Architects and builders now produce homes and commercial buildings to much tighter standards against outside air leaks. Ironically, this causes indoor air pollution to build up more than in previous designs. To counteract this, they use ventilation to improve indoor air quality through the dilution of indoor air pollutants, removal of stale or bad odors, and the introduction of more oxygen.
Imagine a pleasant fall slumber party with several children sleeping in one room. Heating or cooling may not be needed, but they would need fresh air into the room to keep the air from being stale. If they can’t leave a window open, one solution to this situation would be ventilation within the HAVC system.
In properly designed commercial buildings, the HVAC system exchanges a certain amount of the air as it circulates. The amount time needed to completely turn over the air volume of a room, known and the ventilation rate, measures the air exchanges per hour. Another way to express air exchange is in cubic feet per minute (cfm) of outside air per occupant.
Ventilation affects indoor air quality in the home and the work place
In office environments, a U.S. department of Labor website states: “Inadequate or improper ventilation is the cause of about half of all indoor air quality (IAQ) problems in non-industrial workplaces.” Indoor air can be two to five times more polluted than outdoor air, and in extreme cases, improper ventilation causes contributes to “sick building syndrome”, according the U.S. Environmental Protection Agency.
To first establish some vocabulary, the term “vent” is used as both a noun and a verb. As a verb, “vent” means to move air from one place to another, either by passive or active means. As a noun, a vent describes a port where air enters or exits the air conditioning and heating system. In most HVAC systems, there are two types of vents: (1) intake or return air vents (for example in a wall) where return air gets pulled from the rooms back into the HVAC system and (2) exit or supply vents in the ceiling, floor, or walls that deliver the air to the rooms after it has been warmed or cooled. The broader term “ventilation” encompasses vents, related equipment, design, and processes.
Understanding the source of indoor air pollution
For ventilation purposes, indoor air pollutants and undesirable contents can be divided into two categories: those produced or increased by inhabitants (cigarette smoke, dry cleaned clothes, carbon dioxide from exhalation) and those produced by the structure (off gassing from construction materials, fibers, furniture, etc). By understanding the source and targeting certain type of pollutants or gasses to be diluted through ventilation, the cleansing can sometimes be timed to coincide with the presence of inhabitants, thus saving energy.
Most residential central air conditioning and heating systems are closed, which means that the air re-circulates inside the envelope of the structure. In homes, windows and doors provide manual intentional ventilation when it is desired. Vent fans in a bathroom and vent hoods in kitchens provide one-way ventilation (pulling air out) with no influx of fresh air. The exhaust flue from a furnace simply gives combustion gases a safe exit, also one-way.
Unless ventilation is specifically incorporated into the HVAC system, leaks in the home or building “envelope” to the outside are the only steady source of fresh air. This haphazard influx of fresh air from the outside also increases the heating load in the cold months and the cooling load in the hot months. Further, unless the fan inside the HVAC system is running (which changes the inside pressure), the ventilation unintentionally supplied by leaks in the envelope is only passive. This includes dormant chimneys, which, unless they are sealed, generate an unintentional updraft to the outside.
During times of mild temperature, there are stretches of time when neither heating nor air conditioning are needed. On the pleasant side, these are the days to enjoy open windows, doors and lower utility bills. However, there are times or situations when heating and cooling are not needed, but the windows cannot be opened. Examples: commercial buildings with fixed windows or security concerns. In these cases, to keep from becoming stale, air needs to move through the HVAC system to be filtered and may need ventilation to introduce fresh air or dilute pollutants.
Energy consumption during ventilation in different climates
The fresh air being introduced into the HVAC system during ventilation may need to be heated or cooled to save energy. Climatic conditions partially dictate whether your system (and monthly bills) would benefit from a heat recovery ventilator (HRV) or an energy recovery ventilator (ERV). A heat recovery ventilator uses a heat exchanger between the incoming and outgoing air. HRVs bring fresh air into the building while recovering heat from the exhaust. An ERV works in the opposite way (cooling the inbound air) except they also affect the humidity level of the air. Due to high student occupancy rates, these devices have been tested extensively in schools. A good color schematic complete with air flow can be viewed on this U.S. EPA website.
Since residential buildings have been built with less air leaks, the need for HRVs and ERVs has increased there also. Why? It does little good to build a tighter home only to have to open a window for ventilation during cold weather. For these reasons, HRVs would be used in climates with long heating seasons, and ERVs are for warmer and more humid climates with longer cooling seasons.
In summary, the components and dynamics within a structure and its heating, ventilation, and air conditioning system are complex and influence each other. HVAC contractors have the experience and tools, such industry standard design manuals or computer software, to coordinate all these factors. Just as load calculations determine what kind of equipment to use, proper ventilation requires a “whole house” approach that considers the equipment, building, and the environment in which it operates.