SolarVent Intake Ventilation

Good intake ventilation is very important in achieving the desired performance results from your SolarVent solar powered ventilation product.  SolarVent fans will operate at peak performance when supplied with the proper amount of fresh air intake ventilation.  Failure to supply adequate intake ventilation may result in less airflow performance and reduced cooling capacity.  We recommend becoming familiar with your home or commercial application's ventilation design to ensure that the proper amount of intake ventilation is installed for your SolarVent solar powered ventilation system.

How Much Intake Ventilation Do I Need?

The national building code recommends a minimum of one square foot net free area intake ventilation for every 510 m3/hr of powered ventilation airflow.  Since SolarVent solar powered fans are variable speed in terms of airflow, the calculation for required intake ventilation must be made at the fan's highest rating to ensure adequate ventilation under the full range of operation.  The table below may be used to estimate the minimum required amount of intake ventilation for all SolarVent solar powered ventilation products.

Minimum Required Net Free Area Intake Ventilation

When using any type of powered ventilation product for an attic application, all passive roof vents will become sources for air intake.  Unlike passive ventilation where airflow must travel from the lowest point in the attic to the highest, powered ventilation uses air intake from all areas of the roof cavity both high and low.  If built to national building code standards, your home will have a minimum of one square foot net free area passive ventilation for every 28 square metres of roof space.  Estimates of total passive roof ventilation based national building code standards are shown in the table below.

Estimated Passive Attic Ventilation Based on National Building Code

What is Net Free Area?

Net free area is a term used to describe the amount of ventilation space available in any type of vent that may be used to freely pass airflow.  This is essentially the empty spaces you see in the installed screens and louvers on most passive vents.  As the screen mesh size decreases, net free area decreases as well and airflow through the vent becomes more difficult.  As a general rule, screen mesh sizes of ¼" mesh and larger have a negligible effect on net free area intake ventilation.  The chart below shows the effect of screen size and obstructions on net free area for various types of soffit vent designs.

How Do I Add More Intake Ventilation?

In some cases, it may be necessary to add more intake ventilation to meet the recommended requirements for installation of a SolarVent solar powered ventilation system.  This is accomplished by either adding soffit vents, gable/wall vents, or passive roofs vents.  The decision as to which type of vent to install should be based on proper balancing of airflow, as well as space availability.  The chart below gives the net free area for typical roof and gable passive vent sizes.

When adding additional intake ventilation, install the new passive vents at least 10 feet away from the SolarVent solar powered fan.  More intake ventilation is always better when working with active ventilation systems.  SolarVent recommends installing more than the minimum requirement if possible should you need to add additional ventilation.

My Home has Ridge Vents

When installing SolarVent products in conjunction with ridge vents, the solar roof fan unit should be installed a minimum of 5-6 feet away from the ridge vent to allow for balanced airflow through the roof cavity.  Experience shows that this distance will typically allow enough air diffusion space for proper balancing of air intake ventilation from both the ridge vent and other passive roof vents.

Please note that while the SolarVent solar roof fan is operating, the ridge vent will work in reverse acting as air intake vent.  In fact, every passive vent on your roof will act as an air intake vent while the fan is operating.  This is not a problem, just a different method of operation for your roof's ventilation system.  When the solar roof fan is not operational (at night, during rainstorms, etc...), the ridge vent will function as normal.

What about Roof Cavity Depressurization?

Roof Cavity depressurization occurs when there is not enough roof air intake ventilation to support the volume of attic air being removed.  While depressurization can occur naturally (i.e. the sudden cooling of your roof during a rainstorm), it may also occur when using powered ventilation.  Since airflow follows the path of least resistance, for any powered ventilation system to create depressurization and pull conditioned air into the roof cavity, it must have enough power to overcome both any roof intake ventilation present, as well as that of frictional effects created from pulling air through the small cracks and opens between the attic and living space of your home.

Although by appearance they may seem the same, AC powered attic fans and SolarVent solar powered roof fans are distinctly different.  AC powered attic fans have a very large source of grid power from which to use, as well as a constant speed AC motor.  When an AC powered fan does not have enough "free" air intake to support the motor's operating speed, the fan simply uses more power in order to maintain motor speed, thus pulling more air from any air intake source available.  If the AC powered fan is not installed with enough roof air intake ventilation, roof cavity depressurization may occur.

By contrast, SolarVent solar roof fans have both a limited source of solar power and use a variable speed DC motor.  Given the same situation, our solar roof fans will simply reduce in speed rather than pull more air because additional power is not available.  Additionally, our proprietary UltraFlō™ technology also helps minimize any chance of depressurization by virtue of our unique airflow dynamics.  If installed properly with the recommend amount of intake ventilation, roof cavity depressurization will not be a problem with SolarVent solar powered ventilation products.