Which Type Is Right For Me?

Bathrooms fans use controlled mechanical ventilation which exhausts moist stale air from a home, while introducing fresh outdoor air to improve indoor air quality. This process helps in a number of ways:

  • Reduces the chance of moisture damage to the structure of the home
  • Reducing the chances of mold and mildew growth
  • Dilutes and exhausts carbon dioxide build up from occupant respiration and other indoor pollutants
  • Improves indoor air quality for health benefits such as reduced incidence of allergies and asthma 

It is especially important to add controlled mechanical ventilation to a home that has been weatherized, as a tight building envelope allows for little natural air leakage in or out of a home. 

Even older home homes can benefit from controlled mechanical ventilation. Natural ventilation is primarily caused by the “stack effect” within a home, where warm buoyant air rises, drawing in fresh air through leaks toward the bottom of the home and exhausting air through leaks toward the top of the home. The stack effect is strongest when the indoor air temperature is much greater than the outdoor air temperature. Therefore, an old leaky home will only naturally self-ventilate under certain conditions, such as cooler, or very windy days. On still days during the spring and fall or days with similar indoor and outdoor temperatures little if any natural ventilation will occur.

There are two types of ventilation systems

  • Exhaust-Only Controlled Mechanical Ventilation
  • Balanced Heat Recovery Ventilation

The table below, taken from page 36 of the 2020 Vermont Residential Building Energy Standard Handbook, illustrates the sizing required based upon the number of bedrooms (indirectly the potential number of occupants). The handbook is also a great source of information on ventilation and combustion safety including additional information on qualifying equipment, flow rates, equipment, controls and installation guidance.

Number of

Minimum Rated
Capacity (CFM)

Minimum Number of












Centrally ducted systems: 1,
All other systems: 2 or more



Centrally ducted systems: 1,
All other systems: 2 or more

Homes over 3000
sq. ft.

0.05 x sq. ft. of conditioned

Centrally ducted systems: 1,
All other systems: 2 or more

The details and benefits of each type are discussed below.

Exhaust-Only Controlled Mechanical Ventilation

  • Exhausts potentially harmful moisture from the home
  • Inexpensive to install, only requires a single exhaust duct

Exhaust-Only Controlled Mechanical Ventilation systems consist of a high quality, energy efficient ENERGY STAR Certified bath fan with either integral control or a separate control (programmable timer) device.

Installation tips

  • Separate control devices should be installed in a location that cannot be readily accessed by occupants.
  • The timer control should be programmed to operate the fan 6 hours per day. This can be 15 minutes out of every hour of the day or the fan control can be programmed to operate the fan for longer intervals when the home is more likely to be occupied (e.g. between 4:00 PM through 8:00 AM).
  • A manual wall switch for the fan should also be installed allowing occupant override of the timer to turn the fan on when the bathroom is in use.
  • Wherever possible, exhaust ductwork should be smooth wall pipe, sloped to its termination at the outside of the building.

While not eligible for BED rebates, kitchen range hoods vented to the outside are an integral part of home ventilation, removing moisture produced by cooking.

Receive $110 toward the purchase of an eligible bath fan and control. 

Bathroom FanBathroom Fan

Balanced Heat Recovery Ventilation

  • Provides fresh air where it is needed, to the home’s occupied spaces, especially bedrooms.
  • Spot ventilation provided by bath fans, kitchen range hoods and clothes dryer vents is still necessary to remove moisture from the home

While exhaust-only mechanical ventilation systems do an excellent job to rid the home of excess moisture, balanced heat recovery ventilation systems not only perform this function, but also provide fresh air where it is needed, in the occupied spaces in the home. This is accomplished through ductwork connected to a heat recovery ventilation unit which brings in fresh air from outside the home, supplies the fresh air to each room, while exhausting stale air back outside the home. Furthermore, heat that would be wasted by exhausting heated air from the home is recovered and transferred to the supply air by the heat recovery unit.

Rebate: BED does not have a set rebate for Balanced Heat Recovery Ventilation, but  can perform a custom analysis of a proposed system to determine if the energy savings are sufficient to justify a custom rebate.

Contact BED Energy Services to get started with free, unbiased advice. We’ll walk you through the decision-making process and help you maximize your energy investments.