In the mid 70's, as homes became tighter and tighter to conserve energy, we started to devise mechanical means to bring fresh air into the home on a predictable basis to ensure air quality. There are many ways to achieve this, and I am not going to discuss all of the different systems in detail. The building codes eventually "required" some means of changing the air at prescribed rates in all new construction and homes that were being substantially remodeled.
Sometimes the system is a stand-alone unit with its own ductwork----like a "Heat Recovery Unit." These systems are really cool because in the Winter they exchange the heat in the outgoing air into the cold incoming air (and just the opposite in the Summer).
Other times it is tied into a bathroom or laundry exhaust fan and there will be a timer located in the home that can be set so the fan turns on periodically.
Another method is for it to be tied into the forced air furnace system. This type of system utilizes an air intake installed in the return air duct of the furnace with an automatic damper that is controlled by the furnace circuit board, timer and thermostat. Here is what the automatic damper control might look like at the intake pipe in an attic.
This is a very efficient way to bring fresh air into the home if you have a forced air system----although all of the different methods have their issues. One of the advantages of this system is that the air in the home is filtered year round----not just in the winter when the furnace is operating. And, because it utilizes the ductwork of the furnace heating system, fresh air is distributed to all the rooms in the home.
Several times a year I find this particular type of system compromised because of lack of communication between the roofer and the heating contractor. For the air intake to work properly it has to be able to bring in air when the automatic damper opens and the furnace blower kicks on to pull in air. The roofer knows that the air-intake pipe needs a roof cap----so they install a typical roof vent cap----with a back draft damper in it----designed to move air out of the house not into it. While the vent cap itself is most likely adequate----the damper must be removed for any air to move into the system. A simple enough repair---but the damper in this cap has been installed this way for 10 years.
The flat plate-like flap of the damper can be seen behind the screen in the picture.
PS, for those of you that are new to my blog (or for some other "unexplained" reason have never noticed)all pictures and smiley-face inserts (emoticons) (when I use them) have messages that show up when you point at them with your cursor.
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WA State, Home Inspector Advisory Licensing Board