Intro: MPAX: Massively Parallel Air Extraction System A.k.a. a DIY Whole House Fan
House too hot in the summer? Not enough breeze at night when you open the windows? The a whole house fan is for you.
Step 1: What You Will Need
Some 12V PC fans (I used 20).
A 12V SMPS with sufficient current capacity.
A spare loft hatch.
A loft with 240V power sockets installed in it.
High current cable, connectors
Optional: Arduino and electronic components to make it variable speed.
Step 2: Loft Hatch
I happened to note my loft hatch has a detachable door. So I bought a second one, and drilled circular holes in it. That way I can install the extraction hatch in the summer, and in the colder months put the well-insulated hatch back in place.
Step 3: Fans
I bought 20 Yate Loon fans (2200 RPM, 77 CFM, 0.3A each). These were known to provide good static pressure as well as flow speed, which is what is needed for extraction.
They came with tiny 3 pin connectors. I also obtained male versions of these connectors which I used to wire all the fans together.
Step 4: Installing the Fans
The fans were connected together with plastic connector blocks, using the fan screw holes. These were done on both sides of the fan array, connected in one direction on the back and in the opposite direction at the front, for stability.
In between the fans I placed some strips of that non-slip rubber mat you can buy to seal unwanted air gaps.
I created a wooden frame for the fans to sit in, which was then attached to the hatch door. More rubber mat was used to ensure there were no gaps between the fans and the frame, and the frame and the hatch.
Step 5: Wiring of the Fans
Important: when you are considering 20 x 0.3A fans, you are dealing with high currents. You cannot daisy chain these fans in series. If you do the thin fan wires will get hot. They need to be connected in parallel, and you need to use sufficient thickness of cable for the main supply. e.g. 10A twin core power cable. The connectors also should be rated sufficiently. I used 10A connector terminals.
I used a 12V switched mode power supply e.g. for monitor/laptop rated 10A.
Step 6: Final Touches
Finally because the fans would stir up dust in the loft, I attached some nylon sheet to the fans so that when fans are off, it will drop down onto the fan hatch and prevent dust coming back down from the loft.
Step 7: Electronics and Remote Control
I remote controlled the system via a remote control plug socket, so I could turn it on and off.
Full speed on this system was rather noisy, but effective however!
I decided to do some electronic tinkering and make a device to slow the fans down and provide remote control.
I made a DIY 10A 'buck convertor'.
But basically, a variable pulse width modulation input to the circuit produces a variable output voltage between 0-12V. The PWM capability of the Arduino nano was used for the purpose, ramped up to a higher frequency than default.
I also attached an RF receiver which was able to pick up the signals from my remote for the plug. RCswitch library was used to read this input. I programmed in the following features:
- Faster, slower in small increments.
- Run for an hour at full speed, then resume previous speed. (e.g. if wanting it to be quieter later at night).
- Turn off for an hour, then resume previous speed. (e.g. if someone has lit a bonfire).
The code I used is attached.