Part IV: Solar Water HeatingChapter 7. Active Domestic Hot-Water Systems

Build a Copper-and-Aluminum Fin Collector

A finned-tube collector you can fabricate, with notes on bonding the fin so heat actually moves.

Difficulty: Advanced
Cost: $200–500

published

A fin collector is a row of copper tubes with aluminum sheet locked to each tube, behind glazing, with insulation at the back. Sun warms the fin. The fin warms the tube only where the metal actually touches. A black sheet that merely leans on the pipe heats itself and leaves the water cold.

This is an advanced shop project. A homebuilt collector has no independent rating. Many building departments will not accept it on a permitted domestic hot-water system. If the permit office wants a listed collector, buy one and follow the off-the-shelf path. The heat is the same either way. The paperwork is not.

The loop around this collector still has to be a drainback or a closed loop. A glazed copper collector that stagnates with no flow can exceed 300 degrees F. That is not a place for plastic tubing.

Difficulty

Level: Advanced
Time: Several weekends in a shop, plus roof and plumbing time
Skills: Soldering copper, forming sheet metal, and pressure-testing before the glazing goes on

Cost estimate

$200–500

Copper, aluminum, a wooden or metal case, high-temperature insulation, and twinwall can land inside the range. Tempered glass, a lot of fittings, and roof flashings can push past it. The valves and the tank are not in this number.

Piece	Typical cost
Copper risers, headers, and fittings	$80–200
Aluminum sheet for fins	$40–80
Case, high-temperature insulation, glazing	$80–200

Materials

Item	Qty	Notes
Copper risers	On about 4 to 6 inch centers	Half-inch tube is a common choice. Wider fins run cooler at the tip and collect less per square foot.
Copper headers	2	One size up from the risers. Plumb a reverse-return or a diagonal feed so the end riser is not starved.
Aluminum fins	One per riser bay	Form a groove that clinches the tube. Ordinary plumbing solder will not stick aluminum to copper.
High-temperature contact compound	A tube	Rated for stagnation temperatures, in the groove, not as a substitute for a tight clinch.
Case, mineral wool, glazing	1 box	Foam that melts has no place behind a stagnating absorber. Tempered glass or a glazing you trust in hail.
Lead-free solder and a hand pressure pump	1 setup	Use water for the test. Do not pressure-test with compressed air.

Clinching the fin to the tube

Cut the aluminum so each fin spans the gap from one tube to the next, or a bit less so the sheets do not buckle against each other. Form a groove that wraps more than half the tube and crimp it so you cannot pull the fin off by hand. Clips every few inches make a weaker bond than a full-length crimp. If you can see daylight between fin and tube, fix that bay before you paint.

Paint the sun side flat black with a paint that tolerates high heat. A selective surface helps a commercial collector and is easy to ruin with a fingerprint and a hardware-store rattle can. Flat black is the honest DIY finish.

Headers need a path that drains if this collector will drain back. Slope the whole plate slightly toward the return, and slope both headers so they cannot hold a puddle. A collector that holds water is a burst collector on the first hard freeze, even if the rest of the loop is perfect. Read pipe sizing before you pick the header diameter, and pumps before you pretend a heating circulator can lift to a roof.

The fin has to be clinched to the tube. A gap leaves the aluminum in the sun and the water unheated.

The box, the test, and the roof

Insulate the back and the edges with mineral wool or another insulation that stays in place when the absorber is stagnating. Keep the insulation dry. A wet board does not insulate, and a case with no weep hole becomes a tank. Seal against rain and give water that gets in a way out the bottom.

Glaze after the plumbing test, not before. Fill the absorber with water and bring it up with a hand pump to a pressure the tube and fittings are rated for, below the setting of any relief valve you have already installed. Look at every joint. A leak you can see on the bench is an afternoon. A leak behind glazing on a roof is a teardown.

Solder with lead-free solder if this copper can ever touch domestic water. Flux is corrosive and it belongs off the finished joints, wiped while the work is still something you can reach. Ventilate. A charcoal-hot joint is a burn and a fire starter if the bench is cluttered.

Mounting is structure and flashing, not a bead of roof cement under a bracket. Read working safely and stop if the roof is steep, brittle, or unknown. The collector is not a step.

Confirm the loop and the permit before you buy copper
Drainback or closed loop, from the system choice guide. If the inspector wants a listed collector, switch pages now.
Clinch every fin, then paint
No daylight at the tube. Four to six inch spacing. Flat black on the sun side.
Solder headers so the plate can drain
Reverse-return or diagonal flow. Slope headers. Lead-free solder if potable water is even a possibility.
Water-test, then insulate and glaze
Hand pump, rated pressure, every joint dry. Mineral wool, a weep at the bottom, glazing that can take hail.
Mount it as a piece of the roof, not a piece of furniture
Structure, flashings, and a pipe path that keeps the slope you just built. Then commission the loop on its own page.

Difficulty

Cost estimate

Materials

Clinching the fin to the tube

The box, the test, and the roof

Confirm the loop and the permit before you buy copper

Clinch every fin, then paint

Solder headers so the plate can drain

Water-test, then insulate and glaze

Mount it as a piece of the roof, not a piece of furniture

Further reading