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.

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

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.

Be the First to Share


    • Made with Math Contest

      Made with Math Contest
    • Cardboard Speed Challenge

      Cardboard Speed Challenge
    • Multi-Discipline Contest

      Multi-Discipline Contest

    12 Discussions


    1 year ago

    Neat idea... does it work well/was it worth it? Any noticeable drop in house temperature in the evening?


    3 years ago

    For info, since the whole house fan pulls in flies and pollen at night, I have this year fitted a pollen screen roller blind to filter the incoming air. Pairs really well with this idea since the air flow through a pollen-grade window screen is too small without mechanical ventilation to be effective (or so I find).


    4 years ago on Step 7

    If there is interest I can put more details on the components etc, but it is pretty much a project in itself, and an 'optional extra'.

    2 replies

    Reply 4 years ago on Step 7

    Why didn't you just use a box fan? What are the advantages/disadvantages in using a bunch of small computer fans?


    Reply 4 years ago on Introduction

    Box fans more common in U.S. than U.K! I could have probably sourced a fan like this with some effort and integrated with the hatch, but was concerned about weight, also using DC fans I can and have rigged up a system to remotely control the speed of the fans. With a 240V fan it would run at a single speed.

    Also the low profile of PC case fans was ideal to not clash with the loft ladder which sits above the hatch when closed.

    But if you can find a lightweight box fan and you don't want to vary the speed, then it could work.


    4 years ago on Introduction

    You might consider using the version of these fans that run directly on 120v AC. It would allow you to avoid the power supply and the the losses related to the power conversion.

    1 reply

    Thought about this but was driven by cost, availability, noise considerations. 240v 120 mm fans are about £10-15 each,my DC fans about £2 each. (and I live in UK so 240V fans needed). Maybe offset a little since I could probably use half the number of fans. But I also felt more confident working on low voltage DC application.


    4 years ago

    All those fans, running at "normal" speed (fed 12 volt) put out 1500cfm. That should work for around 100 sq feet of living space.
    For every square foot of living space, 1.5-2cfm is normal ventilation, 2-3.5cfm moves enough air that you can feel a breeze.
    And you need to make sure your attic has enough ventilation(fan cfm ÷ 750 = necessary square feet of venting; 2 sq feet for the authors 1500cfm of fans).

    1 reply

    Reply 4 years ago

    Hiya, thanks for the useful tips. Certainly in free air they pull a lot of breeze. Window open, your curtains are billowing in the breeze. But close window less than few inches open and flow drops off a lot because axial fans have limited flow under back pressure.
    That is a good point on attic / loft ventilation, my loft has ventilated eaves around entire edge, so 'fully connected' to outside as far as fans concerned. Zip ties also good idea for connecting fans. Thanks for looking and for useful comments!

    Hi - plastic connector blocks, usually used for joining wood at right angles e.g. a cupboard shelf, very cheap usually a few £ for a big bag. You can see on the other photo how they join the fans together. Just so happens that the hole spacing is about that of two fans placed next to each other (thought it leaves a small gap) so works well. I originally tried joining the fans with duct tape but the stretch of the tape means it isn't rigid enough, plus when the tape dries out and peels it starts to catch the fans during operation.


    Reply 4 years ago

    Plastic zip ties(~100 for $5) work really well, if you need to eliminate the space between fans.
    Super strong, semi permanent (can be cut off fairly easily to replace a failed fan), no stretch, and not affected by the heat and cold.