What is static pressure in hvac

Did you know that static pressure is an essential aspect of air conditioning and heating maintenance? But, what is static pressure in HVAC system? Why do you need to know about it? Read on below to learn more about static pressure in HVAC.

Definition of static pressure in HVAC

Static pressure in HVAC refers to the airflow resistance in the ductwork and other components. High static pressure levels indicate that something is wrong with your equipment. It is crucial that this problem is identified early on for you to take the necessary steps to repair or eliminate it. This will let your system perform better and last longer.

Cooling or heating systems are meant to transport a particular amount of air, whatever the size of the unit may be. Ductwork is similarly responsible for allowing the system to work efficiently and properly. If all the components of the system are installed properly, it will maintain the static pressure at the ideal level. The factors will ensure that your home stays comfortable all year-long.

What is ideal static pressure in HVAC

The ideal static pressure in HVAC is .81” w.c. or water column or .81 on the manometer.

What is external static pressure in HVAC

The external static pressure in HVAC refers to the suction pressure or the total pressures taken where air gets into the equipment, and the discharge pressure or where air goes out of the equipment. The two pressures are added together to determine the total of two pressure force for the fan to pull and push against.

What is total static pressure in HVAC

The total static pressure in HVAC shows how hard the system needs to work in order to have the air moved through house or building or the amount of existing resistance to flow. When the static pressure is higher, the system also needs to work harder for the air to move through the house or building.

What causes static pressure in HVAC

Filter choices, poor system design, and improper ductwork installation can all cause static pressure in HVAC. A combination of the factors is actually at play in most households. Until the static pressure issue or issues are addressed, you cannot expect your system to work to the fullest potential. It might even end up failing sooner than expected.

What causes low static pressure in HVAC

Low static pressure in HVAC may be very rare but it can be due to any of these two causes:

  • The trunk lines were oversized by the installer. This can be seen in several older houses. Whatever the reason might be, whoever was in charge of installing the trunk lines ended up oversizing them.
  • There are numerous energy efficiency improvements that were made at home. Maybe your home envelope was tightened in an effort to save energy. There are extreme cases when your upgraded house might no longer be compatible with the old ducts. Their size was right before but they are no longer enough now.

Whatever it is, you might find it hard to stay comfortable. If the airflow is not strong enough, one side of the room might be hot while the other is cold. The best fix for is to have the ducts redesigned and reinstalled.

What causes high static pressure in HVAC

The following are among the possible reasons why high static pressure occurs in HVAC:

  • Dirty ducts

Airflow resistance can occur if there is clogged ductwork, resulting to high static pressure. Excessive debris and dust narrow down the ducts’ interior that prevents balanced movement of air.

  • 1-inch pleated filter

Standard 1-inch pleated filter in an HVAC system creates significant restriction of airflow. The resistance is greater of this is thicker. A better alternative is using a media filter with low-pressure drop. UV lights can be installed close to the supply ducts. You can also upgrade to HEPA or high-efficiency particulate air.

  • Poor design of ductwork

Incorrectly installed or badly designed ducts can either be too narrow with excessive sagging, dipping, or bending that creates airflow resistance.

  • Extremely small intake of return air

Undersized return air intake can make it harder for air to be drawn in by the blower motor into the unit. This issue becomes apparent after repairing and sealing ducts or installing a new HVAC system.

How does static pressure affect airflow

The stated causes above are signs in new systems that the equipment is improperly sized. If your HVAC system is already a bit old and these problems show up, it might be an indication of high static pressure that you need to correct.

You can change the filter, clean the coil, or look for any obstructions present in the ductwork. Did the zoning damper get stuck closed? Have you got a zoned system? These are the kinds of obstructions that can lead to high static pressure and affect airflow.

Can static pressure be negative

Yes, you can encounter negative static pressure on the air mover’s inlet side that includes all return ductwork. Pressure on the two sides of an AC or furnace filter will have negative static pressure. The downstream side will have lower pressure or more negative static pressure compared to the upstream.

How to measure static pressure HVAC

It will usually only take less than 5 minutes to measure the static pressure of residential HVAC system. Below are some sample instructions for an external coil and a furnace.

  1. Find the right locations for drilling the test ports on supply side between the coil and the furnace as well as o the return side between the furnace and the filter. The test ports should be centered to make it look neat. Stay away from the circuit boards, condensate pans, cap tubes, or coils to prevent unwanted damage. Make sure that you also look and check before you start drilling.
  2. You can now proceed to drilling the test ports. For this, you need to use a 3/8” drill bit that comes with metal piercing tip. The bullet-tip drill can produce a round and clean port. See to it that you use the drill bit sheath to ensure that you don’t drill into the coil. In case there is a duct liner inside, you also make sure that you penetrate this to guarantee a good and proper reading.
  3. Push one tip of the tubing onto the static pressure end. Put the other tip of the hose on the gauge’s high pressure port marked as +. If needed, see to it that the gauge is level. You can also zero this out with the use of a small screwdriver to have the screw on the face adjusted. All digital gauges zero in different ways to you might need to refer to the owner’s manual.
  4. Read the positive or supply static pressure. To do this, you need to inset the tip of the static pressure into the test port and the tip should face into the airflow. The tip’s magnet will hold this in place as the value gets read and recorded. The measurement you will get is the pressure that the fan sees on the system’s supply side.
  5. Read the negative or return static pressure. To do this, you should move the tube to low pressure port from high on the gauge. The static pressure tip should be inserted to the test port on return side and the tip should face the airflow. Proceed to reading and recording the negative static pressure. Make sure that the port plugs are inserted to the test ports when you are done with testing. 
  6. You can now calculate the Total External Static Pressure of the system. All you have to do is add the two values you got from the steps above. Since the positive and negative signs indicate the specific type of pressure that is being measured, these can be simply ignored once you add together the two values.

How to fix static pressure HVAC

To prevent mechanical failure and avoid costly HVAC repairs, it is important that your equipment is serviced every year. There are several steps you can take to fix static pressure in your HVAC:

  1. Ask the technician to check the static pressure at the correct spots along the air handler, ducts, and plenum.
  2. Make sure the technician seals all the holes.
  3. Request the technician to clean the outside unit’s condensing coil and the inside evaporator coil. This will help lower the risks of mechanical damage and improve the system’s energy efficiency.
  4. Keep a close eye on the air filter and change it often. Air filters are much cheaper compared to the costs of a new blower motor or compressor. While most of the latest smart thermostats feature Dirty Filter notifications, it is still important to have the filter checked with your own two eyes monthly even if your thermostat doesn’t tell you yet to do so.
  5. You can also consider using a variable speed blower motor.

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