70 September 2013
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Figure 3 shows the prototyper set up as a fresh air
controller; Figures 4 and 5 show the vent boxes Joel
designed that have an aerodynamic vane that gently
opens when the attic fan turns on, then closes to keep out
the rain when the attic fan shuts off. Of course, they have
a bug screen to keep out the critters at all times.
These vent boxes are easily removable for those times
of the year when it is just too hot or cold outside to
warrant using this system.
The fresh air controller relies on us having accurate
measures of indoor and outdoor temperatures and
humidity, so we'll know when to turn the attic fan on and
off. Let's look at measuring these parameters and the
sensor we will be using.
Sensing Temperature and Humidity
Relative Humidity
Humidity tends to be important to our comfort. You
can feel cooler in Death Valley at 105° than you might
feel in a Louisiana swamp at 85°. The reason is that we
sweat to cool off. For sweat to work, it has to evaporate.
If the air is dry, it can evaporate quickly; if the air is very
humid, it might not evaporate at all.
To use a modest simplification: The amount of water
vapor that air can hold depends on the air temperature.
Warmer air can hold more water vapor than cooler air.
This is why it tends to rain when a warm front with a lot
of moisture runs into a cold front. The warm air cools
down and the water vapor condenses into liquid that
drops out of the sky.
We use the term RH (Relative Humidity) to mean the
amount of water vapor (humidity) that may be present at
a given temperature before the air becomes saturated and
water begins to turn to liquid — the dew point.
An RH of 100% means that the air is saturated,
while 0% means there is no water vapor present. People
tend to be most comfortable at an RH between 30%
and 70%.
■ FIGURE 4: Inside view of a vent box and the controller.
■ FIGURE 5: Outside view of a vent box. ■ FIGURE 3: Fresh air controller prototype.
