The basic principle of a solar water heater is simple. Water or an antifreeze solution flows through pipes in a large, flat, enclosed box known as a flat plate collector, or through a series of vacuum tubes in an array known as an evacuated-tube collector. As the liquid moves through the system, solar heat is transferred to it. In a thermosyphon system, the solar-heated water flows into a storage tank and is used directly. In a drainback system, the solar-heated liquid—which can be water or antifreeze solution—flows into a heat exchanger inside a water-storage tank, where it heats potable water; the solar-heated liquid is not used directly.
Storing the hot water in a separate tank is necessary because it takes longer for the water to heat up, and because a large supply is needed to last through the night and early morning. Solar water heaters are usually paired with a conventional gas or electric water heater—either a tank or a tankless, point-of – use type—to ensure uninterrupted hot water during cloudy periods or times of heavy use, but the conventional heater won’t turn on unless it’s needed, which saves considerable money. And solar hot water heaters work anytime the sun is out, even in winter.
A number of different manufactured hot water collectors are available, but you can build your own for a fraction of the cost using wood, copper pipe and polycarbonate glazing—all materials available at home centers. The concept is simple, and can be modified to fit your house and needs. There are also many alternative designs available on the internet (see Resources, p. 152).
Hot water collectors can serve a number of different purposes. If you have enough sun, they can provide all the hot water for your household, but even on cloudy days the water will warm up enough to reduce the amount of energy you need. Using them with tankless heaters saves even more money, eliminating the need to keep a conventional water tank full of expensively heated water all day and night. Hot water collectors can also be used to provide heat for pools and hot tubs, and to heat water for use in a heating system.
This attractive solar heater array provides hot water for two families.
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If you can solder pipe and cut wood, you can build a collector like this, and start saving money on water heating costs right away. |
This flat plate collector can be used with several different types of solar hot water systems. In warm climates it can work with a thermosyphon storage tank (see p. 98); in cooler climates where freezing is a problem it can be used as a rooftop or wall collector with a drainback system (see p. 100), or other type of pump-controlled system. It’s also possible to use this collector with a system containing antifreeze, but water heaters with antifreeze require special plumbing and safety features to avoid contaminating the water supply, and should be discussed with a plumbing inspector or left to an expert.
The collector is constructed of wood with a layer of insulation to help retain heat. The panel covering it is made from polycarbonate, a type of clear acrylic that resists the UV damage that clouds and cracks regular acrylic. Cool water comes in through a %" pipe at the bottom and is gradually heated as it rises through a manifold of /4" copper pipes. Heat is collected and transferred to the pipes by thin aluminum panels lining the box and shaped over the copper. As the water warms in the pipes, it rises to the top and flows into the upper part of the storage tank as cool water from the bottom of the storage tank flows in to replace it. This water movement continues until the water in the storage tank is hotter than the
water in the collector, at which point the thermosyphon action stops or the thermostatically-controlled pump switches off.
The collector can be mounted on the ground (as we did—see p. 50), the roof, or the side of the building, at an angle based on the latitude (see p. 51). However, these collectors can get quite hot during the summer months, and they are often placed at a steeper angle so that they face the low winter sun more directly, and deflect some of the intense heat from the high summer sun.
Use polyisocyanurate rigid insulation (usually called “polyiso”) for the insulation in a solar collector as it has the highest R-value and is also the most heat-resistant type of rigid insulation. Polyiso has a variety of trade names; just look for the insulation with the highest R-value.
To simplify construction, the size of this collector is based on a sheet of plywood, but it can be built a different size or orientation or ganged together with other collectors to make a larger array. Deciding how big a collector you need is mostly trial and error based on your usage and climate, but the square footage of a sheet of plywood is a good starting point for an average household. If it’s not enough, you can always add another one.
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The frame that houses this solar water heater is constructed from 2 x 4 and 2 x 6 dimensional lumber. Cut the pieces of the frame to length, then join them with 21/4" deck screws driven into pilot holes.
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1/4" |
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Cut the %" plywood to size, then set it inside the frame as a spacer for the 1/4 x 1/4" nailers. If you can’t locate 1/4 x 1/4" trim stock for the nailers at your building center, rip-cut 1/4’4hick strips from a piece of 1x stock (a tablesaw is best for this, but you can use a circular saw and straightedge cutting guide, too). Screw the nailers to the inner perimeter of the frame with 11/4" deck screws. Check to make sure the plywood is flush in back as you fasten the nailers.
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Flip the frame over and set the 1/4" plywood back into the frame against the nailers. Screw the plywood in place with 1/4" machine screws driven through the back panel into the nailers.
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Cut panels of %”-thick closed-cell, foil-faced insulation
and fit them inside the frame between the 1/4" nailers, flush with the tops of the nailers. Cut a panel of 1/4" plywood to fit inside the frame over the insulation and the nailers and attach the plywood to the nailers, driving a 11/4" screw every 10 to 12" around the perimeter. Caulk the gap between the 1/4" plywood and the frame with clear, 100% silicone caulk.
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The “guts” of this solar water heater
is an array of copper tubing through which the water runs to absorb heat while it resides inside the box. The matrix of copper tubes is assembled using 1/4-to-1/4 reducing Ts connected by short lengths of 3/4" tubing on the ends and long lengths of 1/4 tubing filling out the space from end – to-end. Making this copper “manifold” requires that you be able to solder copper plumbing pipe. Cut all the copper pieces to length, then clean, flux and assemble them into the grid as seen in the Diagram on page 92. Make the inlet and outlet pipes a few inches longer than you need—they’ll be cut shorter after pressure-testing the completed manifold.
continued
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Solder all the pieces together. Protect the floor from drips of solder and flux with a dropcloth. Wait at least 5 minutes to touch the copper after soldering it—it will be very hot.
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Before placing the manifold in the collector box, test the manifold for leaks. Solder a cap to the outlet at the top, then solder a 3/4 x V2" reducer/male coupling to the inlet at the bottom. Wrap the threads with teflon tape, then attach the hose transition (available at home centers). Attach a hose and turn the water on. Leave it on for a few minutes; if there’s a leak in one of the joints, you’ll hear air and then water hissing out. If there are no leaks, drain the water and cut the cap and the reducer coupling off, then shorten both pipes to 5" measured from the last T.
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Mark the location of the cold-water inlet and the warm-water outlet on the sides of the box by setting the copper grid into the box and extending the inlet and outlet port locations onto the outside of the frame. Drill the holes with a 1-diameter bit.
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Mark the manifold locations on the plywood, then cut 4"-wide pieces of aluminum flashing and staple them so they are centered under each length of copper. These will help transfer the heat to the copper pipes. Use stainless-steel staples.
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-| л There are a few suppliers for preformed aluminum fins (see AvI Resources, page 152), but you can easily make your own using aluminum soffit panels, a plywood jig and a sledgehammer. Build a jig to make your aluminum fins using two pieces of 5/8"-thick plywood or hardwood screwed to a plywood base. Space the gap between the two plywood pieces by using the %" steel rod and two scraps of aluminum soffit as a guide. Buy solid (not vented) soffit panels with V-grooves and then cut the panels using a razor knife and straight edge into 63/4"-wide strips with the V-groove in the center. The V-grooves are then formed into round channels that fit tightly over the copper pipe. Form them by pounding a 558"-dia. steel rod down into them with a sledgehammer (or similar heavy weight).
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Fasten the manifold in place with copper pipe straps,
equidistant from the edges. Spread a bead of silicone caulk on both sides of the copper pipe to fill the void along the bottom edge, then push the aluminum fin sections over the pipe and staple or screw them down as close as possible to the pipe. (The silicone helps to transfer heat from the fin to the copper.) Butt the fins against each other.
-t s’* Create cross-bar supports for the polycarbonate glazing by installing two lengths of У2" EMT conduit across the box. Drill У4" holes halfway through the 2 x 4 sides, centering the drill bit 3/8" down and drilling from the inside. Square the holes with a chisel, then place the conduit in the holes.
Lay the glazing on top of the box and mark the spots for У4" nailers at the sides and the center overlap. Fasten the strips with 6d galv. casing nails.
continued
Screws with neoprene washers
-| ^ Place closure strips at the top and bottom and lay the
X J corrugated glazing in place. Caulk the overlap between the two panels with a thin bead of clear silicone. Predrill the screw holes on the sides, then enlarge the holes in the glazing with a 3/4" bit so that the glazing can move with temperature changes. Fasten the glazing with neoprene screws every foot on the sides. In the center, predrill with a smaller bit and use V2" machine screws every two feet. Squirt a dab of silicone in and around these holes before tightening the screw to seal around the screw.
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-Ш { Cover the ends of the glazing with 3/4" wood strips. Predrill Av and fasten the strips with 2V2" deck screws. Caulk the strip at the top against the 2 x 6 cap using paintable acrylic caulk.
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•% ^ Solder on the last copper fittings and the PEX (crosslinked X / polyethylene) adapters. PEX is easier to snake through the house than rigid copper. It also makes it possible to adjust the collector angle if you need to, and will flex easily if the collector moves due to soil movement or accidental bumps. Attach it to the brass PEX adapter with a brass ring and a special crimping tool. Finally, seal the inlet and outlet holes with caulk and paint the exterior of the box. Wrap pipe insulation around the exposed copper and PEX lines, both to retain heat and to protect the PEX from UV degradation. Complete the connections inside, then fill the system (see p. 99).
