When it comes to budgets, our household spending is something we all can relate to. Just before the next check arrives there are several feelings we can experience. If you've spent less than what you've made, all is well. If you've spent more than what you've made you may feel the immediate pain of an overdraft charge, or the long term suffering of painful and expensive growing credit card debt.

Fans have budgets too. If the system total external static pressure operates at less than the maximum rated pressure set by the manufacturer, all is well. If static pressure exceeds the "budget" or specified pressure established by the manufacturer, things are bound to get ugly quick.

Fan Budgets
Each manufacturer publishes engineering data depicting the airflow performance of each fan under certain conditions. Most of them list this data in what they call Fan Performance Tables. Some manufacturers include these tables in their installation instructions and list the maximum rated static pressure on the equipment's nameplate data. However a few only list this essential data in their engineering material used to select equipment. In other words, they make you hunt for it.

Typically, the maximum static pressure indicates that the fan will deliver 400 CFM per ton of cooling with the fan set in high speed mode. Although, a variety of other fan speed selection options may be found across the industry.

When the nameplate data states a maximum static pressure rating of .50-in., this is another way of saying the manufacturer allows you to add up to .50-in. of pressure drop to their equipment and the equipment will operate as it is supposed to. Add together the pressure drops of the coil, filters, and the air distribution system and if the total of these pressures is less than .50-in. system airflow will be satisfactory.

When the total system pressure exceeds the rated fan pressure capacity, the system suffers because it's forced to operate outside of the manufacturer's published specifications.

In cooling mode, the direct result is poor heat transfer, and sluggish refrigerant operation. This is the number one cause of premature compressor failure (I still wonder why our manufacturers continue to pay us to replace compressors that we destroy by our poorly performing installations). Until the compressor gives up, your customers are stuck with limited comfort, excessive utility expense, and a growing distrust for your company.

Heating systems with excessive total external static pressure — the heat exchangers life is shortened and the BTU delivery of the system decreases significantly as well.

If installed system total external static pressure is at or below the rated static pressure, the fan will have the capacity to move the required airflow. This enables the service tech to charge the system properly and adjust the combustion efficiency to the max, allowing the best possible system performance, increased efficiency, and maximum comfort.

The bottom line is that if a fan pressure budget is met, adequate airflow is moving through the equipment per manufacturer's specification and the system can perform at its best. Is there any chance the manufacturer tests and rates their equipment in the factory at anything less than maximum airflow specs? No way. In order to maximize our performance in the field, the same airflow is required in the laboratory.

System Design
Static pressure is a pretty simple concept when you look at it from this perspective. ACCA Manual D includes a worksheet you can use to calculate the pressure drop of each component of the system and then size your ducting accordingly.

The coil, filter, and each duct system component is deducted from the rated total external static pressure. The outcome is available static pressure that can be spent on the duct system. Duct sizing is then determined by the remaining static pressure.

Component Pressure Budgets
In one form or another, each manufacture is required to publish engineering data depicting the pressure drop associated with their component under varying airflow conditions. This enables the designer to include the pressure drop of the component in the design and hopefully deliver working system that will move the required airflow and operate at its maximum potential.

Unfortunately, due to the quest for "high efficiency" components, the pressure drop of nearly every accessory we can add to a duct system is increasing at an alarming rate.

The louder each component manufacturer screams "high efficiency" typically, the higher the pressure drop becomes. We're nearing a point that a system containing all "high efficiency" components may barely exceed a 50% system installed efficiency in the field.

The 100% Rule
At National Comfort Institute, we have reviewed thousands of air balance reports from high performing systems in the field and our research has uncovered some interesting static pressure patterns.

As we model static pressure profiles from systems that exceed an installed system efficiency rating of 90%, we have discovered notable facts than can be used in designing systems for maximum performance.

To express our findings in terms each of us can easily understand, we view the air handler rated total external static pressure as 100%. This means that if pressure exceeds the budget, or is over 100% of the rated pressure, airflow and system performance will decrease. System performance decreases slowly at first, but then takes a serious dive once total external static pressure exceeds 110%.

If the system total external static pressure is at or below 100% and the fan speed has been adjusted to deliver system required airflow, performance can reach its factory rated potential. Get the picture?

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 robf@ncihvac.com or call him at 800/633-7058. Go to NCI's website at www.nationalcomfortinstitute.com for free information, technical articles and downloads.