One of the most common downfalls of installed HVAC systems is their inability to distribute the correct amount of air to where it’s needed most. When systems are restrictive, or blowers aren’t powerful enough, the air simply doesn’t make it to where it needs to go. This issue commonly manifests in the form of comfort complaints.  In addition, most systems suffer from low air flow, only delivering a fraction of what they should. This can also mean that the capacity of a system is much less than what it should be. If it’s only moving 75% of the air, it can only deliver, at best, 75% of the rated capacity. This means that it will run longer in order to satisfy the load, costing more in operating costs. A good HVAC system begins with the selection of a good piece of equipment. The unit selected must have the ability to push the amount of air that your building needs. The blower, and its abilities, is of the utmost importance to the success of an HVAC system, as our ductwork sizing is largely dependent upon the specifications of the blower found within the furnace or fan coil unit.

There are two general types of blowers that you’ll see: The difference between the two becomes obvious when looking at their airflow tables (see below). The OP blower will have a decrease in airflow as static pressure increases. The OR blower, however, will maintain the same (or close to the same) airflow over a range of static pressure. It achieves this by adjusting its rotating speed to “match” the resistance it has to work against. The static pressure shouldn’t actually change that much after your system is installed. (As filters get dirty, their static drop will increase. The amount of increase will vary depending on how much air you’re trying to move through the filter. This is why it’s a good idea to oversize filters.) So, don’t get the wrong idea from my statement above, “as static pressure increases.” Part of the HVAC design process involves the selection of the static pressure at which the design will be based. The idea here is that we select the destination point, or “design static,” and then design our system so we arrive on target.

That said, you’ll notice that there are more static pressure options from which to choose if you’re using an OR blower. It’s also important to understand that an operating-range blower has to consume more energy to rotate faster in order to overcome the additional static pressure. There is a price to pay, so to speak, for achieving your desired airflow at a high static pressure. When using an operating-point (OP) blower, your budget is etched in stone. You have one static pressure option, per blower speed setting, that corresponds to one airflow quantity. When you overspend your OP static budget, your airflow decreases, along with capacity, efficiency, and durability. So, the OP blower ASP selection process is dictated by the quantity of airflow you need, along with the blower speed you choose to design with. This chart shows the blower details for a 1.5-ton fan-coil unit. Notice that there are 5 different speed settings: 1-5. If our goal was to have 600cfm moving through the system, we would have only two operating-point options.

The first option would be to set the blower speed to Med High #4, and design the duct system to create a total external static pressure (TESP) of approximately 0.65 IWC. concord ac unit warrantyThis would allow our blower to move the 600 cfm we need (interpolate between the 0.60 and 0.70 columns). frigidaire energy star ac unitThe other option would be to set the blower on the High #5 speed setting, while creating a TESP that is off this chart – probably somewhere in the 1+ range. carrier hvac parts edmontonSince the data isn’t listed, this would be a risky route to take. So, we’d be left with only one option. Our available static would be 0.65 IWC. When using an operating-range blower, the ASP selection process is a little different.

You actually get to select it from the range of static pressures listed on the blower’s airflow chart. You’ll see that there’s a limit to the OR blower’s ability to fight against high static pressure. At some point, typically around 0.75 IWC (inches of water column), the airflow will begin to drop off, which may be below your desired airflow. Manual-D recommends that you stay away from the “top-third” of a blower’s ASP range listed by the manufacturer. My real world recommendation is to choose the lowest ASP that you possibly can, typically between 0.50-0.75 IWC. After you lay out the ducts, fittings, and components, you can adjust your ASP until your FR is within the acceptable range. Sometimes, you’ll find that you may need to use a larger piece of machinery in order to find a blower that is strong enough for your needs. I frequently run into this when trying to use small furnaces (40kbtu) that have large cooling loads (2.5 tons, for instance). It is difficult to find a 40kbtu furnace that can move 1000 cfm at a reasonably high static.

This chart shows the blower details for a 120,000btu furnace. This unit has a variable speed motor that operates in an operating-range configuration. For each combination of ON/OFF switching displayed, the blower will deliver a different amount of airflow based on the TESP listed at the top of the chart. If we had a 3-ton air conditioner attached to this system, we would want to deliver about 1200cfm of air through the system. We would select the ON, OFF, OFF option, combined with the SW4-3 option shown in the footnote. This would enable us to deliver an airflow quantity just shy of 1200cfm. When choosing the ASP for our Friction Rate calculation, we’d be able to choose any of the static pressures shown, as long as the associated airflow would meet your system’s needs, since the blower will adjust its speed accordingly. The thing you must keep in mind is that high static = high energy use, because the blower is working harder. This may also decrease the blower’s life expectancy.