SOLAR AIR HEATER
11/01/2015
INTRODUCTION
Here is our solar air heater. The panel is a horizontally-oriented, 4 ft. x 8 ft., ‘down and back’ 2-layer screen design, mounted in the vertical plane (90 degrees tilt). The collector stands 23 inches out from the S/SE wall of our out-building on an angle iron mount, facing approximately 160 degrees azimuth.
The 2x4 blocks fitted into the stud track were sort of a compromise; they created too many thermal bridging paths to the outside. Reconsidering, it may have been better to first insulate all around with 1 in. polyiso, and then fit an inner frame made from 1x4’s for the screens.
The screens sit on angled 1x2 boards, screwed to the mounting blocks and to the center divider wall. The absorbers are simple 1x2 frames with screen stapled to both sides. I couldn’t find charcoal aluminum screen except in big rolls, so I used bright aluminum screen, spray-painted flat black. I left the painted screens outside to bake in the sun for a couple days before putting them into the panel. I thought about using charcoal fiberglass screen, but I wasn’t sure how it would hold up. Fiberglass may be OK; the SunTuf glazing is supposed to filter out UV anyway.
The intake and output vents are located on the back - intake at lower-left, behind the baffle plate, and output at upper left. Short 6-inch duct sections have folded-over tabs, screwed to small pieces of 3/8 inch plywood. The plywood pieces are glued to the insulation board and screwed through the insulation into the plywood back plate. An aluminum baffle plate baffle directs the incoming air onto the top surface of the lower screen absorber. The air comes out (mostly) under the screen at the far end, into the turn-around plenum. The plenum is just an open area with a folded-over double layer of screen 1 inch beneath the glazing. The plenum screen is stapled to around the outside of the panel frame and to the top of the lower screen frame. I thought about covering the plenum with a piece of painted-black aluminum flashing instead of screen; I’m still not sure what would work best here. After making the U-turn, the air hits the top surface of the upper screen, finally coming out underneath at the outlet vent in the upper left corner. Short sheet-metal ramps guide the air onto the leading edge of the absorbers.
Two snap-discs are mounted to a hand-bent aluminum bracket near the outlet vent. The two snap-discs are 90/110 and 110/120, with an inside selector switch (explained below in ‘Controls’ section). The wiring run through the back plate into an outdoor plastic J-box attached to the back of the panel. I forgot to put in the snap-discs until after the glazing was installed – dumb mistake. I had to reach in through the output vent to screw the bracket to one of the 2x4 blocks in the side wall. The wiring to the inside is not run yet in the photo, but it comes out of the J-box and follows the mount structure, then goes inside via wall enclosure.
GLAZING
The glazing material is Palram ‘Suntuf’ polycarbonate. I wanted to use twin-wall, which I thought would perform better and be easier to work with, but it was not available locally at Home Depot or Lowes. (Note - I decided early in the project to only use readily available materials. I tried to purchase everything from the nearby Home Depot 1 mile away.) When I asked about the twin-wall, the crew at Home Depot didn’t seem to know anything about it, so I went with the Suntuf instead. I was at the store a couple months later, and there was a bin full of 3/8 inch twin-wall, right next to the Suntuf! Maybe my story about building a solar air heater inspired them. I plan to try the twin-wall next time.
Inside the back room, more flex sections take the air from the bulkhead plate to the Suncourt TF106 6-inch, 410 CFM centrifugal fan on the output side, and to a wood air filter box on the intake side. The air filter box holds a standard 14 x 14 pleated paper air filter, with a hinged door covering the access slot (door added after the photos). More flex duct carries the heated air from the fan to the upper register box on the inside wall, and return air from the lower register box to the filter box. The duct that runs up the wall is 6 inch rigid, wrapped in insulation blanket, with the whole thing surrounded by some 2x4 ‘crash bars’. Hopefully this will protect the ductwork from heavy items stacked nearby. In case you are wondering, a graphic artist once worked here. The little character on the wall is watching us work. The following pictures show the internal duct paths and the air box:
When the control loop closes, the relay is energized, feeding 120VAC to a dedicated 120V receptacle for the fan. The room thermostat is a re-cycled 1970’s electric baseboard heater thermostat. It’s crude, but it’s perfect for the solar heater, since it consumes no power; just open or closed, depending on where you set the dial. Calibration is not great, although repeatability is fine. The thermostat has an OFF position to disable the system during summer months. I purchased the Suncourt centrifugal fan, 24V transformer, and 24V SPST relay from Home Depot, and ordered the White-Rogers snap discs on line from SupplyHouse.com. The room thermostat came from the junk box. The following photo shows the grilles where the air feeds into the front room:
I thought of different and better ways to do almost everything during construction. I also had to change plans many times while standing in the aisle at Home Depot, deciding which ‘almost what I wanted’ item would work best. More hot-air panels - of course! This business is habit-forming, and a lot of fun too. I am already sketching out a horizontal tube-type (downspout) design using some slightly squished 3-inch corrugated aluminum ducts in a 3 5/8 inch stud-track frame with twin-wall glazing. I want to try routing the air through the end wall instead of the back, with the goal of smoothing air flow inside the panel. I also want to incorporate continuous insulation and explore some ‘ultralight’ construction techniques to reduce thermal mass and bridging, and speed up response time. I want to design a clean, sealed air flow path, with as little wood as possible inside the panel and no painted surfaces contacting the heated air stream. Our panel put out a slight smell initially, which seems to be decreasing by the day, but I don’t want to gas anybody. I have some ideas for a staked, low-impact ground mount (I have rock-hard clay a few inches below the surface – no fun to dig). The new zero-pass projects I see on SimplySolar look interesting. I am always looking for ways to streamline the assembly process and minimize hardware (our panel took a long time). We will see.
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