Since 1991 in Washington State, there has been a requirement provide a mechanical means of changing the air in homes. This came about as we started making houses too tight for them to do it on their own—-the way they would when they were just naturally leaky as a sieve.
There are several ways to meet this requirement—today I only want to discuss the method I saw at a recent inspection.
It is very common to utilize the bathroom or laundry fan in conjunction with a 24 hour type timer so that the timer can be set to automatically turn the fan on at set intervals. This particular installation was a version of this method where the fan is designed to operate in conjunction with an air intake tied into the forced air heating system in the home. There is an automatic damper installed so that the duct is only open to the outdoors when the timer tells it to be open and won’t open when the furnace is running. When the timer is set it will turn an exhaust fan on (in this case the laundry fan) and open up the damper at the furnace. This allows outdoor air to be pulled through all the ductwork into the individual rooms on its way to the exhaust fan in the laundry—-and in this way effectively changes the house air. This picture shows the air intake pipe and its attachment to the furnace plenum. It also shows the automatic damper location in the intake duct.Automatic damper at air intake
The interesting thing about this set-up was that the air intake was run up through the garage ceiling into the attic space—-instead of to the “actual” outdoors. Now some would argue that the attic is “sort of outdoors.” But let me ask a question—-how outdoors is it if it is 120 degrees Fahrenheit in the attic? Now in the winter this might not be too much of a problem and might actually reduce the heating needs of the home but what about all summer when the fan turns on and pulls hot air into the home. This gets even more problematic if there is air conditioning in the home and we are attempting to cool the home while at the same time heating it when the vent fan is running.
In this picture you can see my infrared thermometer pointing at the air intake.
Warm air intake pipe
(Please note that these infrared thermometers are notoriously poor at reading accurate temperatures of metal surfaces and the duct is likely considerably higher.)
I did not wait to see just how hot this duct would get but there was 80 degree air being delivered to the rooms. The sun beating on the black roof would likely keep up with the demand of the fan. Of course the simplest solution would be to only run the fan at night—but that would not fix the issue of the intake in the attic being inaccessible.
These intakes typically need a screen over them which need to be periodically cleaned.
Obviously even if it is just hot outdoors—like 100 degrees—we might end up with some amount of “heating” of the home while the vent fan is running, but nothing quite like the free heat provided by a solar heated attic.
Charles Buell, Real Estate Inspections in Seattle
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