Translations by Emily Barnes Franklin – thank you!
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Click here to go back to all FAQ’s.
“Lasko FF305 Air Flex 2-in-1 20-inch Box Fan and Air Purifier in One with MERV10 Air Purifying Filter”
Won’t be as good – but something is better than nothing!
The filters here are 13-A – so I would buy one and swap out the MERV 10 that it comes with.
No affiliation, just these are hard to find.
Quick Tip if your windows and doors open – at the VERY LEAST, open during breaks and lunches. Source:
Professor Cathy Noakes:
“If its cold, crack the windows rather than opening them fully and then open them wider to air the room periodically say at break times. The colder it is, the smaller the gap you need for ventilation. And let them layer up too.
Ventilation is driven by pressure differences and they are created by temperature differences (hot air rises) and wind. So for the same size gap you get more flow when there is a bigger temp difference between indoor and outdoor or there’s a higher wind speed.
So a fairly small gap in winter can give you as much ventilation as fully open window in summer. It is hardest to ventilate when its a warm still summer day with no difference in temperature and no wind.”
Some recommended ways to help with natural ventilation from the CIBSE Natural Ventilation Chair.
Original Source is here – 41 pages of great information. But if you have no time, here’s the most relevant 4 pages to you or me, the common person. Some samples below:
“Openable windows and vents should be used more than normal, as long as security is
considered and the open windows do not cause a hazard to anyone moving outside. If
possible windows should be open at least 15 minutes prior to room occupation.“
“In cooler weather even a small opening can deliver significant ventilation flows, and this can minimise risk to occupants of the space.”
“Where there are both high-level and low-level openable windows in a room, it is
recommended to open the high-level windows during cooler weather in the first instance; incoming air will be warmed as it flows down into the room, thereby reducing cold draughts.”
Next, we are going to make a fan shroud..a fancy way of saying we are going to block off the square corners, so just the round fan is exposed. This was discussed here by Jim Rosenthal.
Short version – the round fan does not reach the square corners, so there’s a gap there, and is not efficient. David Elfstrom, “At the tips of the fan blades there is negative pressure which sucks air in through the front (outlet) of the fan. By covering up this area, more air will flow through the filter on the inlet side and noise is reduced.”
As you can see below, the green arrows are where the air is flowing out of the fan, and the red arrows are where the negative pressure is sucking air back in the opposite way. Picture from here.
When you make the shroud, you will be putting it on the outside of the fan. On the side that the air is blowing from. And you only need one shroud, not one on the inside AND the outside. Just the outside.
You could do this with tape:
So – there have been two fans that have had their fans optimized by David Elfstrom. They are here. After this, will show you how to find the shroud size for your specific fan.
All others, you could can use tissue paper to determine how big the hole in the middle should be. As the fan is on, pull a piece of tissue across the front of the fan, slowly. You will very quickly see the part of the tissue that is being sucked back INTO the fan. You’ll want to cover that part with the shroud. In the picture below, you’ll see the green arrows up, and red arrows down – the red arrow down is the part you want to cover.
After determining the size, draw a circle on your cardboard. If you draw an “X”, corner to corner, that will give you the middle. Then, you can use this technique with a pin and string to draw that circle.
Some people put contact paper over their cardboard parts – like the shroud.
The Corsi/Rosenthal Box is based on a 20″ x 20″ box fan. This is because the size perfectly matches with 20″x20″ filters.
Some countries have no box fans. Round fans have been used to build the Corsi/Rosenthal Box.
- 20″ Box Fan – very important that the On button/ Speed Control is on the outer edges of the fan – or on the front where air comes out. You do not want it on the back where air comes in.
- Lasko 20″ Box 3-Speed Fan
- Utilitech 20-in 3-Speed Indoor Box Fan – these also come in black.
Safety note: the box fans have been tested and found to more than up to the task. With that said, as an additional note, it might be good to get one with a fused plug, which the Lasko and Utilitech fans have for sure.
Any type of duct tape is recommended. Keep in mind that you can use the duct tape color to accessorize your DIY Air Cleaner. Please see the Gallery for lots of creative fun ideas.
20″x20″x1″ or 20″x20″x2″ MERV 13 filters are recommended. MERV 13-A filters are available here. Why 13-A? Because they are top of the line.
If MERV 13-A filters are not available, the next filters recommended are MERV 13 filters from 3M . Specifically, 3M 1900 filters are excellent (the people on the thread are IAQ experts) .
These can gotten at:
- Home Depot
- Ace Hardware
What is not recommended?
FilterBuy’s filters sourced from Amazon. There have been knock-offs sold on Amazon, and so it is recommended to avoid those.
Last tip – if you are making the 4 or 5 filter box, you could make the sides out of 20″x30“x1 filters – and get a huge gain in filter area. You can find 20″x30” filters at Walmart. Tip courtesy of ocnj of Twitter.