Installing a good mechanical ventilation system means getting an energy-efficient, quiet fan; designing and installing the ducts properly; and choosing appropriate accessories and controls for your system.

The Home Ventilating Institute (HVI) rates and certifies home ventilation equipment and provides sizing recommendations. Look for the HVI-Certified label, and refer to the HVI’s online rating and sizing information.

Fan Efficiency

With the quality of the air your family breathes at stake, it’s worth spending a little extra money to get a really good fan. Inexpensive fans are not intended for continuous use. The motor largely determines how well a fan will work, and how much it will cost to run over its lifetime.

Fans with shaded-pole motors are the least expensive to buy. These are the fans that builders usually install, especially in bathrooms. But these fans generally don’t last long when operated for extended periods; they make so much noise (5.0 sones or more) that you won’t want them on for long; they’re inefficient; and they cost a lot to run.

Fans with permanent split-capacitor motors will cost more, but their motors are about twice as efficient as shaded-pole motors, and should last much longer. These fans are usually rated for continuous operation and often have squirrel cage blowers that are designed to make 90% less noise than fan blades. Some of these units are rated as low as 0.3 sone.

If your ventilation system is part of a heating-and-cooling system, you can improve it somewhat by using a highly efficient fan motor. The most efficient motor on the market is the ECM. It doesn’t lose efficiency when it runs at low speeds. This is important if you want to be able to run the fan at low speed for central ventilation and turn it to high speed for heating and air conditioning.

Look at the wattage of the fan you want to buy, and divide this by its capacity in CFM. For higher-capacity spot fans, you want one that uses less than 1 watt/CFM. Low-capacity fans (50 CFM) should use less than 0.4 watt/CFM. If the wattage isn’t shown on the label, get the information from the manufacturer or the Home Ventilating Institute. If you shop for ENERGY STAR-labeled fans, you’ll find they’ve done the watts/cfm calculations for you.

Ventilation Ducts

Well-designed and properly installed ductwork ensures that the fan will operate at peak capacity, and that moisture and indoor pollutants will actually be taken out of the house. Duct runs should be as short, smooth, and straight as possible. Bends lower the fan’s capacity, increase noise, and create a place for moisture to condense. According to the International Residential Code and ASHRAE 62.2, all ducts should terminate outside the building—not in an attic, soffit, or crawl space.

Most ventilation systems use either smooth-metal or flexible duct. Smooth metal (galvanized sheet metal) has about half as much resistance to airflow as flex duct, but it’s harder to route through the house because it’s rigid. For kitchen range hoods, use only smooth-metal ducts; the fan is typically not strong enough to overcome the friction loss of flex duct. Flexible duct is prefabricated metal, polyvinyl chloride (PVC), or Mylar on a metal frame. Metal flex is better for ventilation systems because it is more rigid, making it less likely to sag or kink. Flex duct almost always requires hangers or other supports every 3 or 4 feet to keep it from sagging or kinking.

Ducts must be sized correctly to produce the desired rate of airflow. Your installer must carefully calculate resistance to airflow in order to determine the correct duct and fan sizes. Because flexible duct has more resistance to airflow, it should be sized larger than smooth-metal duct. Manufacturer’s specifications for duct size are too small in most installed conditions.

Grilles and Dampers

Grilles are openings through which ventilation air enters and exits rooms. Dampers inside ducts allow airflow to be adjusted. Mechanical or gravity dampers are set at the time of installation (though they can be reset if needed). A motorized damper is operated by controls that open it when the fan is on, and close it when the fan is off. Backdraft dampers keep air from moving the wrong way through a duct system; their use is required by national building codes.

Timers and Other Controls 

There are many kinds of ventilation controls. Programmable timers allow you to ventilate the house at scheduled times. Controls that detect high humidity, motion, and pollution can activate the ventilator when needed. Variable-speed controls allow you to adjust the amount of ventilation to your needs. Manual overrides can be installed to allow intermittent spot ventilation or to halt ventilation if outdoor air is polluted by a nearby wildfire, for example.

If your heating-and-cooling fan is one of the efficient ones described above, you can link ventilation with the heating-and-cooling system’s thermostatic control, or with memory devices that remember when and for how long the ventilator recently ran. Some inexpensive new controls can be retrofitted to your existing furnace or A/C. They operate the fan regularly even when the thermostat doesn’t call for temperature control. They can even halt ventilation if the outdoor air is too hot, cold, or humid and resume it again when conditions moderate.

Including a good timer and damper control on your system can make it much more efficient, and easier to maintain. No matter what type of control you select, make sure it is easily accessible, and keep the instructions in your Homeowner’s Manual.

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