Static pressure is the amount of pressure in your ductwork as the blower motor fan pulls air in from the return ducts and pushes it out through the supply ducts.
The higher the static pressure, the harder the system must work to circulate air. That’s why HVAC pros often refer to pressure that is too high as “drag.”
Another way to describe it is the amount of pressure required to push and pull against to circulate air through the system and your home. Too much is no good.
This post mixes an explanation of static pressure technology with very practical and applicable advice on Measuring Static Pressure, Causes of Static Pressure Changes and Problems, problems that are Signs of High Static Pressure, and How to Be Proactive in Preventing High Static Pressure problems.
Who Should Read This Article?
>Homeowners should read this article to understand conversations they might have with an HVAC salesperson discussing new equipment or with a technician they might hire to check for air leaks in ductwork or to seal ductwork with known leaks.
>Those interested in an HVAC career, perhaps considering an HVAC school near you, might find static pressure to be an interesting topic to research. You can learn more about static pressure below.
Expanded Definition: What is Static Pressure?
Static pressure is also called air resistance, and it changes throughout the ductwork system as the air meets resistance from the filter, coil, dirt lining the ductwork and other obstacles.
As you might expect, static pressure is greatest right where the air leaves the air handler through the supply plenum into the ductwork trunk. Stand next to a fan – the airflow is intense; move away, and the pressure of the air hitting you drops.
The blower motor in the furnace or air handler forces air through the plenum and into the supply ducts.
Pressure is lowest in the return air plenum right before it re-enters the air handler/furnace to be treated again.
For clarification, the supply ducts in your HVAC system are the ductwork that carry heated or cooled air from the air handler to the rooms of your home. Return ducts are those through which the air handler’s blower motor pulls stale air, cool air during a heating cycle and warm/humid air when you’re running AC, back to the air handler to be heated or cooled and dehumidified.
Info Tip: An air handler serves a heat pump. A furnace functions as an air handler when heating the house or supporting the air conditioner.
Did you know? Heated or cooled air leaving the air handler is called “treated” air; air returning to it is called “untreated” air.
Measuring Static Pressure
An HVAC technician uses an instrument called a manometer to measure static pressure. This diagram shows spots where pressure should be tested in an upflow gas furnace setup. Diagrams are similar for downflow and horizontal-flow systems.
Measuring Static Pressure
Richardson says, “When measuring static pressure, the unit of measurement used is inches of water column, which is often shown as an abbreviation such as “in. wc,” “in. wg” or “in. H2O.”
You should expect spots where static pressure is high and where it drops over the course of the ductwork, but it should be within acceptable limits, as properly determined with testing, in order to ensure all the rooms in your home receive adequate treated air without hot/cold spots.
Info Tip: Small holes are drilled in the ductwork, and the sensor of the manometer is inserted to measure static pressure while the fan is running.
Did you know? Each HVAC manufacturer calculates and publishes pressure drop ratings for the equipment they produce. This allows technicians measuring static pressure to know whether the pressure drop before and after a coil, for example, is within acceptable limits or if the coil is creating unduly high static pressure and needs to be cleaned or changed.
What Causes High Static Pressure in HVAC Systems
There are a few reasons for static pressure increases and drops. Some are expected, and some show the need for maintenance and/or repair.
Distance from the blower causes static pressure drops. That’s normal, as long as the drop isn’t greater than recommended by the equipment manufacturer.
The Air Filter: The thicker the air filter, and the dirtier it is, the more it will impede airflow. This causes higher static pressure.
Static pressure will build up as air cannot pass through the filter in adequate amounts. The pressure will be significantly less on the other side of the filter. This is a problem that will reduce the system’s energy efficiency.
Dirty Air Filter
That’s why it is imperative that you check your air filter often and change it when needed.
That is also why we recommend using a media filter with low pressure drop. This means that there is less of a drop from before the filter to after the filter. While a filter with a MERV of 12 or above will filter out more allergens and other pollutants, it will also slow down the air, producing a large pressure drop. This might cause your system to work too hard – and can definitely cause hot/cold spots in your home. David Richardson, writing in the RSES Journal, states that, “restrictive air filters have the potential to turn a 16-SEER system into a 10-SEER system.”
The Indoor Coil: If you have a heat pump or AC as part of the HVAC system, an evaporator coil will be located in or adjacent to the air handler in the line of airflow. The coil also slows airflow. If the blower is properly sized, this isn’t a problem – unless the coil has a clog or two from excess dirt or is generally quite dirty. So, annual coil cleaning is recommended to prevent undue air restriction and static pressure increase.
Evaporator Coils Clean
Not only is efficiency diminished, “high static pressure is the leading cause of compressor failure,” according to one pro.
Bad Ductwork Design: Poor design includes ducts that are too small (see next), but also too many turns and too little ductwork.
Return Ducts are Too Small: When too much air has to pass through a duct that is too small, high static pressure is the result. This is a major problem! Find the return ducts in your home. Do you hear whistling or other airflow noises within them? They could be undersized. Ask a qualified and experienced HVAC technician to calculate the proper duct sizes for your equipment and then measure the ducts to see if they are the right size.
Overcoming the challenge of ducts that are too small ranges from fairly easy – installing additional return ducts – to difficult and costly like complete ductwork replacement. Again, talk to a technician that knows ductwork to get answers you can use to make an informed decision.
Did you know? Ducts that are too big can lead to low static pressure, which causes hot/cold spots too in rooms far from the air handler. There isn’t enough pressure to push air all the way to the distant air grates.
How Does Static Pressure Affect Airflow?
In new systems, these are signs of improperly sized equipment. If you’ve had your HVAC system for a while and these issues develop, it could be a sign of high static pressure that needs to be corrected by changing the filter, cleaning the coil and/or looking for obstructions in the ductwork. Do you have a zoned system? And is a zoning damper stuck closed? That’s the type of obstruction that will cause high static pressure.
Signs of High Static Pressure
Look out for these symptoms of the problem:
- Hot/cold spots developing when you didn’t have them before is a definite sign of high static pressure – a restriction of airflow – somewhere in the system.
- Higher energy bills – If your system’s energy efficiency drops, something is wrong, and the same issues (filter/coil/obstructions) should be considered.
- Whistling ductwork – Interesting idea, but a definite sign of high static pressure. The pressure is building up somewhere and being forced through seams or gaps in the ductwork, and a whistling noise, like when your vehicle’s window isn’t sealed closed, is the result.
- A failed compressor or blower motor – If the static pressure is too high, the HVAC equipment will work harder than it should, and mechanical failure will result. Don’t just replace the compressor (or entire condensing unit) or the blower motor – find out what caused the issue too. It could be high static pressure.
If you want to avoid mechanical failure and the cost of AC repair or furnace repair, then have your HVAC equipment serviced annually. Ask the tech to check static pressure at the appropriate spots along the plenum, air handler and ducts. And be sure the tech seals the holes!
Ask the technician to clean both the inside evaporator coil and the condensing coil in the outside unit. This will cut down the risk of mechanical damage and will boost the energy efficiency of the system.
And definitely keep an eye on the air filter, changing it often. An air filter is a lot cheaper than the cost of a new compressor or blower motor. Sure, many new smart thermostats have Dirty Filter notifications. But check the filter with your own eyes every month, even if the nest, ecobee or Honeywell doesn’t tell you to.
Consider a Variable Speed Blower Motor
There is an optimal static pressure in each system that will yield maximum energy efficiency. A single-stage blower can’t adjust airflow to hit that “perfect” static pressure. Neither can most multispeed blower motors. Only a quality variable speed blower motor can adjust in small increments to optimize static pressure. And that is why we recommend using a variable speed blower motor. Most air handlers with variable speed blowers have control boards that monitor static pressure. They adjust the motor speed in order to maintain the right airflow (measured in cubic feet per minute/CFM) necessary to deliver the static pressure that produces peak energy efficiency. As a bonus, optimizing static pressure also reduces the risk of damage to the blower or compressor.
Here are questions we hear from homeowners digging into ductwork issues.
How do I know if my ductwork is the wrong size?
Sizing ductwork properly is one of the most challenging jobs for trained technicians, so it might be hard for many homeowners to figure it out. Our best advice is to listen for whistling noises or contractions in the sheet metal that sound like they are collapsing because of suction. Of course, calling an experienced HVAC pro is a good idea too.
What problems does high static pressure cause?
They are detailed above, including poor energy efficiency, blower motor and/or compressor failure, loud ducts and hot/cold spots in your home.
Are static pressure problems covered by the warranty?
They are not covered by the equipment warranty unless you can show the manufacturer’s recommendation for duct sizing is wrong. In most cases, the liability belongs to the HVAC company that sized and installed the ducts. Don’t work with a company that won’t give you a warranty on its work of at least 12 months. If your energy costs are higher than you expect, or if you have any of the other issues we’ve discussed, call another HVAC contractor to inspect your system and the ductwork.
What is the best air filter for an HVAC system?
We recommend using a media filter with low pressure drop. Check your owner’s manual or talk with the installer about choosing an air filter. Yes, air quality can improve with a high-MERV filter, but it might cause static pressure problems for your HVAC system.
What is ideal static pressure?
The optimal reading is .81 on the manometer, or .81” w.c. (water column).
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