1/
I’ll soon be publishing an open-source build: the P14P12+HEPA.
It’s a DIY ductless HRV I’ve been running in my own kitchen.
It confirmed something I already knew: adding a filter always impacts how a system performs. 🧵 2/
When testing the unit on the workbench, everything looked balanced.
But once installed in the window, the system created excessive positive pressure.
Not just suboptimal — unacceptable for a real-life setup.

🧵
1/ This student invention — a window-mounted ERV called SWERV — is exciting. It’s part of a broader shift I’ve been documenting toward decentralized, ductless ventilation solutions for real-world indoor air needs.
medium.com/@levangelisted… x.com/duncan__c/stat… 2/I’m currently writing a series on air exchangers (HRVs/ERVs), with a focus on window-based and ductless systems.
So far, I’ve tested 3 DIY systems I built myself, and 1 commercial device: the Purifresh.

1/ Air pressure in ventilation systems is a crucial aspect when discussing the control of ventilation systems (HRV, ERV, etc.), which is closely related to managing indoor air flow. @GregEqEd question is very relevant. He brings up some key points that I had already planned to address in my HRV series. In fact, I recently decided to add a dedicated post on air pressure within the HRV series, but after seeing this question, I’m providing an immediate answer. A more detailed post on the subject will be coming in the next few weeks. medium.com/@levangelisted… 2/ In your question, I see three key points:
1. You ask if balanced pressure wouldn’t be preferable to positive pressure.
2️. Y ou rightly assume that the more positive the pressure, the less likely it is that dirty air will enter.
3. You seem to question the idea that we can know if we’re in positive pressure just by looking at the fans, since, as you say, “air also moves elsewhere.”

🧵 1/14 Final results of my DIY air exchanger system with AirFanta HEPA filter. Tests are now complete (anemometer + CO₂). Full technical sheet below 👇

Designed to counter outdoor pollution (wildfire smoke, smog, etc.) by creating positive pressure using filtered air. More details in the following tweets
2/14 Why a HEPA filter in a DIY air exchanger? Because the goal is to bring in clean air — even when the outside air is polluted. 🔥 Wildfire smoke
Smog Or simply living in a polluted city — or surrounded by neighbors who smoke Positive pressure + HEPA = winning combo

1/ DIY Air Exchanger with HEPA — Initial Results
Built & installed in ~4 hours. A rough-looking prototype, made just to explore its potential.
I already tested:
🔸 Airflow (Testo 405i)
🔸 Noise (dB meter)
🔸 Power (wattmeter)
CO₂ decay tests now underway — slower process, used to refine airflow estimates. 2/Preliminary airflow (rounded):
• Silent setting (~35 dB): 40 CFM, 2W
• Max setting (~48 dB): 75 CFM, 10W
(CO₂ tests may slightly revise these down)
I'm weighting results with a formula where the anemometer has more influence, since CO₂ is an indirect proxy of ventilation.

1/ Many criticize the sporadic use of UV-FAR lamps due to the ozone risk. But if air quality is truly a concern, I invite them to focus more on the following issues: 🧵 2/ How many people live in appartment, without an extractor hood and ventilation, with windows almost always closed? In my appartment , the base ACH is about 0.3 ACH. I can see that my neighbors generally keep their windows closed, and the hallways are often very polluted.

1/When it comes to infection prevention, particularly for COVID-19 and other pathogens, it’s important to consider all scientifically-backed options, including air purification systems and UV disinfection. 2/ Organizations like ASHRAE 241 and the Harvard Healthy Building COVID-19 Risk Calculator recognize UV disinfection as a viable technology for infection control in indoor environments.
healthybuildings.hsph.harvard.edu/tools/covid-19…

ashrae.org/about/news/202…

Helplessness is one of the most challenging experiences for humans, just as it is for animals. This has been extensively studied, particularly in dogs. 🐶 #Helplessness #Psychology
👇

https://twitter.com/amandalhu/status/1844937057501855972

In the face of helplessness, Stoic philosophy offers a path: focus on what we can control, create an action plan, and accept with serenity what lies beyond our control. 🧘‍♂️ #Stoicism #Philosophy

There’s a controversy around Nukit, who sells UV lamps to reduce viral transmission risks. What started as a simple customer post sharing their use of these lamps at an airport has spiraled into accusations of privilege, racism, and even doxxing. what’s really going on 👇



The controversy began when a customer, @Kevski_Bray, shared a photo at an airport cafeteria, using a Nukit UV lamp during a mandatory trip. The post went viral, garnering hundreds of thousands of views, but the reactions were overwhelmingly negative

https://x.com/ElaineVigneault/status/1785675244273127563

UV disinfection settings, pictures from study ….

Far-UVC: Technology Update with an Untapped Potential to Mitigate Airborne Infectionshttps://www.ncbi.nlm.nih.gov/pmc/articles/PMC10704226/

Ventilation is a case-by-case situation and it could be very challenging, you give an example. The background noise is in the calculator i propose. You can use it to try a solution or a compromise for a specific situation.

For some challenging situation, UV will be a most 🧵

https://twitter.com/GeoffHanmer/status/1833981712881328141

I encourage people to use calculation tools to find personalized solutions for their workplace, school, or child’s daycare, etc. I helped my friend find solutions, and it was very challenging.

I recently received a DM about the 45 dB figure I mentioned regarding noise levels in schools. This is a serious topic because noise pollution can significantly impact health and learning. Let me clarify where this figure comes from
🧵 The 45 dB figure is often discussed in the context of minimizing classroom noise to avoid disruptions and support effective learning. It’s a general reference rather than a strict standard.

Updated Air Purifier Analysis
Refined analysis for classrooms (38 dB background noise, 200 CFM, 28 students).
Focus on dust CADR for ASHRAE 241 and ensure total noise max 45 dB. Compare quieter vs. louder models with new data. Check it ou #ASHRAE241 #45dB
👇

Thanks to @joeyfox85 and other person, I’ve adjusted the analysis to reflect that #ASHRAE241 considers infectious aerosols as 0.3x smoke + 0.4x dust + 0.3x pollen. Intertek tests show 5% of the dust CADR. This adjustment ensures more accurate evaluation and easier to meet goals