To be considered a background ventilation system, the fan must be capable of running on a regular schedule or continuously, if necessary. There are several timers that can control fan speed, operating times, or both. When you’re choosing a controller for your ventilation fan, check with your supplier to make sure that the controls are compatible with the equipment you are using. Regardless of the control you choose, I recommend first setting the fan to run about half the time, at least during the hours people are regularly home. Then you can adjust it to run more or less, as needed.
I usually install Airetrak or Aube controls in the bathroom, as they have a built-in boost override and the timing functions are hidden from view. You can put the 24-hour dial timer in the bathroom, but it’s ugly and it shouldn’t be used for day-to-day on/off overrides. I try to put that timer in a laundry area, utility room, or basement stairwell or near the electrical panel – somewhere convenient and accessible but not that visible
If you have a ducted, central-air system for heating and/or cooling, another inexpensive (between $200 to $300 for parts) ventilation option is a supply-only system ducted through the furnace or air conditioner’s air handler. This system pulls in fresh outdoor air whenever the air handler runs and distributes it throughout the house.
Although a simpler version of this system is common in some parts of the United States, most installations lack three important elements. First, a motorized damper is essential on the fresh-air-inlet duct – preferably where the duct enters the house. This prevents air from coming in through the duct when ventilation is not needed. Next, a timer is necessary to ensure that the air handler runs regularly, ventilating the house when there is no call for heating or cooling. This ensures a minimum ventilation rate year-round. There are a couple of timers that can handle this function.
Finally, insect screening at the inlet hood is important, and it must be kept clean. With a return-air fresh air system, it is also very important to locate the outdoor-air inlet far from potential sources of pollution, such as a dryer, combustion appliance vent, the garage, or the driveway.
If your house has an existing forced-air system, a return-air system may offer better overall ventilation than a bath-fan system, though it may be less cost-efficient. For example, a 20-watt bath fan running 100% of the time may cost less than $20 per year in electricity; a furnace fan running a third of the time may cost between $30 and $120 per year. If you have an old furnace with a high-wattage blower motor, a return-air system is less desirable due to the higher cost of operating the blower motor. And if your central-air handler is noisy, your ventilation will be, too. However, this system has two major advantages: First, fresh air is drawn from a known place and can be filtered. Second, unlike a bath-fan system, it circulates fresh air throughout the house.