Solar Stock Tank -- Construction Details
This page provides the details on how to build a solar heated and well insulated horse/stock watering tank.
The combination of solar heating and good insulation eliminate the need for electric tank heaters in all but the most extreme weather.
The pictures below go over the construction of the tank in detail.
Its a simple project to build.
Click on pictures for full size images.
Build the base frame sufficiently large to allow for 2 inches of rigid foam board insulation around the sides and back. In the front, allow about 3 inches of space behind the glazing. The easiest way to get the size right is to place your tank on the shop floor, and build the frame around it with the right clearances.
With the frame size established, glue the corners and nail or screw.
Check the frame for square by measuring the diagonals.
Glue and nail the box verticals in
place. The height of the verticals should be measured carefully to allow
for the height of the tank plus two layers of 2 inches of rigid foam board insulation
under the tank. So, if the tank is 22 inches high, the verticals would be
22 + 2 + 2 = 26 inches high.
Now, assemble the top frame at the top of the 4 verticals. The top frame is exactly the same size as the bottom frame.
Next, cut out the plywood sides and back for the box.
On the front of the box, add the 2X4
filler to provide a surface for the side to screw to, and a place to attach the
collector glazing -- see the middle picture above.
On the back of the box, add a 2X4 filler along the back vertical to provide a surface to nail the sides and back to -- the added 2X4 should be flush with the outer edge of both the side and back of the top and bottom frame. The picture shows this as a 2X2, but it should have been a 2X4, and should be installed so that it is flush with both the back and side edges of the top and bottom frame.
This added 2X4 provides a surface to glue and nail the sides and back to.
See the middle picture above.
Box frame with sides and back installed. The collector glazing will
go in the front opening.
Cut out the top.
The top should be cut large enough to overhang 1 inch on each side, 1 inch on the back and 2.5 inches on the front. The larger front overhang allows for the thickness of the outer layer of glazing.
Cut a hole in the top that is sufficiently larger for the stock to drink through. The drinking opening for this tank is 16 inches deep by 22 inches wide.
The bottom can be cut to the same size.
I used MDO (Medium Density Overlay) plywood for the top to provide for more weather resistance. The MDO holds up very well in outdoor weather, but is about twice as expensive as plywood.
Paint everything inside and out. Home Depot had a returned gallon of this lovely color for $5. It is a combination exterior paint and primer -- what a deal.
Seal all the edges to prevent air infiltration.
Put a bead of silicone seal all the way around the surface that the top will sit on and let it cure -- this forms a sort of gasket to reduce air leakage once the top is screwed down. The top should not be glued down, as you will want to be able to remove it.
Flip the box over, and screw and glue the bottom to the box frame using deck screws. Seal all these edges.
Cut out the rigid foam board insulation panels to fit the bottom, back and sides.
Test fit them to make sure they fit against the plywood back, bottom and sides.
Put some dabs of Great Stuff polyurethane foam in a can on the bottom. This will act as an adhesive to hold the insulation board in place.
Place the insulation board into the Great Stuff, and then weight it down to let the foam cure.
The first layer on the back and sides is 1 inch thick insulating boards (could be 1.5 inch thick) that is pushed in between the box frame 2X4's.
A 2nd layer of foam board will be installed inside this.
Now add a 2nd layer of 2 inch thick rigid foam board over the bottom, sides and back.
Use the Great Stuff as adhesive in the same way as the first layer.
If this seems like a lot of insulation, you may be right -- see the discussion at the end on insulation levels.
I used a mix of insulation board types because I had some of the grey polyisocyanurate insulation left over. The extruded polystyrene (usually pink or blue) is probably a good choice for this application, since it is very resistant to absorbing moisture, and will keep its insulation value even in wet or moist situations.
This tank uses two layers of SunTuf glazing on the south tank wall. The sun shining through the glazing onto the black painted tank wall heats the water significantly on a day with sun. The double glazing reduces the heat loss at night to about half what it would be for single glazing.
Completed box -- ready to install the galvanized tank and the lid.
Note that the last corrugation on the left and the last corrugation on the right have 3/4 by 3/4 inch (true size) vertical strips of wood that seal up and support the edge of the glazing.
Paint the side of the tank that will be facing the collector glazing with black paint to make it a good solar absorber. I use high temperature, flat black BBQ paint that most hardware stores sell. Set the tank aside and let the paint fully dry -- if you can put the painted side in the sun for a day that will help fully cure the paint.
Place the tank in the box with the back of the tank against the back of the box. There should be an inch or 2 or 3 between the inside glazing the tank wall.
I installed a layer of the Reflectix bubble insulation on the underside of the cover. The idea is that the Reflectixs will help to reduce radiation losses from the water surface and also provide a better seal between the cover and box -- besides, I had it on hand :)
The top is secured with 10 deck screws to allow it to be removed easily.
The finished stock tank in the shop.
Stuff fiberglass insulation in the
open areas to insulate further and
reduce air circulation.
Prepare a place for the tank to sit. A good way to do this would be to make a bed for the tank of a few inches of washed gravel -- this would keep it out of the mud. The collector side of the tank must face rough south -- anywhere from SW to SE is ok.
Since we did not get the tank done until mid winter, there was snow on the ground when it was placed. We put some foam board scraps under the tank to keep it off the snow a bit. As the snow melted in the spring, the tank settled unevenly and now sits at a bit of an angle.
Gary March 15, 2009