Ducted Air Intakes and Increasing Passive Ventilation In Vehicles

Air Quality Management Goals

The ASHRAE recommends that the air inside a human inhabited space should cycle 30% of air volume every hour. The lions share of energy expended in an HVAC system is traditionally conditioning the air coming in from outside to replace this air mass. Using heat exchangers, thermal mass, pressure differentials, and other natural features in a space to preform these tasks based on the passive impact of the environment will significantly decrease overall air quality management energy costs.

Thermal Chimney Effect

As an air cavity warms up it becomes more buoyant, causing the warm air to rise and create a vacuum behind it. This affect can be harvested to create meaningful passive air movement inside a space and is technically called natural convection.

See the source imageOne of the simplest ways to get natural convection in a vehicle is to puncture the roof at the highest point and at a lower point, preferably at a shaded point (underbody). By simply protecting the intake vents from direct sunlight and insect intrusion just simply adding a few vent holes will create a noticeable change in air quality. As the sun heats up the vehicle air will naturally be sucked into the vehicle and exhausted out the top. Adding a fan on the roof duct obviously adds to the effect, and the solar heat actually decreases the amount of energy required to run the fan. 

Creates a cavity on the roof as a plenum
A basic vent hood
You can further increase this effect by creating an airspace on the roof, called a plenum, and painting it a dark color. This creates a solar chimney effect by producing a hot zone which produces more suction the warmer it gets. One simple solution is to use a vent cover painted black to produce a serious differential. This cavity also provides a waterproof cavity that allows the roof vent to be open even in rain or other inclement weather. Puncturing a vent hood and ducting to the outside allows other zones to directly vent out without being drawn through the living space as well, which is how I prefer to ventilate non-living spaces such as under beds and inside cabinetry. The taller the vent hood is the more vacuum it will create. 

Image result for solar chimney
A wall cavity driven chimney
A more compact and effective method of producing natural ventilation is to produce a cavity that runs the entire height of the structure. By merely ducting a vertical cavity that accepts air from the climate controlled space at the low side and exhausts at the top the solar chimney can
produce MUCH more vacuum. This can easily be a cavity inside of an insulated wall, or just a ducted chimney much like a home downspout. You could combine both of these effects by ducting the chimney out the top of the vent hood. By placing intake vents all around the vehicle you can ensure that wind helps assist air through the space; there are various ways to direct intake air as well such as omnidirectional wind scoops.

Simple PVC ducted intake vents

No automatic alt text available.I've decided to duct my intakes with simple PVC pipe, which is vastly inferior to repurposed rectangle SUV vent which will be my future material for this. PVC will be run inside all cabinets and underneath furniture. This airflow is to combat mildew and mold growth while also giving me a central point to place an air exchanger, dehumidification equipment, particulate filters, and the like. 

Louvered metal insect screened vents are attached to the PVC pipe and run through the space directing air from outside into various air zones.

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