By Robb Aldrich
Residential ventilation can be a tricky topic, and I often try to discuss it in terms of practical, cost-effective, and holistic solutions. However, it is also important to clarify the purpose of proper ventilation and what an ideal system might look like in a new, efficient home. In both single-family and multifamily homes, an ideal system would be similar – keeping in mind that practical issues can be quite different.
The purpose of ventilation is to remove contaminants that can compromise health, comfort, productivity, durability, etc. There are other ways to lower contaminant levels aside from ventilation that should be noted. Emitting fewer contaminants from materials and activities is obviously good. We all should be doing that. Also, actively filtering, adsorbing, or otherwise removing contaminants from indoor air can certainly help, and that is another lengthy topic entirely. For most new residential buildings, mechanical ventilation is still the primary means to remove contaminants for now, or at least it’s the primary method that designers and developers need to plan.
If building a new, efficient home in an idealized Shangri-La, my ideal ventilation systems would look like this.
Each bathroom would have an exhaust fan that draws roughly 80 cubic feet per minute (cfm). For decadent bathrooms with multiple showerheads, big whirlpools, etc., you may want more, but for most of us, 80 cfm is sufficient. These bathroom fans would remove moisture, odors, etc. to the outside. They would have humidity sensors, or at least controls in place to continue running several minutes after a shower. They would and should still have the ability to be manually operated by the occupant.
Kitchens would have range hoods with good capture efficiency that exhaust 200 to 250 cfm directly outdoors. Because these hoods would be in new, efficient, and air-tight homes and apartments, you cannot draw out 200-250 cfm without providing makeup air. This would probably require a separate duct to outdoors with a damper, a fan, and an electric resistance element to temper the air when it’s very cold outside. As we are still in my Shangri-La, this heating element would only operate when outdoor temperatures dip below 40F, and it would modulate to maintain discharge temperature of ~70°F as to not waste electricity. The range hood – and ensuing makeup air system – would activate automatically when heat or cooking contaminants are detected above the range.
In addition to local ventilation, continuous ventilation is still needed to dilute contaminants throughout the home. For this you want an energy recovery ventilator (ERV) or a heat recovery ventilator (HRV). These ventilators save or “recover” a significant share of the heat that might otherwise be exhausted by the ventilation process, heat that would need to be created by the building’s heating systems. You want a H/ERV with low power consumption and good heat recovery effectiveness (at least 80% sensible, 60% total for an ERV).
At a minimum, the H/ERV air flow should deliver what is required by the ASHRAE 62.2 standard. In our ideal home, however, we want to be able to boost this rate by two to three times. Even better, there would be a control system that senses concentrations of key contaminants and can boost the ventilation rate automatically. In an ideal multifamily building, each apartment would still have its own H/ERV so there are no issues balancing flow rates across dozens or hundreds of apartments.
Fresh air from an H/ERV would be distributed around the home by ducting or some other mechanical mixing system. If local exhaust is accounted for, there is not much difference where you exhaust and supply. It does not take much air flow to equalize contaminant levels throughout a home.
For all ventilation systems, fans would be powered by very efficient, variable-speed motors. All systems could be boosted or manually controlled by occupants as needed.
All this said, there are significant challenges of the ideal system outlined here. From cost, to wall penetrations, maintenance, and space (and back to cost), the scenario is not practical or affordable in many situations – especially multifamily buildings. Some of the “ideal” controls mentioned don’t exist (yet). I don’t see systems like this often; when I do they’re typically in custom, high-end homes. But when thinking about indoor air quality and energy efficiency, this is what I would aim for if designing a new home for myself. And I think it’s useful to keep an ideal in mind when designing something realistic.
Robb Aldrich is a Principal Mechanical Engineer for Steven Winter Associates, Inc.
Many thanks to our sponsor: