Allow me to begin with a simplistic analogy used to explain “What is Static Pressure?”
A fan or blower inside a furnace is setup to deliver a specific amount of air. Let’s compare that airflow to a man pushing a shopping cart. A cart holding five pounds of groceries on a smooth flat surface offers minimal resistance.
However, add resistance by attaching a five pound weight to his ankle, and you will discover the man pushing the cart will either move slower, or require more energy to move at the originally desired speed. A HVAC system is similar. Static Pressure is resistance against air.
Your HVAC system is made up of duct-work, filters, dampers, and mechanical HVAC equipment to mention just a few. Therefore, static pressure, or system air resistance is a compilation of the entire system. No matter how well an overall design, one flaw can significantly cripple an entire system.
I had a neighbor who repeatedly called for service because their home HVAC system was under-performing. Three years after living in the home, a company was contracted to clean the duct-work. The technician discovered one of the two return air ducts was completely blocked off by the upper floor. Somehow during construction the duct-work was never completed. The blocked return added unexpected static pressure to the system. It restricted all the air intended for two returns down to one, increasing the static pressure. The construction over-sight forced air furthest from the restricted return to travel further and differently than designed.
Static pressure is dynamic and changes with CFM
Static pressure is directly related to duct surface material, duct distance and design, HVAC equipment obstacles and volume of air.
MicroMetl offers a “Air Duct Calculator” (See Video – How To Use A Duct Calculator) which allows a quick and easy static pressure calculation to be performed with the use of just a few design variables.
The higher the static pressure, the more cost required to overcome it. HVAC systems are designed for a specific heating & cooling load. The load consists of room and building material & design, windows, occupancy, etc. It is imperative that duct-work and all associated system elements are properly calculated. If the CFM is lower than required, inadequate circulation & distribution of air may occur. If the CFM is too high, the additional static wastes energy, high velocities cause occupants to detect a breeze, and heightened decibel levels may become detectable throughout the system.
Don’t Confuse High Static Pressure With High Building Pressure
There are times when a high static HVAC fan is required to over-come high static HVAC designs. More powerful fans are required to adequately push air throughout a system, and back to the HVAC unit. The additional fan power is designed to properly distribute the air, and in no way in itself results in building pressurization.
Static Pressure is the resistance air encounters while traveling throughout a building’s HVAC system.
Building pressure is a result of a higher or lower volume of air supplied into a space, than air returned to your HVAC source, resulting in negative or positive building pressure. Two previous articles are available for additional information… What is a HVAC Power Exhaust — Achieving Proper Ventilation, and Barometric Relief – What Is It, And How Much Air Does It Remove?
The Measurement of Static Pressure
A Manometer is an instrument which originally utilized a column of water (shown here incorporating red dye) to reflect and measure static pressure. Air pressure physically elevates the water in a measurement of inches. Thus, the origin of static pressure is still referred to in inches of water column as a unit of measurement. The term manometer is used nowadays in reference to any static pressure measuring device.
Don’t Forget the Air Filters
Did you know? As an air filter gets loaded with more and more particles, the filter actually becomes more efficient. However, it also increases resistance and reduces airflow.
It is said that nearly half of the energy used in a home goes to heating and cooling. A dirty filter will slow down air flow and make the system work harder to keep you warm or cool — wasting energy.