$2K Solar Space + Water: Building the Storage Tank

This section shows how to build the heat storage tank for the DIY solar combined space and water heating system. This single tank provides heat storage for both pre-heating domestic water, and for space heating. 
 

The tank is basically a reinforced plywood box with an EPDM rubber liner and insulation.  The tank is easy to build, inexpensive, and has a long and reliable track record.    For domestic water heating, a large coil of PEX pipe is immersed in the tank to act as a heat exchanger to preheat cold water on its way to your regular hot water heater  -- this is very simple, durable, and works very well.   For space heating, water is pumped directly out of the tank, through the radiant heating floor loops, and right back to the tank -- no heat exchanger is used.   Back to Table of Contents...

On this page:

Overview

Laying Out the tank

Locating the Tank...

Building the  Tank...

Build the base

 

Building the tank walls

 

Building the primeter frames

Installing the EPDM Liner...

Building the Tank Lid...

Disclaimer
 

From here, the inside gets high temperature insulation and then an EPDM rubber liner.  An insulated and tight fitting lid completes the tank.

While the tank construction is simple carpentry, there is a lot of water pressure on the bottom and sides, and the tank must be built with some care.

The tank is filled with plain water, and the water in the tank never changes after the initial charge -- it is used strictly to store heat.

This is actually the same tank I have been using on my $1K Solar Water Heating System, with a few small changes to add the space heating function.  It would have been nice to add somewhat more volume to the tank, but the existing tank is in very good shape, and while the volume is on the low side, its OK.  See the Overall Design section for information on sizing the tank.

You will need to work out the dimensions for your tank so that it will have enough storage for your collectors, and so that it will fit in the space you have available.

This is what we did for our tank:

   - The 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 - 2*3.5 - 2*0.75 = 39.5 ) before any insulation is placed inside.  With 2 inch insulation inside the tank, he 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.

Be sure to allow enough vertical for the lid and clearance to remove it -- about 8 inches is just adequate.

Note that if you use a full sheet of plywood for the base, and full sheets for the long sides, you can get the inside dimensions up to about  (35 wide)(83 long)(44 high)/(231 in^3/gal) = 553 gallons.   This is typically enough to support 200 to 300 sf of collector area.  For tanks of this size, some extra precautions must be taken with the tank structure -- see the Solar Shed tank for more details...

Note also that if your tank is to be located in a place where appearance is important, there are several examples of tanks with nicely finished exteriors in the $1K Example Systems...

• The tank must be mounted below the collectors so that water in the collectors will drain back to the tank when the pump is turned off.
   
• The tank should be close to the backup hot water tank, so that not much heat will be lost in the plumbing connection between the solar storage tank and the backup hot water heater.
   
• The tank must be installed on a level surface that is capable of carrying the weight of the 1500+ lbs or so of tank water.
   
• The tank base must be protected from water -- don't install the tank in a puddle!

Our tank is located in the crawl space on good, firm, dry dirt.  The whole crawl space floor is covered with 6 mil poly to keep the humidity level low in the crawl space. 

The tank base sits on top of 2 inches of rigid foam board insulation (XPS), and and additional 2 layers of 6 mil poly.  If anything shortens the very long life of these tanks, the most likely thing is moisture from the outside.  With proper installation, the life of these tanks is measured in decades.

With the crawl space poly film pulled to the side,
leveling the spot for the tank.

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.

If the tank cannot be made large enough to accomodate the large PEX coil, a more compact heat exchanger can be fabricated from copper pipe or tubing.  Since copper is a much better conductor of heat than PEX, much less is needed -- maybe as little as 1/5th?

This is what we are building in this section.

Building the Bottom

On our tank, the bottom is made from a 4 ft by 4ft half sheet of 3/4 inch exterior plywood. 

2 by 4's are used to form a perimeter frame around the outer edge of the base.  These 2 by 4's support the lower edges of the tank sidewalls.  They take a great deal of outward pressure from the sidewalls, and should be carefully glued and screwed to the plywood base.

As mentioned above, its important that the base of the tank be kept away from any wet spots that may exist where the tank is placed.

The tank base with the 2X4 perimeter
frame already in place.
The tank walls are to the left. 

Build the Tank Side Walls

The tank sidewalls are made from more of the 3/4 inch exterior plywood.

The pictures to the right show the vertical edges where two sidewalls meet being reinforced with a triangular shaped wood member that is ripped from a 2 by 4. This method works fine, but the alternative method shown in the box just below seems simpler, easier, and structurally better, so I'd consider using it.

Adding the corner reinforcement to the vertical
edges of the tank sidewall.

Here is an example of this joint in Ken's tank...
 

Build the Tank Perimeter Frames

The pictures to the right 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.   This lap joints should be carefully glued and screwed with a high quality waterproof glue.
 

The upper and lower peripheral frames that support the tank sides are very important.  They are resisting a lot of water pressure -- the water even in this small version of 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.

Note that if you build a larger tank with longer sides, then tension ties should be used to tie together the middle of the 2 by 4 perimeter frames on the long sides.  See the Solar Shed tank for an example...

Install the Inside 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 more 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.  If you need a really well insulated tank, the inside is probably the best place to add insulation.  Only the first layer (closest to the hot water) needs to be the high temperature polyiso insulation.

Cut and install the bottom sheet first.  Then cut the side pieces and install them.  I used a little Great Stuff polyurethane foam to tack them in place, but once the tank is filled, the water pressure holds the insulation in place.

Fill any gaps or cracks with Great Stuff.  Make sure that there are no cracks or shape edges that the EPDM liner will get pushed into by the water pressure.


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.

Bottom of tank insulation board
in place.

In my case, the tank was going into a crawl space where the only access is the hatch shown in the picture.  I had to cut the base plywood in half to get it down, but spliced it back together, and then glued and screwed the uncut edge frame 2 by 4s on.

Determine the Size of the Lining & Cut it Out

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 inches

 

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.

Good to check this by measuring the actual tank with a flexible tape measure -- you don't want to cut it too small!

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.

Installing the Liner

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.  The staples just hold things in place until the edge boards are installed in the next step.

Note on water used to fill: If you have hard water, it is a good idea to fill the tank with demineralized water or to collect rain water and use that.  Doing this prevents scale from building up in the plumbing and in the collector.

It is possible to make waterproof seams in EPDM using available seaming products.  Using this technique, you could make a custom shaped liner that fit the tank without the need for all the folding mentioned above.  I strongly recommend against this based on one person very experienced with EPDM and seaming trying this and having the lining fail immediately. 

Line up the midpoints of the side walls with
the mid points you marked on the
EPDM sheet, and clamp.

Installing the Edge Boards

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.



Later, all of the plumbing connections to the tank will be made by cutting slots in the edge board for the pipes to enter and exit the tank.

This shows the fillers made from scrap pieces of EPDM used to make the surface under the edge board flat.

It also serves the critical safety function of keeping pets and kids out of the scalding hot water.

For all the reasons listed above, the lid must be well secured to the tank.

My lid (from bottom to top) is a layer of EPDM, a 2 inch layer of rigid polyiso insulation board, a layer of hardboard (could be half inch plywood), and 2nd layer of 2 inch thick insulation.

Wood strips along the outer edge of the tank are screwed into the tank upper frame to compress the lid tightly down on the tank edge boards.



 

Cut out the lid layers

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.

The top layer of insulation can be any type, but the lower layer should be the Polyisocyanurate insulation due to the potentially high water temperatures in the tank.
 

Glue the lower 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.

Place a piece of EPDM that is about 10 inches larger than the lid on the floor.  Put some dabs of silicone calk on the EPDM, and then place the lid with the foam board down on the EPDM leaving about 5 inches of EPDM lining sticking out on all edges.   Now wrap the EPDM up around the edges and staple the EPDM to the top of the plywood.

Now place the 2nd layer of insulation on top of the plywood and anchor it with Great Stuff foam.

Install Lid with Hold Down Blocks

The lid is compressed tightly down onto the tank edge boards by boards resting on the top of the lid in the picture.

Long screws go all the way through the hold down board, tank lid, tank edge boards, and into the tank perimeter frame.  These 6 inch screws are tightened firmly  press the tank lid firmly down onto the tank edge.  Around here, these long screws are sold a "log screws".

The wood blocks compress the lid down
onto the tank edge boards.

Plumbing Connections Through Lid (or not)

On my tank, some of 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.

Note that it would be quite feasible to run all the plumbing connections to the tank through the edge boards in the same way that I now for most of the connections.  If I had it to do over, I would do it this way, and just make the lid a single, removable piece.

The lid has a half inch CPVC pipe through the EPDM liner and up through the lid.  This half inch penetration also serves as a point to check the level of the water in the tank, and also to top up the tank if needed (very rare).

When the half inch pipe is not being used to measure tank level, I cover it with a piece of tape that has a small hole it it to provide venting of the tank.

The white triangle at the back is the fixed part of the
tank lid.  The copper pipe going down through
the lid is the collector return line.
This just as easily could have come through the
tank edge board like all the other lines -- this way
the whole lid would have been removable.