Cooperative Extension Service
Purdue University
West Lafayette, IN 47907

Matching Multiple Ventilation Fans

A. J. Heber,
Department of Agricultural and Biological Engineering

Fans are often used to ventilate agricultural buildings. Adjoining rooms with connecting air paths through open doors or manure channels can operate as a single room with a common negative pressure. The pressure capability or strength of multiple ventilation fans must be carefully matched to allow all fans to operate efficiently. Under ventilation, back drafting, and poor air distribution may result when a fan exhausts air from a room in parallel with a stronger fan, especially in tightly constructed buildings. The fan mismatch problem is not immediately obvious to the casual observer, but may result in depressed animal health and performance in livestock buildings, reduced human comfort in residences, contaminant exposure of products in food plants, and lower air quality in all buildings. But there are ways to check whether the problem exists in ventilated buildings. The following cases illustrate a few common problems and solutions.

Variable-Speed Fans in Livestock Buildings

It is not uncommon to see a variable-speed fan in the second ventilation stage while the first-stage fan is running at full speed. This scenario should be avoided. One fan controller that implements the correct strategy will be described later.

To visualize the consequences of such a mismatch, imagine two straws in an empty soda cup with a well sealed lid. Now, imagine a man and small child attempting to suck air from the cup at the same time. The child's sucking power is no match for the adult's. Neither is a variable-speed fan running at low speed a match to one running at full speed.

Here is what happens when variable-speed fans are mismatched in a livestock building:

Some solutions to these problems include:

1.Operate a variable-speed fan with another fan that is identical and running at the same speed. After the first-stage, variable-speed fan is at full speed and the room needs more air, decrease its speed to 50% airflow and increase the speed of the second-stage variable-speed fan starting it also from 50% airflow. Both fans then always operate at about the same speed. At least one electronic controller on the market is capable of carrying out this strategy (Heber, 1991).

2.Use a variable-speed fan as a winter, minimum ventilation fan only, unless the above solution can be implemented.

3.If a variable-speed fan must operate with a full-speed fan, raise its minimum speed setting to at least 50% to reduce back drafting and performance degradation.

4.Avoid using variable-speed fans in adjoining rooms that have unavoidable air paths between the rooms. Single-speed fans are more likely to match the pressure created by other single-speed fans.

Multiple Single-Speed Fans in Livestock Buildings

The words "weak" and "strong" in this bulletin refer to pressure capability, not airflow. A fan that has three, low-angle blades on a small hub would probably be "weak" compared to an impeller with six, high-angle blades on a large hub. A high-speed fan rotating at 3200 rpm would likely be "stronger" than a fan rotating at 1600 rpm. Likewise, a two-speed fan is weaker at the lower speed.

Staged ventilation systems in livestock buildings frequently operate a fixed-speed, minimum winter fan that is weak relative to other fans, especially large summer ventilation fans.

Some consequences of this type of mismatch include:

Here are some solutions to the above problems:

1. Turn weaker, first-stage fans off while operating higher pressure fans. The higher pressure fans then need to be larger to compensate for the capacity intended from the weaker fan.

2. Seal air paths between adjoining rooms so each room operates at its own negative static pressure.

3. Install fans that have the same design pressure. All fans should operate at about the same speed. One might have one or more single speed fans in the wall that operate at about 3300 rpm and other fans that operate at about 1650 rpm. This should always be avoided because it is likely the fans are mismatched unless the high-speed fan is connected to pit ducts.

Figure 1. Exhaust fans and air inlets in a mechanically ventilated building.

When adjoining rooms have connecting air paths through open doors or manure channels, they operate as a single room with a common negative pressure. If that is the case, then fan matching between rooms becomes an issue.

An analogous situation occurs in a residence with a small fan in the bathroom and a large, whole-house ventilation fan in another room or a hallway. When both fans operate, the small bathroom fan ends up operating at reduced efficiency and reduced or negative airflow. As a result, odors and moisture exhaust more slowly or enter other rooms in the house. Implementing solution #2 in the list above would be to shut the bathroom door (unless the door is the only air inlet to the bathroom).

Food Plant Ventilation Systems

A food manufacturing plant may have several types and sizes of fans exhausting from the same room or adjoining rooms. High pressure power ventilators, tube-axial fans, centrifugal fans, room heaters, and vacuum systems sometimes operate in parallel with low pressure propeller fans.

Fan mismatches like this may result in the following problems in food plants:

The fan mismatches can be eliminated by:

Figure 2. Top view of three exhaust ventilation fans operating in parallel.

Technical Explanation of Fan Mismatch Problems

Static Pressure and Air Flow of a Ventilation Fan

Static pressure developed by a ventilation fan depends on the resistance to airflow offered by the ventilation inlets. The resistance as indicated by pressure drop through the ventilation inlets increases with airflow rate and is represented by the system curve (Figure 3). Notice that doubling the airflow generally increases the pressure drop by four times at any given inlet opening. Increasing the inlet area, e.g., opening inlet baffles, moves the system curve downward to lower pressures.

Figure 3. Characteristic curve and ventilation efficiency ratio (VER) of a 16-in. variable speed fan operating at several control voltages. The lower system curve is created by increasing air inlet openings. (iwg=inches of water gauge pressure and cfm/W is cfm of air flow per watt of electrical power). Source of data: Kansas State University (Heber et al., 1989).

At a given voltage, a fan has its own characteristic curve that gives the airflow it produces over a range of static pressure. A fan always operates somewhere on its characteristic curve. Voltage adjustments by the fan controller create new characteristic curves. The relationship between pressure and airflow is rarely a straight line and the curve sometimes has a very distinct stalling region, especially with propeller fans, Figures 3, 4 and 5. Stalling is described later.

The free air point, where there is no pressure developed by the fan, is located at the low end of the pressure range. A fan develops its maximum air flow at free air (Figure 3). Free air is experienced by circulation fans and by exhaust fans when all wall curtains are wide open.

The cutoff point, where a fan may be rotating but not moving out any air, is at the upper end of the pressure range (Figure 3). Cutoff occurs when a fan exhausts from a space that has no air inlets or when a strong wind induces just enough pressure on the fan blades to cause zero air flow through it. A fan normally operates somewhere between the free air and cutoff pressures and preferably at the point of highest impeller efficiency.

Impeller efficiency is highest at pressures between free air and stalling. More interesting, however, is the fan's ventilation efficiency ratio (VER) which is air delivered per unit of electric power. VER curves are shown in Figure 3 for a 16-in. fan at five control voltages. VER is highest at free air and drops to zero as pressure goes up to cutoff. Therefore, excessive fan pressure means decreased efficiency, wasted electric energy, and insufficient ventilation.

Figure 4. Characteristic curve of a 24-in. variable speed fan operating at several control voltages. Source of data: Kansas State University (Heber et al., 1989).

The operating point of a fan is dictated by the intersection of the system curve and the fan characteristic curve. Some operating points are shown by large dots in Figures 3 to 8. For example, the 94-in variable speed fan delivers about 4100 cfm at full speed (at 115 V) at operating point A in Figure 4.

Fan stalling does not mean that the fan quits turning. Rather, stalling occurs when the fan suddenly experiences a greater drop in air flow with additional pressure. As Figure 3 shows, a small pressure increase, (0.26 to 0.36 in. of water gauge (iwg), for example) can move the fan from operating near its rated pressure and airflow to the stalling region causing airflow and VER to drop by up to 50%.

Figure 5. Characteristic curve of a 12-in. variable speed fan operating at several control voltages. Source of data: Kansas State University (Heber et al., 1989).

Further pressure increases (0.36 to 0.60 iwg) because of wind, insufficient inlets, or fan competition will reduce airflow and efficiency further. It is important to point out that fan characteristics vary a lot from one model to another. Stalling also varies in severity, It depends on blade and housing design and the effect of guards, shutters, and other obstructions to airflow.

Airflow and efficiency reductions often go unnoticed in buildings because fan speed is about the same and the fans appear to be operating normally, The following paragraphs explain how competition from mismatched fans causes a fan to depart from its design point and how such departures can be avoided.

How Multiple Fans Move Air

When more than one fan exhausts air from a room (Figures 1 and 2), the fans are said to operate in parallel. Because they are operating in parallel from a relatively large space, all fans must operate at the same static pressure. A combined characteristic curve for multiple fans is determined bv adding the airfiows at each pressure. For example, the characteristic curves of two 16-in. fans, one operating at 55 V (or 45 V) and the other operating at 115 V, are combined in Figure 6. Each fan operates at a common pressure as dictated by the operating point on the combined curve. All the fans should ideally have the same design pressure and the air inlet system should allow the fans to operate at that design pressure. Pressure mismatches become a problem when the design pressures of multiple fans are significantly different.

Parallel fan mismatches are not serious at pressures at or near free air. However, the design pressure of ventilated agricultural buildings is generally about 0.10 to 0.125 iwg and manufacturers typically design them so the highest propeller efficiency occurs at a pressure of 0.125 iwg. Actual design pressure may differ from 0.125 iwg depending on the quality of the final product.

At lower voltages, variable-speed fans create new characteristic curves that have lower design and cutoff pressures (Figures 3, 4 and 5). Thus, a variable-speed fan operating at its lowest speed has much lower design and cutoff pressures than an identical fan operating at full speed. For example, the 16-in. fan in Figure 3 has a cutoff pressure of 0.9 iwg at 115 V but less than 0.1 iwg at 45 V or low speed. That means the 16-in. fan at low speed cannot move any airflow if it is operating against an external pressure greater than 0.10 iwg.

Figure 6. Combined characteristic curves of two identical fans, one operating at low voltage and one operating at full voltage.

Common Fan Mismatches

The following cases illustrate some common fan mismatches that can occur in livestock buildings. The three commercial fans used in the examples were designed for livestock buildings and were thoroughly tested in the laboratory. Other fans will have different characteristics but the general concepts illustrated still apply.

Case 1. Two Identical Fans, One Operating at Low Speed and One Operating at Full Speed

Figure 6 shows two identical 16-in fans, one fan running at low speed and one at full speed. This case is very common, especially with the advent of integrated controllers that are able to stage more than one variable-speed fan. With the slow fan operating at 45 V, the system curve intersects the combined curve (Point B) at about 0.09 iwg and 3,000 cfm . Since the cutoff pressure of the slow fan at 45 V is only 0.07 iwg and less than the operating pressure of 0.09 iwg, it cannot exhaust any air. In fact, air will flow in reverse through the slow fan so that it becomes an inlet for the full speed fan!

Of course, reverse flow will be blocked by back draft shutters. However, back draft shutters do not always seal perfectly thus allowing some air to flow back through the fan. All this can happen while the fan impeller rotates in the positive direction! Operating by itself, the slow fan moves 1,050 cfm (point D). Turning the full-speed fan on again causes reversal of flow through the slow fan (see dashed line from point B).

Since the building will be under-ventilated with the slow fan at only 45 V indoor temperature will gradually rise causing fan control voltage to increase. When it reaches 55 V we get the new operating point A, which is now at a pressure less than the cutoff pressure of the slow fan (Figure 6). The slow fan now operates at point E on its individual characteristic curve. Since the fan is stalled at point E, the slow fan's efficiency and airflow are seriously compromised.

When the high-speed fan is turned off, the operating point for the slow fan at 55 V changes from point F (0.125 iwg and 600 cfm) to point C (0.05 iwg and 1,650 cfm), as shown in Figure 6. Therefore, operating the full-speed fan causes the flow rate of the slow fan to drop by 64%. Checking Figure 3, the VER drops from 8.5 to 3.0 cfm/W. Blowing 600 cfm at 3.0 cfm/W as compared to 8.5 cfm/W costs $9.53/month at 10 cents/kWh.

Also, the slow fan becomes extremely vulnerable to further airflow reductions, complete shutdown, and even speed reversal from wind induced pressure. At 0.125 iwg, the 0.04 iwg of additional static pressure needed to completely shut down its airflow can be created by a head-on 9 mph wind.

Electronic controllers introduced to the livestock industry in recent years are often used to stage more than one variable-speed fan. After the first stage fan is brought to full speed, rising temperatures bring a second fan on at low speed, thus creating an unhealthy pressure mismatch. The mismatches generally go unnoticed because:

The minimum control voltage of the second fan should at least be high enough to prevent back drafting that occurred with the slow fan at 45 V (Figure 6), and preferably high enough to prevent stalling.

Figure 7. Combined characteristic curve of a 24-in. fan operating at low voltage and a 16-in. fan operating at full voltage.

Case 2: Large Pan at Low Speed and Small Pan at Full Speed

Ventilation systems in livestock facilities often use a small fan for the first stage of ventilation and a larger fan for the second stage. Sometimes, both fans are variable speed and the large fan is started at low speed while the smaller fan is running full speed.

Figure 7 shows the characteristic curves of a 24-in. fan operating at 40 and 50 V individually and in combination with a 16-in. fan operating at full speed. At 40 V the combined operating point is 0.1 iwg (Point B) and 3,000 cfm. Amazingly, the 24-in. fan is completely overpowered by the smaller 16-in. fan! This happens because the 0.1 iwg operating pressure for both fans is much larger than the cutoff pressure of 0.065 iwg for the 24-in. fan. At 50 V the combined operating point is 0.125 iwg and 3,250 cfm (Point A), but the 24-in. fan is only contributing 350 cfm (Point E) and very inefficiently. By itself, with the same system curve (inlets not readjusted), the 24-in. fan at 50 V would move 1,250 cfm or 3.5 times as much air.

Looking at the characteristic curves for the 12-in. and 24-in. fans shows that the 12-in. fan running at full speed can cause flow reversal through the 24-in. fan at its lowest speed (control voltage 40 V).

Therefore, avoid operating another fan at full speed, whatever its size, with a variable speed fan at low speeds.

Case 3: Large and Small Pans Operating Full Speed

Fan mismatch problems are also possible when all fans operate at full speed. Figure 8 shows how the performance of a 12-in. fan is hampered when operating in parallel with a 24-in. fan. The stalling pressure for this particular 12-in. fan is only 0.08 iwg (probably low compared to most 12-in. fans). A typical operating point for both fans running is 0.10 iwg and 4,600 cfm (Point A). The pressure developed by both fans together forces the 12-in. fan to operate at Point D on its characteristic curve, blowing only 375 cfm. Compare this to Point C where the fan operating by itself moves 850 cfm!

Figure 8. Individual and combined characteristic curves of 24-in. and 12-in. fans operating at full voltage.

Table 1

                    Total Flow VER (12 in.) VER(24 in.) Monthly electricity cost
                     (cfm)     (cfm/W)      (cfm/W)      ($.10/kWh) per 1000 cfm
  Both fans         4,600 cfm     9.3          10.3              $7.01
  24 in. fan alone  4,216 cfm    12.6          10.6              $6.79

One might rightly argue that the inlets would be adjusted to smaller openings thus moving the system curve upward. This might make the 12-in. fan operate at Point E (0.05 iwg and 750 cfm). Even so, the fan pressure mismatch still causes a 50% reduction in airflow produced by the 12-in. fan.

This problem can be corrected by choosing a small fan with higher pressure ratings. Another solution may be to shut the 12-in. fan off when operating the 24-in. fan. The results of this strategy are shown in Table 1.


Figure 9. Variable speed fan running at half speed by itself (a) and with another identical fan running toll speed (b).



Severe mismatching of parallel connected fans can be checked in the field. If, while the multiple fans are operating, the back draft shutters of a fan are shut or nearly shut, then there may be a problem. Watch the shutters while turning off the other fan or fans. If the shutters open up when turning the other fan or fans off, then the fan is "weak" compared to the other fans. For fans without shutters, use a vane anemometer or anything that would "blow in the wind" to visualize the exit air velocity from the fan.

This procedure was conducted in a hog producer's building. The shutters of this fan (Figure 9a) were wide open with the fan operating at about 50% speed or so. An identical fan about 20 feet away in the same room was turned on at full speed. The result? The shutters of this fan slammed shut (Figure 9b). Turning the full speed fan off caused the shutters to open up again.

The experiment was repeated with an outside door wide open. This time, the other fan had no effect on this fan. Why not? Because opening the door simulated a very loose building and a free air condition. Fan mismatches are not a problem at free air.

If a fan is operating at a pressure less than the stalling pressure, air will move outward, even near the hub. However, when a fan is stalled, air is sucked back into the fan near the hub. As pressure increases more, the circle of reverse flow grows. A strand of an ostrich feather attached to a dowel rod works well for checking air direction at the fan outlet. The size of the reverse flow region can be readily measured. If the size changes when turning off other fans, then the fan is suffering from a fan mismatch.


Ventilation control strategies in mechanically ventilated buildings can be improved by avoiding fan mismatches. Mismatches occur when the static pressure capabilities of parallel connected fans are not compatible. Sometimes fixed speed fans can be mismatched depending on their respective design pressures. The most serious mismatch occurs when variable speed fans are operated at low speeds when other fans are operated at high speeds. This bulletin described several cases of fan mismatches and explained how the problems can be avoided.


New 8/95

Cooperative Extension work in Agriculture and Home Economics, State of Indiana, Purdue University and U.S. Department of Agriculture cooperating: H.A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914. The Cooperative Extension Service of Purdue University is an equal opportunity/equal access institution.