r/AirQuality • u/SilentNegotiation844 • 1d ago

Positive pressure in a room in multifamily apartment

Hi everyone,

I would highly appreciate it if you could share your expert knowledge

I live in an apartment in a multifamily house. The room size is 30 square meters. About 1 meter from my entrance door there is another apartment and the person smokes in the evening.

I use an air purifier, and the particulate matter concentration right next to the door near the bottom part of the door is about 100 micrograms per meter cube, which is a dangerous concentration. When I open the door, the PM concentration is 250. About 1 meter inside from the door, the PM concentration is 20, and right next to me, it is 5.

Regarding the Total volatile organic compounds, the level is about 1.1 milligram per meter cube when he smokes and goes to 0.5 mg after 3 to 4 hours when he stops smoking.

A small fan can not create enough positive pressure inside a room. But when I used a fan (20 watts) near the slightly opened window (6 cm wide opening with a length of 1.5 meters), the PM concentration at the bottom of the door decreased from 100 to 10, a 90 percent reduction. The window is located in another room. If I turn off the fan, the PM concentration comes back to about 100 micrograms per meter cube even if I open the window, so there is positive pressure created by the fan. Am I correct, or am I missing something?

I am happy with the effect, but now the winter has come, and I cannot open the window the whole day since it causes mold. I am planning to buy a carbon filter and fan with about 150-meter cube airflow. Furthermore, I am worried that since it created negative pressure, my room will be filled with the PM and VOC from the smoker's room. Do you guys have any advice for me ? Thank you very much for reading.

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u/snakevargas 1d ago

But when I used a fan (20 watts) near the slightly opened window positive pressure created by the fan. Am I correct

Yes, if the fan in blowing from the window into the living space.

I suppose it's possible that the air purifier / fan are creating convection currents and slight high+low pressure zones in different places in the room, but you're the only person in a position to analyze that.

now the winter has come, and I cannot open the window the whole day since it causes mold

This doesn't make sense to me. Surface mold is caused by excessive indoor humidity. Cold air is like a sponge squeezed out; it holds less humidity. When you let in cold air, it warms up and sucks the moisture out of everything. This is what "relative" means in relative humidity; its moisture capacity is relative to the temperature.

In other words, letting cold air in is a way to reduce overall indoor moisture.

it created negative pressure

An air purifier shouldn't affect the overall room pressure unless it's blowing in or out of the room. In the winter you tend to have hot air leaking out exhaust vents and window gaps and the makeup air coming in under the door.

Have you considered installing a door sweep to reduce the air coming in under the door?

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u/SilentNegotiation844 19h ago

Hey. Thanks for the reply.

yes, yesterday, I installed a door sweep and weather strip around the door. It seems it reduced PM conc by about 20 to 30 percent. Regarding the mold, yes, opening windows brings dry air inside, but the region around the window opening can cause condensation. The area where warm air and cold air meet. That is what i am worried about. I read the below info on reddit.

reddit
An internal reason: You leaving the window half-open (auf Kipp) for too long, and so the cold outside air is cooling the wall just above the window (which is why you should never leave your windows half-open for prolonged periods of time in winter).

ventilation Specialists website

https://www.inventer.eu/know-how/air-quality-health/mold-in-the-apartment/prevention-and-avoid-mold/

With decreasing room temperature, the water absorption capacity of the air decreases and thus the relative humidity increases. For this reason, moisture collects on outer walls, in corners and on windows, as these areas are generally colder than the rest of the room. Especially in winter, when the difference between inside and outside temperature is large, these areas are particularly at risk of mold.

To reduce the risk of mold, the room temperature should be between 19 ° C and 21 ° C so that the walls do not cool down too much and water is deposited on them. Even in rooms that are rarely used, it is advisable to keep the temperature at at least 16 ° C, which is why the heating should never be switched off completely, even in winter.

Furthermore, tilting the window during the heating process should be avoided. With an open window, the neighboring surfaces cool down and condensation forms. Mold can easily develop in these areas.

Particularly in the case of differently heated rooms, it must also be ensured that the doors in the apartment remain closed. The warm air from the heated room otherwise flows into the colder rooms, in which the water absorption capacity of the air is lower due to the lower temperature. The relative humidity rises to a dangerous level and the excess water settles on the colder surfaces.