DIY: SOLAR-POWERED ATTIC-TO-BASEMENT FAN

General Approach

I might be stating the obvious when I say that the attic only heats up when the sun is shining. But what this means is that I can maximise the utilization of my PV panel by connecting it directly to the fan system, and avoid losses attributable to any kind of power storage system. I therefore needed a fan system that would operate within the parameters of my panel (i.e. 12VDC fans, with a total wattage of no more than 20W). The biggest 12VDC fan I could find at Jaycar was 120mm [4.7"] in diameter, drew 4.56W and had a theoretical throughput of 2.3 m³/min [80 ft³/min].

The ducting between the attic and the 'basement' needed to be at least 4m [13'] in length, not including that required to reach the apex of the roof space, where the hottest air collects. I decided on a 6m [20'] run of ducting, and thus decided that I'd need a fan at each end, one to blow and one to suck, to overcome the drag in such a long run of ducting (complete with at least two 90° bends). I elected to marry the 120mm fans to 150mm [6"] diameter ducting. To further increase throughput I elected to use a pair of the 'two fans + ducting' units in my system, with all the fans wired in parallel. The total power required by my four fans would thus be 18.24W (or 1.5A @ 12VDC).

To maximise the output of my PV panel, I also decided to include a 'mini-maximiser' (a budget form of maximum power point tracking) in my system. This unit ensures I get the most out of my panel by ensuring that the fans can be started, and kept running, in less than full sunshine (i.e. early morning, late afternoon or in overcast conditions), especially given that my 18.24W requirement is very close to the 20W rating of my panel - more on this below. It is also worth noting that this unit draws just under 1W to do its job - the total power requirement of my system is thus around 19W (you can't get a much better utilization of a 20W panel, eh?!).

Materials List

Here's a list of what I used to build my system. You will need to tailor this list to meet your specific needs:

• 4 x 120mm 12VDC Thin Ball Bearing Fan from Jaycar
• 4 x 3m Aluduct flexible ventilation ducting (2 x 6m would have been better, but it wasn't stocked at my local hardware store)
• 1 x 7.5m [25'] roll of 75cm [2.5'] -wide bubble wrap
• 1 x 10A screw terminal strip - cut up to provide two blocks of two connectors
• 7m [23'] 15A auto DC power cable
• 1 x Mini-maximiser kit from the ATA
• 4 x 400mm x 200mm [16" x 8"] sheets of thin plywood, plus enough 25mm x 25mm [1" x 1"] timber to build a surround around two of the plywood sheets
• 2 x 100mm x 500mm+ [4" x 20"+] plastic strips, suitable for creating the flange that marries each end of the ducting to the plywood fan boxes - I used the packaging from a recently purchased roller blind
• Screws and glue for construction, plus brackets for mounting the plywood fan boxes

The total cost to build this system was around NZ$300, excluding the PV panel.

If anyone is interested, the (Ponoko) laser-cutting template I used to make the fan boxes is available here. Please bear in mind that you'll need to source additional materials to make the finished item, as per the following instructions.

Fan Boxes

The fan boxes are where everything comes together on this project. There are two of them - one for each end of the twin ducting run. They are where the ducting is married to the fans; where the power is supplied to the fans; and where air intake/outlet coaming is attached.

The Ø150mm [6"] ducting is married to the Ø120mm [4¾"] fans via a double layer of thin plywood. Each layer of ply has an appropriate sized hole cut in it (Ø120mm [4¾"] for the fan side layer, and Ø150mm [6"] for the ducting side layer). The holes share a common centre point. Between the layers I inserted a flange that exits out the Ø150mm [6"] holes (the ducting side). The flange was made by cutting halfway across the plastic strip at regular intervals (25mm [1"]) and then rolling up the result (see photos). The ply layers are then glued together and edged on the fan side with 25mm x 25mm [1" x 1"] timber all the way around (secured with glue and screws).

Once everything was dry/set and trimmed, I attached the fans over the Ø120mm [4¾"] holes. Most fans only spin in one direction, so you'll need to establish how your fans need to be wired up. Each fan box has a pair of fans wired in parallel to a connector block attached to the ducting side of the fan box (i.e. the fan wires run through a hole from the fan side to the ducting side, where they are then joined to the connector block). One of the fan boxes is set up to blow (i.e. the fans point out to the ducting) - this will be the 'head', or attic, unit. The other of the fan boxes is set up to suck (i.e. the fans point in from the ducting) - this will be the 'tail', or basement, unit.

Ducting, Electronics & Wiring

The bulk of the ducting runs outside the house, from under the eaves down to a hole in the basement wall (which is only visible by the neighbours, fortunately). To help ensure as little heat as possible is lost, and also to protect the ducting from the elements, I wrapped the twin ducting run in bubble wrap. I also threaded a cable inside the bubble wrap to connect the fan sets together, ensuring a good length of cable stuck out each end of the ducting run. I cut a pair of Ø150mm [6"] holes in the eaves and basement wall and threaded the ducting inside at each end. The plan was to have the minimum of ducting needed at the basement end, with as much as possible extending into the attic (in order to get the inlet as high into the attic space as possible).

The final item to assemble before installing everything was the mini-maximiser. The instructions with the kit were perfectly clear, and the unit was up-and-running in no time. I tuned the output to 13V, as recommended in the instructions. I fitted the completed circuit board into a case, with a short length of 'output' cable connected. The output cable will be connected directly to the 'head' unit connector block (together with the cable that run's down to the 'tail' unit, all wired in parallel).

Attic Installation

Installation is fairly simple: I joined the ducting to the 'head' unit with duct tape; attached the mini-maximiser output and 'tail' unit cable to the 'head' unit connector block (wired in parallel); and then attached the 'head' unit to the rafters with brackets and screws. I ran a cable directly from my PV to the mini-maximiser, with a switch in the line that is accessible from inside the house. At this stage I did not fit any coaming to the 'head' unit, pending testing.

Basement Installation

The installation here was similar to that of the attic - just make sure that everything is wired the right way around! At this stage I did not fit any coaming to the 'tail' unit, pending testing.

Testing & Operation

Testing was simply seeing if everything worked once the switch was flicked! Obviously, the 'head' unit should suck air from the attic and blow it into the ducting, and the 'tail' unit should suck air from the ducting and blow it out into the basement.

Once all was verified I fitted coaming (mostly fabricated from corrugated cardboard) to the 'head' and 'tail' units. The 'tail' unit coaming was pretty simple and attempts to direct the hot air to where it can do the most good. The 'head' unit coaming is more complex in that I needed to extend the intake from where the 'head' unit was mounted right up to the apex of the attic space in order to capture the hottest air. I also fitted wire mesh over the intake coaming to keep out any bird that manages to infiltrate the attic space.

So 'does it all work?', I hear you ask. It sure does! The air coming out from the 'tail' unit loses less than 1°C [2°F] on its journey from the attic, and the basement is now ventilated with air more than 20°C [36°F] hotter than ambient. The volume of air moved is totally adequate too, though it's a bit harder to measure this objectively. My only other concern at the outset was the level of noise that might be generated by four fans, but this turned out to be of little consequence. Ironically, the only real noise that comes from the system is a buzzing generated by the mini-maximiser when then insolation level drops too low, causing the fans to spin down, but a little tuning of the mini-maximiser helped minimise this issue.

Success? Failure?

★★★★★

Pros: A very effective system that does its job very well.

Cons: None come to mind!

Enhancement Suggestions: Having a slightly higher rated PV would extend the opertaing time of the system into periods of lower insolation, but too much more would be a waste of generating capacity, and the purpose of the system is to operate in full sun, when the attic is being heated the most. Some kind of remote-control and/or timer switch might also be a nice addition.

Postscript

I've since combined the mini-maximiser and solar battery charging controller into a single box, with switching, etc., as per:

All DIY: Solar-Powered Attic-To-Basement Fan assets by Chris Molloy are licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.