchanger.  This makes for an inexpensive and essentially 100% efficient heat exchanger (see details below).

 

The lack of any liner penetrations below the waterline eliminates the possibility of leaks at penetration points.

 

On the negative side, you need to be certain that the tank sits on level smooth ground, and that it does not sit in a puddle. 

 

Its not as nice to look at as a  cool new stainless steel tank -- but, you could always have the kids draw some artwork on it.

Locating and Installing the Tank

You want to place the tank on a flat, level surface that is capable of supporting the weight, and that will not expose the wood to wet conditions.  Level flat dirt is fine.  A crawl space or basement if fine as long as moisture is controlled (as it should be for the health of your house structure).

I have been asked about sinking the tank into the floor, and on the Solar Shed Project I did this to some extent.  If the ground is dry, and you can isolate the tank from the ground (say with insulation board), I think this is OK.   I prefer the above ground installation as you can be sure thinks will stay dry and you can inspect the outside of the tank easily from time to time.  For sunken or damp locations you might want to consider using Medium Density Overlay (MDO) plywood, which is very resistant to moisture, but costs about twice what ordinary plywood costs.

The other important consideration in locating the tank is the relationship of the tank to the collector and the relationship of the tank to the existing hot water tank or tankless heater.   This is a drainback system, so the tank water level MUST be below the level of the bottom of the collector and below all the plumbing from the collector to the tank.  All of the water in the collector and in the plumbing from the collector to the tank must drain by gravity when the pump is turned off.   The more direct and short this plumbing from the collector to the tank is, the better.  All of this plumbing must slope toward the tank.

The location should also be as close to the existing hot water tank as is practical.  Basically, the cold water supply pipe that now runs to your existing  hot water tank will be broken, and detoured over to the solar storage tank and then back to the hot water heater.  If pipe going from the solar storage tank to the existing hot water heater is long, the water in this pipe will cool off between hot water draws, and the solar energy that went into heating it will be wasted.    Its not the end of the world if you can't locate them right together, but its desirable if you can.

Tank Construction

The finished tank is too large to fit through my crawl space hatch.  So, the tank parts are cut out, and a trial assembly is done in the shop.  It is then disassembled, and the pieces are passed through the crawl space hatch.  The final assembly is then done in the crawl space.  I assume in the sequence below that you will be faced with a similar situation, and will have to take the tank apart after a trial shop assembly -- if not, just go ahead and do the final gluing and screwing right in the shop and move the assembled tank to where you want it.

Building the Tank Box and Frame

The tank walls and bottom are cut from 3/4 inch exterior plywood.  The top and bottom peripheral frames that support the sides and hold everything together are made from good 2X4's.   As you build the tank, bear in mind that it will hold about 1500 lbs of water, so work carefully, and pay careful attention to making good joints.   This is really simple carpentry, but it must be done correctly.

You can adjust the tank dimensions to fit your space available and storage needs.  Be sure to account for the volume of insulation that you will be adding inside of the tank walls when you make your tank capacity calculation.  Another consideration in choosing the tank dimensions is that if you plan to use the pipe coil heat exchanger described below, you want the tank to be large enough to hold the pipe coil.

The dimensions I used are as follows:

   - Base of tank is a 4 ft by 4ft half sheet of plywood -- this makes the the inside of the box 39.5 inches square (48 - 3*3.5 - 2*0.75 = 39.5 ) before any insulation is placed inside.  If you use 2 inch insulation inside the tank, then the inside of the tank will come out just over 35 inches square with the liner installed.

 

- The height of the tank walls is 35 inches.  If you add 2 inches of insulation in the bottom, and allow about 2 inches of airspace above the water, this gives you a water depth of 31 inches. 

 

- The overall height with the lid thickness and the insulation under the base is 41 inches.

 

- So, the volume of my tank is:  (35 inch)(35 inch)(31 inch)/ (231 in^3/gal ) = 164 gallons

None of this is at all critical -- within reason, you can change the dimensions to suit your space.  If you want to use the 300 ft coil of PEX heat exchanger (see below), then the inside of the tank can't be a whole lot less than 35 inches across.

Click on pictures for full size

The bottom is cut out allowing sufficient space for the 2X4 frame that surrounds the tank walls.   The 2X4 edge frame is temporarily screwed to the bottom.  I made the bottom 4ftX4ft to make maximum use of the plywood sheet.

The walls are cut out of the same 3/4 inch plywood. 

Note the vertical corner reinforcement piece being installed.  There is one of these at each corner.  While I made these pieces triangular, they could be square, and it could be the same thickness as the insulation you plan to install inside the tank.

The pictures above show the upper perimeter frame.  This frame supports the top of the tank walls, and provides a flat surface for the lid to seal down on.  Use 2X4's without a lot of defects in them.  The lap joint shown provides a good strong joint which is needed to resist the water pressure.  
 

The upper and lower peripheral frames that support the tank sides are very important.  They are resisting a lot of water pressure -- the water in the tank weighs about 1500 lbs.  The lap joints shown for the top frame provide the needed shear area for a strong joint.   This lap joint results in the 2X4's on two sides being down 1.5 inches from the top -- to make a level surface for the lid to seal down on, just add another section of 2X4 on these sides.  Then add metal splice plates in the corners to further strengthen these joints.

Tank Insulation

Next, the tank insulation panels can be fitted. 

Use polyisocyanurate insulation to withstand the high temperatures that the tank may see.  The polystyrene pink, blue, or white insulation board will not stand up well to temperatures over 130F, and should not be used.  Many lumber yards carry polyiso insulation, although they may not know it by that name -- ask to see the actual sheets -- Polyisocyanurate will be spelled out somewhere on the sheet.  The polyiso is only a bit expensive than polystyrene, and in addition to be heat resistant, it has a higher R value per inch.

I used 2 inches of polyiso insulation inside the tank sides and bottom.   This provides about R14.  Additional insulation can be added either inside or outside if that is not enough.  Using insulation inside the tank has the advantage that there is no thermal bridging from the framework.

The tank sits on an additional 2 inches of polystyrene insulation in the crawl space.

The lid is insulated with 2 inches of polyiso, plus a sheet of polystyrene over it.

At this point, just cut the panels to size and make sure they fit -- do not secure.

In my case, the bottom of the tank was too large to fit through the crawl space access as one piece, so I cut it in half and added metal splice plates to put it back together.  NOTE -- don't cut the peripheral 2X4 frame in half.

Giving everything a coat of paint at this point is also a good idea

Cutting Out the EPDM Liner

Cut the EPDM liner so that a single piece can be used for the entire liner.

The liner dimension are  about

2 * Sidewall Height + Bottom Width + 10

 

For example, if the inside height is 33 inches, and the bottom width inside the walls is 37 inches, then the size of the EPDM piece is 2*33 + 37 + 10 = 113 inches.

If the tank is not square, then bottom width will be different in the two directions, and the liner will be rectangular rather than square.

Where these dimensions are measured inside the insulation.

The 10 inches allows for the EPDM to lap over the top of the insulation and the peripheral frame.  This is important, as the EPDM provides a sealing plane for the top to fit on.

Lay your EPDM sheet out on a big flat surface (like the lawn).  Mark of the dimensions established above, then check them, then check them again,  then cut the EPDM with a pair of scissors.

Before you fold up the EPDM, mark the center of each side with a marker or tape.  This make make it easier to do the final installation in the tank.  If the tank is rectangular, mark which side is the longer one.

Move the Tank to Crawl Space

Now the tank can be disassembled and move into the crawl space.

My crawl space floor. Cut back the existing poly on the floor, and level and compact the dirt to make a good base.

Down the hatch with all the parts.

Install and seal down new 6 mil poly in the tank area.  Install the insulation board over the poly.  Splice the bottom back together if you had to cut it in half.

In my case, the crawl space has a layer of 6 mill poly in place already.  I cut this back to level the dirt underneath and compact it.  I then resealed up the existing poly with caulk.  I then added a new 6 mil layer of poly in the tank area. Then a 2 inch thick layer of extruded polystyrene is placed on top the poly.  The tank base goes on top of the insulation.    This gives a dry, level, and low heat loss surface for the tank to rest on. 

If you had to cut the base in half to get it through the hatch, then reattach the halves.

The pictures show the tank being assembled in the crawl space.

Be sure you use glue and screws for all the joints.

Our crawl space is so dry, I only painted the bottom and lower sides, but it would be good to paint everything.

Installing the Insulation

Once you get the tank box assembled, re-install the insulation panels.  I glued them in place using the polyurethane foam in a can.  Be careful with this stuff in that if you get in on your skin you will be wearing it for a week.

Fill up any remaining cracks and gaps with more foam in a can.

Installing the Liner

Next, install the EPDM liner that you cut out earlier.

Mark the mid points of each side of the wood upper tank frame with a marker.

Push the liner into the tank, and line up the mid points on the EPDM liner that you marked earlier with the mid point marks on the frame.  If you have rectangular tank, make sure you get the long side of the liner with the long side of the tank.  Clamp the liner mid points to the tank sidewall midpoints.

Push the liner down to the bottom of the tank.  Then, take your shoes off, and work from inside.

This will look like a real mess when you start, but just work all the excess material into a single fold at each corner.  Some clamps are helpful to keep things in place.  Make sure that there is enough slack in the EPDM that it won't be stretched by the water pressure -- you want the liner to bear against the tank wall everywhere -- no bridging.  There will be fold lines in the EPDM -- this is OK.  When you get things in place, climb out of the tank, and put about  half a foot of water in -- then work the EPDM around and make certain that the EPDM is supported by the tank walls and bottom with no bridging.

Once everything is the way you want it, run a bead of silicone between the EPDM and the top frame, and then staple around the top edge with stainless steel staples.  

At this point, you want to install an edge board all the way around the ledge on the top of the tank.  The board holds down and seals against the tank liner you just installed, and provides a flat plane for the lid to seal against.   I have been using the plastic deck boards that lumber yards sell for this.  It seems to hold up to the warm hot conditions inside the tank much better than wood. 

Before installing the edge board, use some scraps of EPDM to even out the top surface of the EPDM all the way around the top of the tank.  The folds that you put in the tank will mean that in some places you have 3 thicknesses of EPDM lapped over the top ledge of the tank, and in other places there is just one layer.  Use the scraps to even this up.  Apply the scraps with silicone caulk.

Once the surface of the EPDM covering the top frame of the tank is fairly even, put a bead of silicone around the top frame, then screw the edge board down over the silicone and EPDM.  Use either stainless steel screws, or coated deck screws to prevent corrosion.

The two ends of the heat exchanger coil (see below) will exit the tank trough openings you cut in this edge board later.

Heat Exchanger

The heat exchanger for this solar water heater is a 300 ft coil of High Density Polyethylene (HDPE) plastic pipe.  This coil of pipe is immersed in the storage tank.    Cold water bound for the hot water tank first passes through the HDPE pipe coil where it picks up the stored solar heat in the storage tank.  If the water needs further heating, the hot water tank tops it off.

The HDPE pipe coil itself holds 12 gallons of water.  So, for each new hot water demand, there will be 12 gallons of water already in the pipe coil that has been heated up to the full temperature of the storage tank.   So, for the first 12 gallons of demand, the heat exchanger is essentially 100% efficient.   If more than 12 gallons is needed, then the 300 ft pipe coil acts as a conventional heat heat exchanger and the outlet temperature will drop somewhat below the storage tank temperature depending on the flow rate.  I have not had a chance to test how much the temperature drops, but based on previous tests with smaller coils, I believe that the performance will be good.

Since most hot water demands are less than 12 gallons (showers, dish washing, ...),  the heat exchanger will be 100% efficient with water delivered at the full temperature of the storage tank.  Pretty nice for a $70 heat exchanger.

Using HDPE pipe here is pushing the envelope somewhat in that it is not designed for handling high temperature water at high pressure.  This is one reason that the tank temperature is limited to 140F.   If you have high water pressure, you will want to consider changing the pipe coil to PEX as discussed below.

If the use of the HDPE poly coil makes you uncomfortable, or you have high water pressure that might exceed the HDPE ratings at high temperatures, then you could use a coil of PEX.  Here is one outfit that has a potable water rated coil of 1 inch PEX for $170

http://www.blueridgecompany.com/radiant/hydronic/453/rht-radiant-pex-b  with free shipping! :)  It would also be possible to use a shorter or smaller diameter coil without compromising performance significantly.   Even if you use PEX for the heat exchanger coil, I favor keeping the maximum tank temperature down to the 140F area.  This (usually) eliminates the need for an anti-scald (tempering) valve, and generally makes the life easier for the tank liner and pump.

The picture above shows the 300 ft coil essentially as it comes.  It is best to just leave it in this coil shape rather than trying some kind of recoiling to fit your tank -- this can lead to a real mess.  In my case the full size coil would not quite fit through the crawl space hatch, and in this picture I am dividing it into two connected coils to pass each one through the hatch separately.

The pipe I bought is rated at 100 psi.  I intentionally chose the lowest pressure rating just to see how it did and whether any problems will result.  You can get the HDPE all the way up to 200 psi, and these higher pressure ratings will provide more margin of safety.

Be sure that the pipe is NSF rated for potable water -- do not get "utility" or irrigation grade pipe.

Heat Exchanger supports

The heat exchanger should be supported up off the bottom of the tank to get it into the hotter part of the tank when the tank is stratified.

I used 3 full size concrete blocks to do this.

To prevent an abrasion between the blocks and the pipe coil or tan bottom, I siliconed  scraps of EPDM on the top and bottom of each block. 

In general you need to be careful about materials used inside the tank.  The temperature is high and its wet :)

Silicone, EPDM, polypropylene cord all seem to be OK.  Most kinds of metal fasteners don't do well -- including galvanized. 

The picture above shows the pipe coil sitting in the tank and resting on the 3 support blocks.  Short lengths of half inch CPVC pipe are used to space the coils apart -- this allows for better heat transfer from the storage tank water to the pipe coil.  The coils are tied into shape using the white propylene cord.   The two ends of the 300 ft coil are unwound just enough to lead them over the top of the tank sides -- they will pass through sealed openings that are above the EPDM tank liner.  

When filling things up, fill the pipe coil first, then the tank.  If you don't do it this way, the pipe coil will just float up on top the tank water.

Note that there are no penetrations of the tank liner, and there are no plumbing fittings inside the tank.  That is, the ends of the HDPE pipe are taken outside the tank where the connections to the house hot water plumbing are made.

Since the fluid in the tank is plain old water, there should be no issue of needing a double wall heat exchanger, but this is something you might want to confirm with your local code folks.

The two ends of the 300 ft pipe coil must be brought out of the tank to connect into the cold water pipe going to the existing hot water heater.   To do this, I cut two slots in the edge board that was just installed. Each slot is just wide enough for the 1 inch pipe to exit through.  I located these slots to allow the HDPE pipe to exit the tank in a straight line tangent to the pipe coil circle.  This allows you to have the pipes exit without kinking or bending them.    The pipe is about 1.25 inches outer diameter, and the edge board is a bit less than an inch thick, so the pipe will stick up a bit above the edge board -- this is OK -- the EPDM lining on the lid will accommodate these small bumps.

Picture shows the two ends of the 300 ft HDPE coil exiting from the tank.  The light gray board is the tank edge board, and the pipe exits the tank through slots cut in this board.  The small plywood board that the HDPE pipe goes through at the edge of the tank holds the HDPE in place. 

The 1 inch HDPE is adapted to the 3/4 inch copper pipe with a standard 1 inch barbed to 3/4 inch threaded, right angle adaptor.   At the time this picture was taken, clamps were still being used to hold the lid down.

The Lid

The lid is important.  It must seal down to the tank well, otherwise it will leak a lot of water vapor out, which is a big heat loss and also not good for your crawl space or basement.  The lid also provides the insulation to prevent conductive heat loss out the top of the tank.

My lid (from bottom to top) is a layer of EPDM, a 2 inch layer of rigid polyiso insulation, a layer of hardboard (could be half inch plywood).   The lid is large enough to go to the outer edge of the edge boards you installed earlier.

Cut out the plywood tank top large enough to cover the edge boards on top of the tank.  Then cut the rigid foam board insulation to the same size. 

Glue the foam board insulation to the plywood lid using the polyurethane foam in a can.  Use some weights on the plywood to keep the foam from expanding until it sets.

On my tank, the plumbing connections between the tank and the collector go through the lid.   The part of the lid that these connections go through is permanently sealed to the tank.  In my case, I used a triangular shaped piece at the corner of the tank closest to the collector for this fixed portion.   Your plumbing arrangement will dictate which portion of the lid you want to make fixed to bring your collector plumbing in.

This shows the part of the lid that is fixed in place (white triangle).
Having a fixed part of the lid allows the main part of the lid to be removed without disturbing the plumbing connections.
The orange PEX-AL-PEX  lines connect to the bottom of the collector (one to left side and on to right side).   They are

T'ed together, and the single line then drops down through the tank lid and connects to the pump.
The electrical wire for the submerged pump goes down through the tank lid right next to the PEX.
The white CPVC line comes from the top of the collector.  This line penetrates the lid, and ends
above the waterline in the tank.  When the drainback process starts, air goes up this line to allow the
drain back to occur -- so the line must end above the tank waterline.

So, decide where the fixed portion of the lid will be on your tank, and cut off that portion of the lid -- so, now you have a lid in two parts -- one fixed and one that can be removed.  I would make the fixed part as small as practical.  The electrical wire for the pump power will also go through this part of the lid.

Cut a piece of EPDM that will cover the removable part of the lid, and make it large enough so that it will wrap around the edge of the insulation and up a little ways onto the plywood.  This will let you staple the EPDM to the plywood to hold it in place.

Lay the lid on the floor with the insulation up, apply some bead of silicone, then lay the EPDM over the insulation.  Flip the lid over, and pull the EPDM up over the plywood and staple it in place. 

Do the same with the fixed part of the lid.

Temporarily clamp the fixed part of the lid over the tank, and work out exactly where your plumbing penetrations will be.  You will need one penetration for the collector supply line (from the submersible pump), and a 2nd line returning water from the collector to the tank.

You will also need a hole for the pump electrical connection.

Drill these holes for a snug fit of the plumbing lines. 

The hole through the EPDM layer should be just large enough to allow you to push the plumbing lines through the EPDM -- this way the EPDM will seal around the lines.

Install the fixed portion of the lid with long screws.  Very long coated deck screws work well.

Use a light bead of silicone between the edge board and the lid.  The silicone does not adhere strongly to the EPDM, so you will not have any trouble removing this portion of the lid later if need be.

At this point, the large and removable portion of the lid can be put in place and secured.  Initially, you may just want to use a clamp on each edge to keep the lid down.  When you are sure that everything in the tank is done, you can screw the full lid down with long screws. 

You can also add another layer of insulation board on top the tank to further reduce heat loss.

This is a picture of the tank with lid on.   The lid is the part above the light grey tank edge board.
The lid layers from the bottom are:

1) EPDM membrane,

2) 2 inches of insulation,

3) half inch plywood or OSB,
4) 2nd layer of insulation lightly secured to plywood.
 

The lid is still being held down by clamps when this picture was taken.

 

Plumbing Connections

Update November 11, 2008:  Details on the new pump ...

Collector Plumbing

A submersible pump is located near the bottom of the solar storage tank.  Water from the tank is pumped by this pump up a line that goes through the lid of the tank.  This line is T'ed off into two lines -- one line feeds the bottom of the left half of the collector, and the other line feeds the bottom of the right half of the collector. 

Water returning from the two halves of the collector are joined into a single pipe at the collector, and this pipe runs down to drain back into the airspace above the water in the storage tank.

When the pump turns off, all of the water in the collector and collector plumbing drains back into the storage tank -- this provides freeze protection.  No antifreeze is used.

I have used a solar PV panel to directly power a small submersible pump, and this also worked well.  In this case, a thermal snap switch should be installed in the power line to the pump to shut if off when the tank temperature reaches 140F.  The thermal snap switch could probably be installed between the EPDM lining and the first layer of insulation, and near the top of the tank.

With either type of pump, when the pump starts up, it must have sufficient pumping head capability to pump water from the water level in the tank up to the top of the collector.  So, if the top of the collector is 8 vertical feet above the level of the water in the storage tank, the pump must have a startup head rating of at least 8 ft.  Once flow is established, and all the pipes are filled, the required pumping head drops down to just the friction losses in the system.

The pump should be able to produce a flow through the collectors of around 0.04 to 0.05 gpm per sqft of collector area.  So, for my 48 sqft collector, about 2 gpm is sufficient. 

The picture shows the March pump I am currently using to pump water from the tank to the collectors.  It is a March 893 12 VDC pump.

The pump is very light weight, and simply hangs from the PEX outlet line.  The wire supplying the pump had to be spiced below the water level in the tank.  To do this I soldered the wires together, than used heat shrink tubing, and finally I put a length of half in CPVC over the connection and filled it with silicone. 

Since the controller provides a 120VAC outlet for the pump, I have a small 12 VDC power supply I plug into the controller -- the pump is then powered by the 12 VDC power supply.  The power supply and pump together only draw 20 watts as measured with a Kill-A-Watt.

I'm still looking for the ideal pump and controller for this system.  It seems like with the modest pumping requirement and modest temperature requirement, there should be a high quality low cost solution out there somewhere?  Let me know if you have seen something that might work better.

Update November 11, 2008:  Details on the new pump ...

If you are using one of the Grundfos or Taco HVAC circulation pumps, please note these installation cautions...

House Hot Water Plumbing

Picture shows plumbing connections between solar tank and the house water heater.

The tankless water heater that provides backup water heating when the solar heated water is not up to the needed temperature.  The tank is on the right. 

Basically, the cold water inlet to the tankless heater is brought over to the solar tank, and then back to the tankless heater.  This way if the water in the solar tank is already hot enough, the tankless heater does not turn on at all.

I included valves that allow the solar tank to be cut out of the system, and the cold water routed directly to the tankless heater -- this way the solar tank can be worked on or drained without turning off the home water.

OK, its not pretty, but its very functional and durable --- who needs pretty in the crawl space?

Controls

A Steca differential controller is used to control the collector pump.  The controller uses a temperature sensor mounted in the collector and a temperature sensor mounted in the tank near the bottom.  When the collector temperature goes above the tank temperature by an amount that you can set, the controller turns the pump on and circulates water through the collector to heat the tank.  When the collector temperature drops below the tank temperature, the controller turns the pump off.

This controller also has a tank overheat feature that allows you to limit the maximum temperature of the tank water to a value that you set.  I use this feature to limit the storage tank temperature to 140F -- this insures a long an happy life for the EPDM liner, HDPE pipe coil, and the pump -- its an important feature.

The wiring is very simple. 

Install the temperature sensor in the collector.

 

Install the temperature sensor in the tank (look for one that can just be immersed right in the water).

 

Hook the two temperature sensors up to the differential controller.

 

Plug the pump into the controller (it probably has a 120VAC outlet for this).

 

Plug the controller into a wall outlet.

 

That's all there is to it.

If a PV panel is used to run the pump, then the PV panel itself acts as the controller.  When there is sun on it, it pumps, and if not it does not pump.  It may be hard to find a pump that has enough startup head and can be run from a modest size PV panel.  But, the March pump I am using now works ok with a PV panel.   In order to keep the storage temperature from going over 140F, a thermal snap switch can be installed in series with the pump and mounted just under the tank liner.  The snap switch wants to to to open when it reaches 140F, and be closed at below 140F.  One caution on using PV to drive a drain back system is that the PV must spin the pump fast enough on startup or after some clouds go by to pump the water all the way up to the top of the collector.  This did not seem to be a problem whit the March pump I am using and a PV panel that is about 15 watts, but I'm not sure this is always going to be true -- particularly if you collector is well above the tank.

Gary September 21, 2008, October 10, 2009, April 23, 2010