After much analysis, the team decided to renovate the existing exhaust vents with the Aeroseal method and install fresh air trickle vents. Trickle vents on the exterior walls replace the existing fresh air system that did not work. Before the renovation, fresh air was supposed to enter the apartment underneath each front door from the hallway, because the hallway was pressurized. This never really worked.
Before settling on the renovated central exhaust/trickle vent approach, the team explored heat recovery ventilation (HRV). Ideally, from an energy savings perspective, each apartment would have an individual HRV, so that heat (or cooled) exhaust air would preheat (or pre-cool) fresh air entering the apartment. Using this system, there would be much less energy loss and better air quality than with a central ventilation system.
However, HRV’s are difficult to install in an occupied apartment. The floor plan at Castle Square was especially challenging, because the HRV duct work would have to travel from the exterior wall of the building all the way to the interior kitchens and baths. New duct work would have been extensive, requiring substantial coring through concrete and the installation of new drywall conduits to exhaust to the outside wall of the building. In an occupied rehab scenario, the impact of dust and other inconveniences could have compromised resident health. HRV’s can be expensive even on new construction jobs, however the resulting cost and disturbance to residents on this job would have been more than $7,000 per unit. Because of these issues, the team decided to rehab the central exhaust system and install trickle vents for fresh air.
It should be noted that there is always the concern that trickle vents won’t work, because they are a passive vent that responds to negative pressure in the apartment. When the exhaust draws stale air out of the apartment, the trickle vent draws the equivalent amount of fresh air into the apartment. If an apartment is not air sealed enough, the trickle vents won’t work, because make up air will come from the easiest route (which is generally not the trickle vent). In a leaky building, the easiest route may be leaks in the exterior walls or windows or even leaks between apartments. Relying on leaks means that air leaking into the apartment would probably not be fresh air (for example, who wants to breath in air from the wall cavity or a neighbor’s apartment!). Our team hopes to avoid this problem, through extensive air sealing between the apartments and the outdoors and between the apartments and each other. With air sealing, the trickle vents should function properly.