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The common knowledge these days seems to be that positive air pressure pumped into the common hallways of highrise buildings will allow smokers and non-smokers to live in the same highrise. Good seals between units is still needed, but the positive pressure prevents smoke from going into the hallway, and then into neighbouring units.

During the summer, many people open their windows. In fact, I like to put a fan in front of the window. Often, this blows cigarette smoke from neighbouring units into my unit. There is simply not enough ventilation to mitigate the heating up of the unit without a fan. Otherwise, it might possible that the positive pressure alone would ensure that smoke doesn't come in from the outside (assuming that the air from the corridor is bearably cool, which it often isn't). For the time being, I try to choose a window that is as far from the smoking apartment as possible, and place the fan in front of that window.

In the spring and fall, however, it might be possible to do without a fan in front of the window, and let the apartment be ventilated by the positive pressure. I don't know where the intake for the positive pressure is, however, and the air quality smells really "non-ideal".

Another complication is that the windward side of the building will experience a high pressure outside, and I wonder it is enough to counteract the positive pressure from the inside.

There seem to be so many variables that can upset the conditions for positive pressure to work. I wonder how well understood is the benefit of positive pressure? Or is it just one of those things that sound reasonable, but difficult to study empirically based on actual highrises?

I eventually want to move, but in the mean time, I am trying various coping mechanisms based on whatever I can understand about positive pressure. A complicating factor seems be the eddies on the leeward side of the building, making it hard to gauge whether to open the windows or which windows to open.

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  • $\begingroup$ perhaps it is just where they located the intake for the positive pressure ventilation... $\endgroup$ – Solar Mike Jul 7 '18 at 6:57
  • $\begingroup$ You mean the explanation for the smell of the air vented into the hallways? I have no doubt that you are right. It's just one of the many factors that complicate the reliance on positive pressure to solve the smoke issue. $\endgroup$ – user2153235 Jul 7 '18 at 12:43
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It is more accurate to think of these as negative pressure systems. But the apartment is the thing that is negative. There is typically a "whole house" ventilation fan that is constanctly exhausting air from the apartment and keeping it at a negative presure relative to outdoors. This draws in air via windows or permamant openings (sometimes trickle vents) for ventilation purposes. The positive pressure in the hallway is to ensure that air does no migrate from the apartment to the hallways, not so that air does not come in the window. Air coming in the window is actually by design. Mitigating odors from outdoors is very difficult and would require a much more complex system.

Another important reason for the positive pressure (other than odors) is for fire protection. In the event of a fire in an apartment, the airflow from the corridor should be keeping the smoke contained to that unit, keeping the path of egress clear. There are typically fire/smoke barriers between units to mitigate the impact of a fire and keep it contained.

Hopefully this quick sketch helps. residential vent sketch

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  • $\begingroup$ Wouldn't this mean that the tissue test should show air flowing from the hallway to the apartment? My tests (limited in sample size) shows that flow is often consistent with the direction of the prevailing wind direction outside. Unless I close the windows and turn on the ventilation, but the source of that flow is obvious. Perhaps my building has an uncommon design -- each unit has its own exhaust conduit and vent to the outside. Under such a design, I'm not sure where the whole-building exhaust would fit. $\endgroup$ – user2153235 Aug 7 '18 at 2:20
  • $\begingroup$ @user2153235 Yes, the tissue should always blow inwards. The effect with an individual fan per unit of a entire building fan is the same. The design vs. what actually happens can vary. I did just find this slide deck which provides some interesting context. link $\endgroup$ – devonp Aug 8 '18 at 3:29
  • $\begingroup$ I have to admit, the schematics showing inflows and outflows make sense. Since my apartment has individual exhaust vents, it seems that only the positive pressure vents in the hallways is common. I suspect that it is so weak that it is often weaker than the external pressure due to wind. Perhaps it's weak because of overall leakage into all the units, which means the doorways between the units and hall are not well sealed (maybe they're not suppose to be), and furthermore, there is a lot of leakage between the units and the outside. $\endgroup$ – user2153235 Aug 8 '18 at 4:46
  • $\begingroup$ Of course, there's loss through the elevator shaft, garbage chute room, and fire stairwells (though in theory, the latter are suppose to be well sealed). To simplify things, however, I consider the net positive pressure from the common area to be the gross incoming air from the hallway vents minus the loss through the various common pathways above. $\endgroup$ – user2153235 Aug 8 '18 at 4:49
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I think that I can say that, at least for my building, the benefit of positive pressure is a myth. I can hear the wind whistling into the hallway from the vent, so I expect positive pressure in the hallways. If I open my windows, and I open my apartment door a crack, I should be able to do the tissue test to indicate which way the air is flowing.

The empirical result is that it flows in the same direction as the external wind. My apartment faces West, and if the wind is coming moderately from the West, then air flows from the apartment to the hallway. If the wind is easterly, the air flows from the hallway into the apartment. Essentially, the positive pressure scheme does very little to control the flow of air between my apartment and the hallway.

My building is just one data point, but if positive pressure works most of the time, then what I observed would have been quite unlikely.

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Properly designed HVAC systems in multi story buildings introduce Make Up air which compensates for air leaving the system thru exhaust air dampers. A mixing box section accepts return air from the conditioned space, make up air from the outdoor air dampers, and the outdoor air intake for make up air is tied to the exhaust air damper position. IN some modes of enthalpy control, one hundred percent outdoor air is taken in and the entire return air stream is exhausted (when the OA stream enthalpy is lower than the return air enthalpy ).Dedicated exhaust air leaving the conditioned space is incorporated into proper design, wherein the outdoor air damper minimum position is set to makeup the fixed exhaust air(from bathrooms, etc.)

Additionally, core and perimeter systems are seperate entities. Building pressure control is achieved, with varying degrees of complexity, by outdoor pressure sensors on all four sides of the building, and integrated with discrete indoor air pressure sensors with the computation output fed into the HVAC unit. IN the case of VAV (variable air volume ) systems, the supply and or return air fans are speed or vane controlled to match the computed requirements, thereby controlling system, and the served space pressure . Bottom line is without knowledge of the systems in the building, all you can do is hunt and peck for your specific situational "fix", in this case regarding smokers smoke. Fire Smoke is typically handled by purge fan systems coupled with the unit mixing box dampers to maintain relative pressures which ensure smoke is drawn out of the building. It's complicated and every building is an individual in and of itself. Hallways and stairwells are always kept at positive pressure to mitigate the introduction of fire generated smoke from entering areas of egress from the building. Yup, HVAC for multi story buildings is a craft, requiring professional design engineering training and skills.

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  • $\begingroup$ Do you know what a paragraph is? It would significantly improve the readability of your answer if it was divided into paragraphs instead of having a monolithic block of text. $\endgroup$ – Fred Aug 7 '18 at 3:33
  • $\begingroup$ Coming from engineering schooling (albeit not the physical kind), I appreciate the technical theory. However, there are many terms that I'm going to have to research. The interpretation of some of the info relies on intricate knowledge of the design of HVAC systems, e.g., "tied to the exhaust air dampener position", "OA stream enthalpy", "incorporated into proper design", "air damper minimum position is set to makeup the fixed exhaust air". It almost sounds like exhaust air is fed back into the building. $\endgroup$ – user2153235 Aug 7 '18 at 12:05

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