Most homes, and especially older ones, are beset by one stubborn room that just never quite warms up—whether it’s a remote second floor bedroom, a kitchen or dining room with large, north-facing windows, or a main floor office in the corner closed off from the home’s central heating system. Operating an electric space heater can help you warm up in the shortterm, but is not an energy-efficient or long-term solution to this problem. One very efficient longterm solution, however, is to build and install hot-air solar panels. Even if you don’t have a cold spot, a supplementary solar heat system can save plenty of energy dollars.
Using solar energy to heat a cold space in your home is a great way to harvest the sun’s energy and supplement your home’s heat in these problem areas. Solar hot air panels are fundamentally different than photovoltaic panels—this style is designed to use the sun’s energy to heat the air inside each box rather than to create electricity. Mounted on a south-facing wall or on the roof, solar hot air panels collect air from inside your home and blow or draw it through the thermal solar panels, which are essentially a series of metal ducts in a black box under tempered glass. As the air moves through the ductwork, the sun’s rays cause it to heat to high temperatures. Then, at the end of the duct, another vent moves the air back into your home’s heating ductwork or an interior vent, sending the now-heated air right into the home.
When combined, these three DIY "hotboxes" introduce enough hot air into this home to carry 30 to 40 percent of the home heating load.
Tools & Materials
- Jigsaw or circular saw with metal-cutting blade
- Drywall saw
- Straightedge
- Aviation snips
- Tape measure
- Eye and ear protection
- Carpenter’s square
- Drill/driver with bits
- 1⁄8" pop rivets (2)
- Pop rivet gun
- high-temperature silicone caulk
- Caulk gun
- Aluminum foil tape
- 1"-thick R7 rigid insulation
- 2 × 6 steel studs
- utility knife
- Tempered glass
- 4" hole saw
- Sheet-metal start collars
- 8" plenum box (2)
- 4" male and female duct connectors
- 1"-wide closed-cell foam gasket
- 4" aluminum hVAC duct
- high-temperature black paint (matte)
- Trim paint roller
- Sheet-metal screws with rubber gaskets
- Chalkline
- Cardboard
- Scissors
- Reciprocating saw
- Roof jack
- Roofing cement
- Flashing
- Rubber gasket rooting nails
- Shingles (if needed)
- unistrut
- unistrut connectors
- Duct collars (2)
- 3⁄8" threaded rod
- Spring-fed 8" backdraft dampers
- 8" blower fan
- Temperature controls
- Clamps
- 1" foam gasket tape
- Work gloves
The temperature control equipment opens and closes the damper and causes the blower fan to turn on and off as needed.
1. Cut and bend the box frame pieces from 2 × 6 steel studs. Each steel stud piece will wrap two sides of the panel with a 90° corner bend. Mark the bend location on both steel studs. Cut a relief cut into the 6" side of the stud with aviation snips at this mark. Bend the stud to an L-shape and use a square to ensure that the corner forms a true 90° angle.
2. Drill 1∕8"-dia. holes in the overlapping top and bottom flanges. Clamp the corners together before drilling and use a square to make sure the corner forms a 90° angle.
3. Fasten the corners of the metal box with two 1⁄8"-dia. sheet-metal pop rivets in the top and bottom. Leave one corner open to create access for the insulation panel insert.
4.Cut the foil-faced rigid foam insulation to match the interior dimensions of the box, using a drywall saw or a utility knife.
5. Apply high-temperature silicone to the bottom flanges of the box (inset). Fit the 1" foil-face rigid foam insulation into the back of the frame, then close up the box and secure the open corner. Cut 5"-wide strips of foam insulation to the length and width of the panel. Place a thick bead of silicone around the outside perimeter of the unit. Insert the strips into the silicone and tightly against the sides of the panel to hold the backing firmly in place. The foil should be facing into the box.
6. Seal the insulation edges. Place a bead of silicone around the inside corner where the insulation strips and backing panel meet, and then seal with foil tape. Flip the panel over. Place a bead of silicone on the intersection of the 2 × 6 stud flange and the back of the insulation and seal with foil tape. Conceal any exposed insulation edges with foil tape.
7. Create inlet and outlet holes in the walls with a hole saw or circle cutter. The number and location of the ductwork holes depends on where each panel fits into the overall array (presuming you are making and installing multiple panels). The first and last panels in the series each will have one end wall that is uncut, while intermediate panels will have duct holes on each end wall (inset).
8. Install a compartment separator in the first and last panels with a piece of foil insulation set on edge. Cut ductwork access holes in the separator. Then, cut out holes for the ductwork that will pass through the separator. Also cut a plenum opening in the separated compartment in the first and last unit.
9. Paint the entire box interior black using high-temperature paint and allow it to dry completely. A trim roller works well for this task.
10. Insert the ductwork. Beginning at the plenum over the inlet duct, guide 4" aluminum HVAC ductwork in a serpentine shape throughout the entire multi-panel installation, ending at the outlet duct. Join ends of adjoining duct sections with flexible duct connectors fashioned into a U shape and secured with metal screws and foil tape (inset). Paint each section of ductwork with black high-temperature paint once it is in place.
11. Paint the last section of ductwork and touch up around the interior of the box so all exposed surfaces are black.
12. Affix the glass top. First, double check that all openings in the panel are adequately sealed and insulated. Then, line the tops of the steel stud frame with foam closed-cell gasket tape. Carefully position the glass on top of the gasket tape, lined up 1⁄2" from the outside of the frame on all sides. Then, position foam closedcell gasket tape around the perimeter of the top of the glass panel.
13. Attach the casing. Work with a local metal shop to bend metal flashing that will wrap your panel box. Attach around the perimeter of the panel with sheet-metal screws with rubber washer heads.
Tip: Be careful when working around the plenum ductwork. If you set the unit down on its backside, you will force the plenum up and break the seal around the opening.
14. Mark off the panel layout locations on the roof. Transfer the locations of the 8"-dia. inlet and outlet holes to the roof as well. The location of these holes should not interfere with the structural framing members of your roof (either rafters or trusses). Adjust the panel layout slightly to accommodate the best locations of the inlet and outlet, according to your roof’s setup. Cut out the inlet and outlet holes with a reciprocating saw.
15. Use a roof jack or Cone-jack to form an 8"-dia. opening. Apply a heavy double bead of roofing cement along the top and sides of the roof jack. Nail the perimeter of the flange using rubber gasket nails. Cut and install shingles with roofing cement to fit over the flashing so they lie flat against the flange.
16. Attach Unistrut mounting U-channel bars to the roof for each panel. Use the chalklines on the roof to determine the position of the Unistrut, and attach to the roof trusses with Unistrut connectors.
17. Hoist the panels into position. Carefully follow safety regulations and use scaffolding, ladders, ropes, and lots of helpers to hoist the panels onto the roof. Wear fall-arresting gear and take care not to allow the plenum ductwork to be damaged. Connect the inlet and outlet ducts on the panel(s) to the openings on the roof (inset). Position the panels so the inlet and outlet openings match perfectly, and attach with a duct collar and silicon caulk.
18. Connect the panels to the Unistrut with 3⁄8" threaded rod attached at the top and bottom of the panel on the outside. Cut threaded rod to size, then attach to the Unistrut with Unistrut nuts. Attach the top clip to the top of the rod and the front face of the panel. Tighten the assembly to compress the panel down to the Unistrut for a tight hold. Seal the panel connections with 1" foam gasket tape around each end of the panels where they connect. Place a bead of silicone caulk on top of the gasket tape and then attach 3"-wide flashing over the two panels at the joint. Attach flashing to the panel with galvanized sheet metal screws with rubber gasket heads.
19. Hook up the interior ductwork, including dampers and a blower fan. The manner in which this is done will vary tremendously depending on your house structure and how you plan to integrate the supplementary heat. You will definitely want to work with a professional HVAC contractor (preferably one with experience with solar) for this part of the job.
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