Static Pressure Profiles
Keeping in mind that the general idea is to keep the system's static pressure at or below 100% of the air handler's rated total external static pressure, we found the following pressure budgets for each of the following system components in well-performing systems across the country. Keep in mind these are ideal residential pressure drops, and there is some wiggle room that we'll discuss in a moment.
Cooling Coils 40%
Ideally, cooling coils should not exceed 40% of furnace rated static pressure. Cooling coils are the most restrictive component we install in our systems. Remember these are remote cooling coils that we add to a furnace (heat pump coils are part of an air handler's internal pressure drop). It appears that cooling coil pressure drops are approaching an unhealthy level. Cooling coils are not optional in systems and in order to offer the higher efficiency equipment, you must use coils matching furnaces and condensing units with which they are rated.
An appropriate budget number to assure a system will have adequate airflow averages 40% of rated furnace static pressure. In other words, if a fan is rated at .50-in., that fan may only afford a cooling coil pressure drop of .20-in.. Unfortunately, few cooling coils are rated at pressure drops at or below .20-in. So in order to assure your system will work, a furnace with higher rated static pressure will be required.
For example a furnace rated at .70-in. can afford a cooling coil rated at .28-in. However, there are many cooling coils bearing a little star sticker rated at .50-in. of pressure drop. Where are the fans that can afford those cooling coils? Who's keeping an eye on system components and equipment that can really work in the field?
Air Filters 20%
Ideally air filter pressure drop should not exceed 20% of furnace rated static pressure. The current trend in air filter pressure drops is skyrocketing. Our quest to become IAQ experts can dump system performance in the tank if we don't keep an eye on the pressure drops of the filters we select for our systems.
Interpreting filter pressure drop from published engineering data is another issue. Most marketing data seems to be published showing the rated filter pressure drop at 150 to 300 ft. per minute velocity. Traditionally, our industry sizes filters at 300-400 feet per minute. So evaluate the engineering data carefully. Contact the manufacturer for help in sizing their filters to meet your static pressure budget. You may find some filters are incompatible with the fans you have selected
The best way to decrease filter pressure drop is to install an additional filter in the system so a portion of the air is directed through the additional filter. In other words, as an additional filter is added to the system, less pounds of air pass through each square foot of filter and the pressure drop will decrease significantly. As a matter of fact, doubling the surface area of a filter will decrease the pressure drop by more than half.
Duct Systems 40%
If 40% of the available static pressure is used up by the coil, and 20% is consumed by the filter, this leaves 40% of the available static pressure for the air distribution system. This includes the pressure drop of the plenums or trunk duct, the fittings and transitions, the takeoffs, dampers elbows, cans, boots, and grilles.
When applying the 40% static pressure rule for duct systems, we often find ducts needing to be sized a little differently than you Daddy did. The old industry rule of thumb of setting your duct calculator at .10-in. per 100 feet of duct friction rate usually doesn't cut it.
You'll also have to consider the type of ducting material that you select and install as well as the quality of the installation. A survey of contractors who measure the performance of their installed systems shows some awfully low friction rates are used on metal duct calculators. For flex duct, the average friction rate is .05-in. per 100 ft. of duct. Round pipe is often calculated at .06-in. and rectangular duct sizing is typically figured at a .70-in. friction rate.
Heat Pump Air Handlers
Heat pump air handlers have a distinct advantage when it comes to static pressure ratings. They do not normally require an external cooling coil. The heating and cooling coil comes with the air handler and is included in the equipment's internal pressure budget.
So when considering a high pressure drop filter, if it fits you application, a heat pump can afford up to 50% of its static pressure rating to be spent on the filter.
An Exception to Every Rule
Just like at home, budgets can be adjusted. Perhaps one month an insurance premium comes due, and you may have no insurance costs for the next six months. It's similar with your static pressure budgets. One system may need 30% of its static pressure for the filter, 30% for the coil and 40% for the duct system. That's OK too.
The only hard and fast rule is the total pressure drop of all the components may not exceed the static pressure rating of the equipment as indicated in the manufacturer's engineering data.
As you begin to apply static pressure budgets you will soon discover a significant portion of the equipment made may not qualify for the systems you design and build. Manufacturers build equipment for applications that may not be adequate for a typical ducted system.
ARI only requires some equipment to operate at only .20-in. of total external static pressure. This equipment may have some "sweet" efficiency numbers but it may not operate efficiently in 90% of the usual field applications it may be installed under.
It's our job to select the right equipment for the systems we deign and install. Choose carefully before ordering the low priced equipment. More and more manufacturers are building equipment that have fans rated at to operate at the higher static pressures to match the higher pressure drops of their system components.
Ask your sales reps for help in selecting components that work well together. A study of matched equipment packages offered at one priced will uncover fans and components that will never work in the field. Don't assume they will until you personally study the fan capacities and add up the component pressure drops.
Remember that system design is only an expression of hope that the installed system performance will agree with the design criteria. Once a system has been built, only by testing and balancing a system can you be assured the system will work as designed. Airflow alone may also be inadequate, finish the testing by adding temperature measurement to prove and rate installed system performance
Measure installed system static pressure to check every system's performance, just like the doctor uses blood pressure on every patient.
Rob "Doc" Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in receiving a free procedure showing you how to measure total external static pressure, contact Doc at firstname.lastname@example.org or call him at 800/633-7058. Go to NCI's website at nationalcomfortinstitute.com for free information, technical articles and downloads.