Pressure Made Simple

Recent articles have generated many questions and comments related to HVAC system and building pressures. Here’s a look at some basic principles of pressure and some clarity to the way our industry reasons with it.

When you start pushing air around a building in order to deliver the right amount of BTUs to each room, it doesn’t take long to “see” that the air in one room is connected to the air in all the other rooms. When you throw air to one room, it squishes out into the hall, and moves back to the return where the fan pulls it back through to run another cycle through the building. As long as the fan keeps running, this cycle continues over and over.

Imagination Exercise
In order to understand this principle better, let’s imagine a small office (with no furniture or people in it, of course). Since both air and water are fluids, imagine the office is filled with water instead of air. Replace the fan with a pump, and the ducts with pipe. Got the picture?

Instead of air, water is being pumped into each room through the supply registers and out of the return grilles. Let’s go a little deeper now; the whole building is full of water, just as it is with air. As water is pumped into one room, the room is pressurized. Because high pressure moves toward low pressure, the water moves toward the return grille. But because water and air follow the path of least resistance, maybe it leaks out of a window, under a door or into another room. Is this making sense? It’s pushing and pulling everywhere. It fills everything.

Just like air, water is all connected. If the entire office is filled with water, and one room had more water pumped in than out of it, the room would have a positive pressure. The pressure of that room would then spill over into other rooms and begin to affect the pressure in the entire building.

Now imagine leaking pipes (ducts). If the supply or return pipes leaked different amounts, what effect would this have on the pressure in the office? If pipes leak, ceiling falls in, or buildings blow up. But when ducts leak or deliver unbalance volumes of air, the result is invisible and the problem is rarely given another thought.

So since we don’t see water flowing out of the windows or doors in a building when we arrive to service a system, we need to measure pressures as part of service and balancing a system to verify the fluid flows and the effectiveness of our HVAC systems. Not to mention the comfort and efficiency aspects of service and balancing.

Interpret Fan Performance with Pressure
Let’s go one step further and imagine a fan operating like a pump. When we measure pump pressure, we can plot its operating performance on the manufacturer’s pump performance data. Given the pump pressure, we can determine the GPM, or volume of fluid the pump is moving.

Likewise, knowing the total external static pressure a fan is operating under, we can plot the fan’s performance on the manufacturer’s data to determine the cfm, or volume of air the fan is moving.

The industry is becoming increasingly aware that airflow through a system is critical to the performance, comfort, and efficiency a system can deliver. Being able to identify fan performance during a service call would be a valuable addition to the information we collect.

When a tech begins to measure total external static pressure, the readings often reveal that the fan pressure exceeds the maximum rated capacity of the fan. Like a pump, when fan pressure exceeds its rated capacity, the volume of fluid it can move decreases. In installed residential HVAC systems around the country, fan static pressure exceeds the rated capacity of the fan more than 70% of the time in cooling mode. The effect is reduced airflow, resulting in reduced system performance and efficiency.

Pressure Drops
When total external static pressure exceeds the rated pressure of the fan, it’s our job to determine why the pressure is so high and to find a way to reduce the pressure.

Let’s look at a typical pressure profile and evaluate what the pressure readings are telling us. Let’s say our fan is rated at a maximum total external static pressure of .5-in. The system measures -.48-in. on the return side of the fan and.25-in. on the supply side of the fan for a total external static pressure reading of .73-in. Since the pressure exceeds the maximum rating of .50-in., the fan is failing to move the required airflow.

With the pressure being the highest on the suction or return side of the fan, in this case we can safely determine that there is an unacceptable restriction on the return side of this system. The next step in pressure diagnostics requires us to measure the pressure drop over the suspected components on the return side of the system.
Pressure drop is identified by measuring the pressure on either side of a system component, and then the two pressures are subtracted from each other to find the pressure drop of the component.

In this case, the pressure drop downstream of the filter measured -.42-in. and the pressure drop upstream of the filter measures -.12-in. The pressure drop over the filter is .30-in. A rule of thumb is that ideal filter pressure drop should not exceed 20% of the rated fan pressure. Since this fan is rated at .50-in., the ideal filter pressure drop over the filter should not exceed .10-in.

With this filter measuring .30-in., something’s has to give. Either the filter should be replaced with a less restrictive one, or additional filter surface area must be added to the system to reduce the pressure drop over this type of filter.

When you measure the pressure drop over any component or section of a system, the pressure drop can be divided into the rated total external static pressure to find out the percent of restriction that one component is of the entire system.
Example: Measured filter pressure drop of .30-in. divided into the rated total external static pressure of .50-in. equals 60%. This is three times the resistance an ideal filter should have.

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 a Extreme Weather Duct Diagnostic Test Procedure, contact Doc at [email protected] or call him at 800/633-7058. Go to NCI’s website at for free information, technical articles and downloads.

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