HVAC system-Choosing The Best Blower

If you have no idea what you’re doing with your HVAC system, you won’t know what you need when it comes time to pick one out for your home. Whether your home is new or old, it may need a new blower on the system due to its inability to distribute the proper amount of air to where in the home it is required. In addition, if your blower isn’t powerful enough or can’t keep up with the house, you may notice some discomforts.

One of the most common complaints about faulty HVAC systems is that the homeowner is uncomfortable. It is due in part to a lack of airflow throughout the house, delivering only a part of what it’s supposed to. It leads a person to start looking for a new blower if that’s the cause of the issue.

It can be challenging to begin knowing where to start, but that’s okay. First, you should know that there are two blowers types: the Operating-Point and Operating-Range. The differences are pretty obvious once you see how the airflow works. Therefore, it’s ideal to understand the difference between the two when it comes time to decide which one to choose.

Two Types of Blowers

An excellent HVAC system starts with picking a good piece of equipment. Then, the selected system can push the level of air the building needs. The blower, along with its abilities, is extremely important to achieving the success of an HVAC system, as our duct sizing is based mainly on the blower’s features within the furnace and fan coil unit.

There are two general types of blowers that you will see:

  1. Operating-Range (OR)
  2. Operating-Point  (OP)

The main difference between both becomes clear when thinking about their airflow tables.

NOTE: Quite often, an Operating-Range blower is known as a “variable speed” blower, or occasionally an “ECM blower.” Bear in mind. However, we now have Operating-Point blowers that are ECM motors. It is excellent, as ECM motors tend to be more potent than their PSC counterparts. But, keep in mind that all ECM blowers are NOT always variable speed Operating-Range HVAC blowers.

The OP blower may decline in airflow as static pressure rises. However, the OR blower will keep the same (or close to the same) airflow over a variety of static pressure. It achieves this by altering its rotating speed to “match” the resistance it must work.

The static pressure shouldn’t change much after any system is installed. (As the filtration system gets dirty, its static drop will grow. How much increase will vary depending on how much air you need to undergo the filter? For this reason, it’s wise to oversize filters.) So, don’t discover the wrong concept from my statement before, “as static pressure raises.” Part of the HVAC design process requires choosing the static pressure based on the design. We choose the destination point, or “design static,” and then design the system so that we emerge on target.

You’ll observe many static pressure options if you have an OR blower. It’s also essential to learn that an operating-range blower must consume much more energy to rotate faster to overcome the extra static pressure. There’s an investment, so to speak, for reaching the necessary airflow at a real static pressure.

Flexibility from Blowers-HVAC system

If you use an operating-point (OP) blower, the budget is etched in stone. You have a single static pressure alternative, per blower speed setting, that matches one airflow variety. But, on the other hand, if you overspend your OP fixed budget, the airflow, efficiency, capacity, and durability will reduce. So, the operating-point (OP) blower ASP selection process is influenced by the amount of airflow you should have, together with the blower speed you decide to design.

Operating-Point Blower Table

HVAC system

This chart displays the blower information for a 1.5-ton fan-coil unit. See that there can be Five different speed controls: 1-5. We’d only have two operating-point options if we aimed to have 600cfm moving throughout the system. The first option is to get the HVAC blower speed to Med High #4 and design the air duct system to produce a TESP (total external static pressure) of roughly 0.65 IWC. While allowing our blower to transfer the 600 cfm, we must (interpolate between 0.60 and 0.70 columns). Another option is to get the blower to a High #5-speed setting, setting up a TESP that’s off this chart – probably anywhere in the 1+ range. As the data isn’t listed, this might be a risky option to take. So, we’d be left with a single choice. Our accessible static might be 0.65 IWC.

The ASP process is slightly different if you use an operating-range (OR) blower. You probably get to select it from the number of static pressures on the blower’s airflow data. You’ll note that there’s a restriction to the OR blower’s capability to fight against higher static pressure. Sooner or later, generally around 0.75 IWC (inches of water line), the airflow will start to disappear, often below your preferred airflow.

Manual-D recommends that you steer clear of the “top-third” of the blower’s ASP range listed by the manufacturer. Instead, I suggest picking the lowest ASP, which you can do, typically between 0.50-0.75 IWC. Then, once you lay out the ducts, components, and fittings, you can adjust your ASP till your FR is inside the acceptable range.

Sometimes, you’ll discover that you might need to use a larger piece of equipment to find a blower that’s sufficiently strong enough to meet your requirements. I often encounter this when using all furnaces (40 kb) with cooling loads (2.5 tons, for example). Getting a 40 kb furnace that can move 1000 cfm at a high static is challenging.

Operating-Range Blower Table-HVAC system

HVAC system

This chart demonstrates the blower details for any 120,000 BTU furnace. This unit features a variable speed motor operating in an operating-range layout. For every single combination of ON/OFF switching shown, the blower will give a different measure of airflow depending on the TESP listed on top of the chart.

When we had a 3-ton air conditioning unit attached to this system, we’d like to deliver about 1200 cfm of air throughout the system. Therefore, we’d select the ON/ OFF option, with the SW4-3 option shown inside the footnote. It will enable us to offer an airflow of just 1200 cm. When selecting the ASP for the Friction Rate calculation, we’d be capable of choosing one of the static pressures displayed if the relevant airflow would fulfill your system’s needs, as the blower will alter its speed. One thing you should bear in mind is that high static = high energy consumption, as the blower is working harder. It can cut the blower’s lifetime. To begin with, I advise 0.50 IWC, altering up as needed until your Friction Rate is inside the ideal range.

Many manufacturers offer a couple of models for the same capacity (when handling furnaces), with much stronger HVAC blowers. It gives them to couple with larger-sized air conditioning units, which demand larger airflows.

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