Gas station without pumps

2015 September 26

Crawlspace ventilation

Filed under: Uncategorized — gasstationwithoutpumps @ 23:03
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I have a crawl space under my house that gets rather damp—my house is built where an aquifer comes to the surface, and in most years the water table is only a few centimeters below the surface.  (Because of the 4-year drought, the water table is currently about a 30cm below the surface.)  A few years back I had a solar panel put on one side of the house, connected to 12V fans in the crawl space to exhaust air from the wettest part of the space, but the fans stopped working shortly after the installation.

I have two conjectures about why the fans failed:

  • The 12V DC wires from the solar panel to the fans were broken somewhere, and so there is no voltage at the fans.
  • The voltage from the solar panel exceeded the voltage rating of the fans enough to burn out the fans.

I’m claustrophobic enough that I’ve never wanted to crawl around down there to try to debug the problem or replace the fans, so the installation has been non-functional for a few years.

So next week I’m having my general contractor/carpenter redo the fans so that they will be more maintainable.  The main thing is to make hinged “doors” for the vents, so that the fans can be accessed from outside the house, without having to crawl the width of the house under a very low ceiling.  Having access to both ends of the cables also makes checking them for continuity easier, and makes it possible to pull new cables if needed without crawling under the house.

 

I’m also going to change how the fans are hooked up.  Instead of using 12V fans, I got a number of little 5V fans that are very quiet (18.1 dB) but have reasonable airflow (13.1 CFM): http://www.digikey.com/product-detail/en/FAD1-06025BBLW12/Q620-ND/2600074 They are rated for 70,000 hours, but are only rated down to –10°C (good enough for Santa Cruz).

Putting 4 of the little fans blowing air through holes in the “door” should provide me with about 50 CFM (cubic feet per minute) at about 24dB.  With two such doors (8 fans) I should get 100CFM at about 27dB.  That is more air flow for the noise level than any single-fan solution that I found.

I’m going to put a 5V switching regulator on each door, so that the voltage from the solar panel can fluctuate wildly and the fans can still run at a constant voltage. The OKI-78SR regulators can accept anything from 7V to 36V and produce 5V output with about 90% efficiency. They only handle 1.5A, but 4 of the fans is only 0.6A, well below the rating. Each regulator will be delivering about 3W to the fans, so the power needed from the panel is only about 7W.  I believe that the panel was a 50W panel, so the fans should run even when the sky is overcast, though not at night, unless I add a rechargeable battery to the system.   (There was originally a lithium iron phosphate battery in the system, with a charge controller, but I repurposed it for other projects and it has since failed.)

I also don’t have to worry about the IR drop in the cabling to the vent fans, as it will not be anywhere near enough to drop the voltage below 7V when the solar panel is producing power.

Here is the circuit:

The Schottky diode is there to prevent damage from accidentally wiring the power backwards—I happened to have a couple on hand.

The Schottky diode is there to prevent damage from accidentally wiring the power backwards—I happened to have a couple on hand.

I wired up one of the regulators on a prototype board:

My soldering is pretty sloppy on the prototyping board—I've gotten so used to doing custom PC boards for everything that running wires and joining them to components seems a bit strange. There's not much point to a custom board for only 2 instances, though, especially with such a simple circuit.

My soldering is pretty sloppy on the prototyping board—I’ve gotten so used to doing custom PC boards for everything that running wires and joining them to components seems a bit strange. There’s not much point to a custom board for only 2 instances, though, especially with such a simple circuit.

In this rather low-quality photo, the 12V input (7V–36V) comes in through the two screw terminal on the right, and the 5V output is on the the two front screw terminals on the left, with ground on the two back screw terminals on the left.  I added male headers for +5V, Gnd, Vin, so that I could later add monitoring circuitry for remote monitoring or logging the voltages, if I felt like adding that.  It might be interesting to log the voltages for a year.

I’m considering adding a resistor and an LED to the 5V output, so that there will be a visible indicator of power, but I’m not sure it will be worth the effort, since the board will usually not be visible and if I open the panel for debugging, I can use a voltmeter easily enough.

1 Comment »

  1. […] Crawlspace ventilation, I talked about wiring up some little 5V fans, a voltage regulator, and a solar panel to ventilate […]

    Pingback by Crawlspace ventilation again | Gas station without pumps — 2015 September 27 @ 22:54 | Reply


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