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Author of the article, happy to answer any questions. A bit of background, my house is in Northern Thailand that has similar levels of air pollution every year

Author of the article, happy to answer any questions. A bit of background, my house is in Northern Thailand that has similar levels of air pollution every year than what we see currently in North America and Canada and it took me a few years to get my house to zero PM even when having the worst wildfires around. So before installing a positive pressure system and making my house air tight in the process, I experienced with all types of air purifiers but did not find anything that works consistently at very high pollution levels. The experience with these wildfires let me actually found AirGradient and we are proud of open sourcing many of our air quality monitors.

sampo 3y ago
I have a pet peeve about graphs with bad / non-existent labeling. Here is how to read the graph: The black line is the outside air PM2.5. The green-yellow-orange-red bars are the indoors air PM2.5.
stephen_g 3y ago
What kind of NOx and SOx readings do you get? (I assume the AirGradient sensors can measure that?). Do you think an activated charcoal filter is a worthwhile addition?
domoritz 3y ago
It’s great that PPS can reduce PM2.5 and CO2 to extremely low levels. I live in an area with industrial pollution (coal etc) and am concerned about VOCs, NOXs, SO2s etc as well. Wouldn’t I bring in more of those if I constantly bring in outside air even when the pollution is high outside? Unfortunately I haven’t been able to exactly identify the chemical that’s causing issues for me yet. I’d love to hear your thoughts on this situation.
WhatIsDukkha 3y ago
What do you need for a 700sqft space built tight? So its going to make the minisplit work harder but to what extent at 30 and 40 degree air outside?
- ahaucnx 3y ago
  I have seen PPS systems work from small rooms to whole sports halls. If you have a large temperature/humidity differential between inside and outside, I would recommend looking for units with heat recovery system.
frognumber 3y ago
I have air filters in all my rooms. I guess my questions are: 1) What should I do to get up to PPS? Should I just run a tube to their inlets through my window? 2) How is heating / cooling handled? The weather here swings wildly. I'd be reluctant to pump hot or freezing air in. Might work well in spring / fall which are peak allergy season, though. 3) What's a good way to monitor air quality on the cheap? I'm looking for opinionated recommendations.
- ahaucnx 3y ago
  1) The first DIY PPS that I built used a small duct fan drawing air from outside and pushing it through a hepa filter to the inside. It is important to use duct fans as you need to create air pressure more than air flow and box fans are not built to create the amount of pressure required. 2) Ideally you get a PPS system with a heat recovery unit. Also important is to have the PPS demand controlled based on an indoor air quality monitor that measures PM and CO2 and then only pushing in the right amount of air to meet your target values for PM and CO2. This way you can save a lot of energy costs and temperature differentials. 3) Thanks for asking ;) We maintain popular open source air quality monitor kits that are very easy to assemble and use industry grade sensors [1] [1] https://www.airgradient.com/open-airgradient/kits/ https://www.airgradient.com/open-airgradient/kits/
  - mlpinit 3y ago
    I was looking for something like this. Friendly note: "Join thousands of makers builing their" that's probably meant to be "building".
    - ahaucnx 3y ago
      Thanks. Fixed.
- rapjr9 3y ago
  I've been looking for an inexpensive PPS with heat exchanger that fits in a window (preferably an Andersen window, the kind that opens sideways, but maybe that's too much to ask). Since there don't seem to be any on the market (for any kind of window) I'm working on a design of my own. I think that two CPU coolers connected by their CPU plates with one pushing air out and one drawing air in would make a cheap heat exchanger. Then possibly running a large hose from the outflow of that to the air intake on a small HEPA air purifier would create a simple, cheap PPS. I know the pressure would not be high inside the house, but it would be a cheap way to introduce fresh air without losing heat/cooling and would help create a small positive pressure, without having to cut big holes in the walls.
_Nat_ 3y ago
Seems like the big drawback of the positive-pressure approach would be flushing the heating/cooling out, too (as you've said in the article). That drawback would seem to be proportional the flowrate of the positive-pressure airflow. So what if you redo your experiment with lower positive-pressure flows? This is: 1. Keep the 280-m^3/hr purifier on the whole time. 2. Start with the 220-m^3/hr positive-pressure system running. 3. Wait for the particulate-level to equilibrate, then record the particulate-level and the pressure-difference (between outside and inside). 4. Reduce the air-flow of the positive-pressure system by, say, 20-m^3/hr. 5. Loop back to Step-3 while the air-flow of the positive-pressure system isn't negative. With the objective to be to determine how low the positive-pressure system's flowrate can get while still maintaining its benefits, reducing heating/cooling losses.
- MichaelZuo 3y ago
  > Seems like the big drawback of the positive-pressure approach would be flushing the heating/cooling out, too (as you've said in the article). There are lots of enthalpy recovery systems on the market, some that can recover >75% of the 'waste' energy, though mostly aimed at larger customers. You need to do a cost-benefit analysis to see if it makes sense to spend the upfront cost.
  - _Nat_ 3y ago
    Yeah, looks like you're right: [waste-heat recovery systems](https://en.wikipedia.org/wiki/Waste_heat_recovery_unit https://en.wikipedia.org/wiki/Waste_heat_recovery_unit ). Guessing that there are two main scenarios there: 1. A leaky house where positive-pressure causes flows to come out of various leaks all around. 2. A well-sealed house where positive-pressure causes flows to leave through a well-defined channel. The first-case might be harder to recover heat/coolness from, as the (high/low)-temperature air might leak in a way that'd be harder to make use of it. The second-case would seem to offer a stream of warmer/cooler air that could be used with a heat-exchanger or heat-pump or something. To note it, well-sealed houses might have issues with relatively poor ventilation, making positive-pressure more desirable -- much like how the OP describes wanting to keep CO2-levels down with their positive-pressure system.
tjmc 3y ago
HVAC engineer here. I'd be interested to know if you've done any measurements with outside air coming through standard F5 filters. This is the typical setup for most ducted AC I come across. Much lower pressure and cheaper than HEPAs. It would be good to know what air quality results as a baseline and also in conjunction with in room purification if you have any data. Edit: I'm in Australia. Here's a comparison of filter types for different locations: https://www.filtermakers.com.au/selection-guide/ https://www.filtermakers.com.au/selection-guide/
- ahaucnx 3y ago
  No, we only tested with true HEPA filters. One important thing to point out with PPS systems is that you need to get the air from fully polluted to zero polluted in ONE pass as unlike air purifiers, there is no recirculation. So the filter quality is super important.
  - tjmc 3y ago
    Agreed. Unfortunately we usually only see HEPAs in hospital theatres, CSSDs and the occasional lab. Air quality standards are still very lacking here. They haven't been updated since COVID or the east coast bushfires in any way.
- trog 3y ago
  Not sure how to read that guide, but would an F5 filter out smoke well?
  - tjmc 3y ago
    No they wouldn't. Typically outside air systems shut off in fire mode, but they often keep running if there's smoke due to air pollution. If you need to deal with this you need a HEPA. An alternative for critical facilities like datacentres, is they often just go into recirculation mode. This wouldn't work in locations with persistent air polution.
_Nat_ 3y ago
Have you explored the effects of a localized positive-pressure system? Like, say the positive-pressure system has a duct that pumps outside-air directly to locations where a resident spends a lot of them time -- like their bed, a work desk, or a couch in a living-room. Could they get reasonable air-quality that way, even if the system isn't strong enough to clean up other areas of their home that they might spend less time in?
- ahaucnx 3y ago
  Yes, this is what I am doing in my house. Pump the cleaned fresh air into the peripheral rooms (mostly bedrooms) and then this actually pressures the whole house (living room, bathrooms etc).
usbinsider 3y ago
What was the filtration level of the filter system? What was the orientation/positioning? I’m wondering if the airflow was in a loop but not mixing the entire room, or the filter wasn’t Hepa (or close), or it wasn’t reaching its claimed throughput/filtration, etc. I recently installed a Merv13+Merv16 filter on my newly upgraded HVAC system and am able to drive PM2.5 to 0 with a minimal fan setting of roughly 2ACH. (Granted, we haven’t had wildfire smoke that strongly since installing it, but I induce bad air from time to time and it cleans it up quickly.)
aulin 3y ago
what about Radon?
Cocktail 3y ago
Do you have any issues with mold after air tightening the house? Here in Finland pretty much every house needs constant airflow/intake and the houses built to breathe to fight against mold. Im curious if youve had any issues, especially due to living closer to the tropical zone.
- regularfry 3y ago
  When I got my basement damp-proofed against a nasty mould problem there were three options for how it could be done. In increasing order if effectiveness, they were: 1) they could re-plaster with a sealant; 2) they could mount plastic stand-off sheets to the wall with plasterboard covering; 3) they could air-tighten and install a positive pressure system. So PPSes are sold as a damp prevention mechanism; as long as you install one when you air-tighten it's doing the job that natural airflow would otherwise take care of. Also worth noting: each successive option added a zero to the price.
- valenterry 3y ago
  He won't because it's Thailand. The air inside the house won't be significantly warmer than outside for a prolonged period of time. Not so in Finland. Your best option is a centralized or decentralized automatic ventilation system with heat exchanging and humidity control. On top of that, the better the insulation the less the air is cooling down near walls/windows and the less likely the humidity is condensing into water causing mold. And then last, the less humid the air inside the house is, the more it can cool down before condensation is happening - so keep the humidity low not only with automatic ventilation but also by making sure to not cause humidity by cooking, showering (and breathing, haha) and if you do, air out for just a few minutes.
  - 2rsf 3y ago
    What do you mean by humidity control? a central one? I am not saying you don't need one, but here in Sweden modern houses do not use humidity controls and are still air tight and mold free, maybe at the cost of letting more air and energy be swapped. Older houses have local vents at the baths which are humidity sensitive and will blow out excess humidity quickly, again at the cost of wasted energy. I suppose that controlling humidity centrally will somewhat save energy, but at what cost and complexity?

sampo 3y ago

I have a pet peeve about graphs with bad / non-existent labeling. Here is how to read the graph: The black line is the outside air PM2.5. The green-yellow-orange-red bars are the indoors air PM2.5.

stephen_g 3y ago

What kind of NOx and SOx readings do you get? (I assume the AirGradient sensors can measure that?). Do you think an activated charcoal filter is a worthwhile addition?

domoritz 3y ago

It’s great that PPS can reduce PM2.5 and CO2 to extremely low levels. I live in an area with industrial pollution (coal etc) and am concerned about VOCs, NOXs, SO2s etc as well. Wouldn’t I bring in more of those if I constantly bring in outside air even when the pollution is high outside? Unfortunately I haven’t been able to exactly identify the chemical that’s causing issues for me yet. I’d love to hear your thoughts on this situation.

WhatIsDukkha 3y ago

What do you need for a 700sqft space built tight? So its going to make the minisplit work harder but to what extent at 30 and 40 degree air outside?

ahaucnx 3y ago
I have seen PPS systems work from small rooms to whole sports halls. If you have a large temperature/humidity differential between inside and outside, I would recommend looking for units with heat recovery system.

frognumber 3y ago

I have air filters in all my rooms. I guess my questions are: 1) What should I do to get up to PPS? Should I just run a tube to their inlets through my window? 2) How is heating / cooling handled? The weather here swings wildly. I'd be reluctant to pump hot or freezing air in. Might work well in spring / fall which are peak allergy season, though. 3) What's a good way to monitor air quality on the cheap? I'm looking for opinionated recommendations.

ahaucnx 3y ago
1) The first DIY PPS that I built used a small duct fan drawing air from outside and pushing it through a hepa filter to the inside. It is important to use duct fans as you need to create air pressure more than air flow and box fans are not built to create the amount of pressure required. 2) Ideally you get a PPS system with a heat recovery unit. Also important is to have the PPS demand controlled based on an indoor air quality monitor that measures PM and CO2 and then only pushing in the right amount of air to meet your target values for PM and CO2. This way you can save a lot of energy costs and temperature differentials. 3) Thanks for asking ;) We maintain popular open source air quality monitor kits that are very easy to assemble and use industry grade sensors [1] [1] https://www.airgradient.com/open-airgradient/kits/ https://www.airgradient.com/open-airgradient/kits/
- mlpinit 3y ago
  I was looking for something like this. Friendly note: "Join thousands of makers builing their" that's probably meant to be "building".
  - ahaucnx 3y ago
    Thanks. Fixed.
rapjr9 3y ago
I've been looking for an inexpensive PPS with heat exchanger that fits in a window (preferably an Andersen window, the kind that opens sideways, but maybe that's too much to ask). Since there don't seem to be any on the market (for any kind of window) I'm working on a design of my own. I think that two CPU coolers connected by their CPU plates with one pushing air out and one drawing air in would make a cheap heat exchanger. Then possibly running a large hose from the outflow of that to the air intake on a small HEPA air purifier would create a simple, cheap PPS. I know the pressure would not be high inside the house, but it would be a cheap way to introduce fresh air without losing heat/cooling and would help create a small positive pressure, without having to cut big holes in the walls.

_Nat_ 3y ago

Seems like the big drawback of the positive-pressure approach would be flushing the heating/cooling out, too (as you've said in the article). That drawback would seem to be proportional the flowrate of the positive-pressure airflow. So what if you redo your experiment with lower positive-pressure flows? This is: 1. Keep the 280-m^3/hr purifier on the whole time. 2. Start with the 220-m^3/hr positive-pressure system running. 3. Wait for the particulate-level to equilibrate, then record the particulate-level and the pressure-difference (between outside and inside). 4. Reduce the air-flow of the positive-pressure system by, say, 20-m^3/hr. 5. Loop back to Step-3 while the air-flow of the positive-pressure system isn't negative. With the objective to be to determine how low the positive-pressure system's flowrate can get while still maintaining its benefits, reducing heating/cooling losses.

MichaelZuo 3y ago
> Seems like the big drawback of the positive-pressure approach would be flushing the heating/cooling out, too (as you've said in the article). There are lots of enthalpy recovery systems on the market, some that can recover >75% of the 'waste' energy, though mostly aimed at larger customers. You need to do a cost-benefit analysis to see if it makes sense to spend the upfront cost.
- _Nat_ 3y ago
  Yeah, looks like you're right: [waste-heat recovery systems](https://en.wikipedia.org/wiki/Waste_heat_recovery_unit https://en.wikipedia.org/wiki/Waste_heat_recovery_unit ). Guessing that there are two main scenarios there: 1. A leaky house where positive-pressure causes flows to come out of various leaks all around. 2. A well-sealed house where positive-pressure causes flows to leave through a well-defined channel. The first-case might be harder to recover heat/coolness from, as the (high/low)-temperature air might leak in a way that'd be harder to make use of it. The second-case would seem to offer a stream of warmer/cooler air that could be used with a heat-exchanger or heat-pump or something. To note it, well-sealed houses might have issues with relatively poor ventilation, making positive-pressure more desirable -- much like how the OP describes wanting to keep CO2-levels down with their positive-pressure system.

tjmc 3y ago

HVAC engineer here. I'd be interested to know if you've done any measurements with outside air coming through standard F5 filters. This is the typical setup for most ducted AC I come across. Much lower pressure and cheaper than HEPAs. It would be good to know what air quality results as a baseline and also in conjunction with in room purification if you have any data. Edit: I'm in Australia. Here's a comparison of filter types for different locations: https://www.filtermakers.com.au/selection-guide/ https://www.filtermakers.com.au/selection-guide/

ahaucnx 3y ago
No, we only tested with true HEPA filters. One important thing to point out with PPS systems is that you need to get the air from fully polluted to zero polluted in ONE pass as unlike air purifiers, there is no recirculation. So the filter quality is super important.
- tjmc 3y ago
  Agreed. Unfortunately we usually only see HEPAs in hospital theatres, CSSDs and the occasional lab. Air quality standards are still very lacking here. They haven't been updated since COVID or the east coast bushfires in any way.
trog 3y ago
Not sure how to read that guide, but would an F5 filter out smoke well?
- tjmc 3y ago
  No they wouldn't. Typically outside air systems shut off in fire mode, but they often keep running if there's smoke due to air pollution. If you need to deal with this you need a HEPA. An alternative for critical facilities like datacentres, is they often just go into recirculation mode. This wouldn't work in locations with persistent air polution.

_Nat_ 3y ago

Have you explored the effects of a localized positive-pressure system? Like, say the positive-pressure system has a duct that pumps outside-air directly to locations where a resident spends a lot of them time -- like their bed, a work desk, or a couch in a living-room. Could they get reasonable air-quality that way, even if the system isn't strong enough to clean up other areas of their home that they might spend less time in?

ahaucnx 3y ago
Yes, this is what I am doing in my house. Pump the cleaned fresh air into the peripheral rooms (mostly bedrooms) and then this actually pressures the whole house (living room, bathrooms etc).

usbinsider 3y ago

What was the filtration level of the filter system? What was the orientation/positioning? I’m wondering if the airflow was in a loop but not mixing the entire room, or the filter wasn’t Hepa (or close), or it wasn’t reaching its claimed throughput/filtration, etc. I recently installed a Merv13+Merv16 filter on my newly upgraded HVAC system and am able to drive PM2.5 to 0 with a minimal fan setting of roughly 2ACH. (Granted, we haven’t had wildfire smoke that strongly since installing it, but I induce bad air from time to time and it cleans it up quickly.)

aulin 3y ago

what about Radon?

Cocktail 3y ago

Do you have any issues with mold after air tightening the house? Here in Finland pretty much every house needs constant airflow/intake and the houses built to breathe to fight against mold. Im curious if youve had any issues, especially due to living closer to the tropical zone.

regularfry 3y ago
When I got my basement damp-proofed against a nasty mould problem there were three options for how it could be done. In increasing order if effectiveness, they were: 1) they could re-plaster with a sealant; 2) they could mount plastic stand-off sheets to the wall with plasterboard covering; 3) they could air-tighten and install a positive pressure system. So PPSes are sold as a damp prevention mechanism; as long as you install one when you air-tighten it's doing the job that natural airflow would otherwise take care of. Also worth noting: each successive option added a zero to the price.
valenterry 3y ago
He won't because it's Thailand. The air inside the house won't be significantly warmer than outside for a prolonged period of time. Not so in Finland. Your best option is a centralized or decentralized automatic ventilation system with heat exchanging and humidity control. On top of that, the better the insulation the less the air is cooling down near walls/windows and the less likely the humidity is condensing into water causing mold. And then last, the less humid the air inside the house is, the more it can cool down before condensation is happening - so keep the humidity low not only with automatic ventilation but also by making sure to not cause humidity by cooking, showering (and breathing, haha) and if you do, air out for just a few minutes.
- 2rsf 3y ago
  What do you mean by humidity control? a central one? I am not saying you don't need one, but here in Sweden modern houses do not use humidity controls and are still air tight and mold free, maybe at the cost of letting more air and energy be swapped. Older houses have local vents at the baths which are humidity sensitive and will blow out excess humidity quickly, again at the cost of wasted energy. I suppose that controlling humidity centrally will somewhat save energy, but at what cost and complexity?