It’s Time to Think Positive!
By: James Hong
Tags: Pollution, Home Improvement, Air Filters, Air Quality, DIY

Fresh air! “Mountain” Photo from Flickr (Barnyz) at https://flic.kr/p/QuafdV

How i’m keeping the air in my house fresh and clean.

Four years ago, due to all the bad air from wildfires, I bought 5 HEPA air filters to place all around my house. I have them running on low year round, but during bad air days I crank them to medium or high.

This year I bought some air quality meters to stick inside my house. I noticed that on bad air days, the air in my house was decent but it was also taking 300 watts of power and was not quiet. I also noticed that after a day or so the air in my house would get pretty stale.

A friend of mine has a fancy house where his HVAC system takes air in from outside, passes it through a large HEPA filter system, and distributes the air throughout the house. This has two benefits: 1) it creates positive pressure so air is always being pushed out of the house to the outside, and 2) it keeps the air inside the house fresh (It turns out that when you close all the windows in a typical house, the CO2 builds up pretty quickly)

The benefits of positive pressure made sense once I thought about it, semiconductor manufacturers and drug factories use positive pressure to keep their facilities clean for a reason.

My friend made an interesting analogy… think of air pollution as red food coloring dye constantly being dripped into a cup of water. Clearing the water of the red dye by running filters in the cup is a lot of hard work and less effective compared to just stopping the dye from entering the cup in the first place… and that’s what positive pressure does. When you suck air in from outside, an equivalent amount of air has to be blown out of the house, and this happens at the points where your house is unsealed and bad air is entering your house. By blowing air out at the points, that bad air never comes in.

I decided to rig up my own version of this by connecting a BlueAir 405 to the window using a garbage bag and found that all of a sudden the air quality in my house got even better. I then experimented turning all my indoor filters back to low again, and even started shutting some of the off, and I found the air quality in the house was still great! The positive pressure was doing all the work.

It turns out that for most homes it doesn’t take much fan intake to create enough positive pressure to keep the house clean. My friend sent me an article that said 50–100 CFM per 1,000 sq ft was typically adequate. Most home HEPA filters run about 100 CFM on medium and in the 200 CFM range on high.

Coway 1512-HHS (Amazon link)

I ended up mounting 2 Coway 1512-HHS units on windows, one upstairs and one downstairs. One is probably enough to create the needed pressure, but I also wanted to distribute the fresh air a bit more. Compared to the BlueAir, the Coway’s form factor worked a lot better for mounting it cleanly (aka my wife was less annoyed with it) and it turns out the filtering efficiency was a lot higher for the Coway than my BlueAir when cleaning air in a single pass. It looks ugly, but I put it in rooms we don’t use much. I heard maybe I can get a custom plastic plate made at TAP Plastics, I’ll probably look into that.

I recommend getting the Coway that has wifi control because once you mount it, you no longer have access to the controls on top. I’m going to have to change the filters periodically, but I think I can just take the window screen off and do that from the outside.

I’m running the two filters on medium, which only draws about 8 watts each and I think that creates about a total inflow of around 200 CFM.

The PurpleAir in my house now usually has an AQI of 0. It goes up when we cook of course, but I left the remaining inside filters on (at low setting) so it goes back down pretty quickly. I wish I had written down all the data through my experiments so consider this article a qualitative argument for this. If someone want to confirm (or dispute) what i’m saying with data, that would be awesome and I’d love to read it!

The other benefit of this is the air in my house is much more fresh, not only maintaining low CO2 levels but also probably flushing out the chemicals off-gassed from furniture. My wife says she thinks the quality of her sleep is better as a result of the fresh air. The only downside to this sort of system is I’m not sure what we will do when the air is really cold outside as this will make the house a lot colder too… but I figure our air quality is fine during winter so maybe I just won’t run it then, or I’ll put a heater in the rooms where I have these set up. Eventually I hope to get my HVAC system to intake from outside like my friend has, but that’ll take a while as I’ll probably do it later when I replace the entire system with a Heat Pump based solution.

If anyone reading this decides to give it a go and wants to use the Coway (which I do recommend), please buy through this link. I don’t care about the affiliate earnings but i’m just curious to know how many people found this article useful enough to give it a go!

PS: None of this was my idea, it was my friend’s.. I’d give him credit but he doesn’t generally want it. Suffice it to say he’s world famous for being smart.