This article was originally posted by Pro Trade Craft and has been lightly edited.
Tightening an old house or building a tight new one means that you can't leave ventilation to random leaks anymore. You have to control it.
Whole-house mechanical ventilation is the intentional exchange of indoor air with fresh outdoor air at a controlled rate using fans. Its purpose is to improve indoor air quality. Historically, mechanical ventilation was limited to local-exhaust (kitchen and bath exhaust fans) for spot control of moisture and odors. Houses commonly had enough natural ventilation, through leaky building enclosures, that ventilation was not necessary.
Houses have become significantly tighter during the past 15-20 years as a result of changing codes, energy efficiency programs and an overall desire to reduce energy use.
Benefits of whole-house mechanical ventilation include:
- A consistent supply of outdoor air for improved indoor air quality and occupant comfort
- Control over the amount and source of outdoor air
- Dilution of indoor contaminants, such as odors and allergens
- Helps control relative humidity and reduce moisture accumulation during the heating or temperate seasons
Types of mechanical ventilation
There are three types of whole-house mechanical ventilation systems: exhaust-only, supply-only and balanced. Each system uses a combination of fans, ducting, dampers and controls, and they each have different pros, cons and costs associated with them.
Exhaust-only ventilation tends to consist of a fan, commonly a bath fan, exhausting indoor air. Outdoor makeup air is drawn into the house through leaks in the building’s enclosure.
- Contaminants may be drawn into the house from an attic, garage, crawlspace or wall cavity
- Potential to draw moist outdoor air into the wall cavity that could condense during the cooling season and cause moisture problems, particularly in warm humid climates
- Can cause or contribute to back-drafting of combustion appliances
- Lowest installed cost and low operating cost
Supply-only ventilation consists of a fan drawing outdoor air into the house. Indoor air escapes through the building enclosure and exhaust fan ducts.
Supply-only can be a dedicated system, or more commonly a central-fan integrated (CFI) system. With a CFI system, outdoor air is ducted to the return plenum of an HVAC air handler that draws in and distributes the outdoor air.
- Minimizes contaminants entering through the building enclosure
- Outdoor air is drawn from a single, known location for best air quality
- Potential to drive moist indoor air into the wall cavity that could condense and cause moisture problems during the heating season in colder climates
- Low installed cost, however for a CFI system, the electronically commutated motor may increase the initial cost and operating cost may be higher
Balanced ventilation systems are a combination of exhaust and supply methods providing approximately equal indoor exhaust and outdoor supply air flows (e.g. an exhaust fan combined with a supply fan or passive inlet vents).
A balanced system may include a heat recovery ventilator (HRV) or an energy recovery ventilator (ERV).
- An HRV transfers a portion of the heat between the exhaust air and the fresh air; an ERV transfers heat and moisture
- An HRV or ERV provides the benefits, but limits the drawbacks, of supply-only and exhaust-only methods
- Generally, an HRV is recommended for dry, cold climates and an ERV is recommended for moist, warm climates
- Highest installed cost for HRV or ERV due to equipment and additional ducting
There are pros and cons to each type of mechanical ventilation but one type may work better for your home than the other two. By consulting a BPI certified professional, you can better your home’s air quality with the right mechanical ventilation system.
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