Using the Lab ROI Tool to Calculate VAV Exhaust Fan Control Savings


Aircuity’s Exhaust Fan Control approach operates the lab’s exhaust fans in a variable volume mode when the exhaust stream is “clean” but automatically switches them to a high velocity, constant volume bypass control when the exhaust steam has a high level of contaminants. This can save a lot of energy and operating expense, but how much? This short article explains how to use the Lab ROI tool to calculate the exhaust control savings in two quick steps or runs of the tool.

Background on Exhaust Fan Energy Use & Controls:

The savings that is generated by Aircuity’s VAV Exhaust Fan Control approach comes from typically operating the exhaust fan at a lower speed compared to the traditional approach of either constant volume or staged fan control. IN constant volume control the fans are just run at a fixed speed independent of the variation in the building exhaust volume. In the staged fan control approach a multiple fan setup is being used and the individual fans are operated either at full flow or are shutoff in a multi-step or staged arrangement. For example, with four fans exhausting from a plenum, the fans are either all run at full flow or if the flow reduces enough, then 3 fans are run or if even less flow is coming out of the building then 2 fans are run, etc. In both the staged and the constant volume control approach a bypass damper is used to draw roof air into the exhaust plenum to compensate for the variation in exhaust volume from the building and to provide a means of controlling the exhaust static pressure in the exhaust ductwork.

It is probably clear why a constant fan control would use more energy than a variable fan control using a variable frequency drive, however why is a staged fancontrol inferior in energy use to a variable volume control, even for the same total flow going through the exhaust fans? The answer lies in the fact that the power use of the fan is not linear to it’s flow. Technically a fan’s power is related to the cube of its flow, however this is only for a fan with a fixed set of orifices or damper openings. In a lab exhaust system like a typical supply fan system connected to a VAV system, the control dampers are varying and the control system is trying to maintain a constant static pressure at least at some point in the ductwork. This means the fan’s load curve is not constant and energy use becomes related conservatively to more like a square law relationship than a cube relationship. In reality there are some reasonable estimates of this power to flow relationship for static controlled fans that use equations having a combination of both linear and cubic terms. (An equation like this is used in the ROI tool for calculating energy savings from variable fan operation.)

Using for example a simple square law power relationship, we can compare 2 of 4 fans running in a staged control compared to all four fans running at half speed in a VAV exhaust fan control setup.  With the same flow, the VAV mode uses half the power of the staged approach! In other words, if a fan’s full power is 1 KW and two of four are running, then we have 2KW of power consumption. However, if all four fans are running with VAV control at ½ flow, the total flow is the still same but each fan is running at (½)2 or 0.25KW. Since we have four fans running the total flow is 1 KW or half of the staged fans approach.

One last point to keep in mind with saving energy with exhaust fan control is that energy will only be saved if the building flow is operating at least some of the time at a reduced level. If all the exhaust fans are running at full flow all the time, because the building flow is varying only a small amount or not at all, then no energy can be saved by a VAV or even a staged fan control approach.

Calculating the Energy Savings of Different Exhaust Fan Control Approaches:

The Aircuity Lab ROI Tool calculates the exhaust fan energy use for either of a constant volume, staged, or VAV (Aircuity Exhaust Fan Control) approach. Typically, the ROI Tool is used to calculate the total energy savings of using a demand based lab control approach compared to a conventional fixed minimum ACh approach. This is done in a single step or run or the tool. After filling in certain assumptions including the exhaust fan control approach, the tool directly indicates the savings of demand based lab control approach.

However, the tool can also be used to calculate the saving in energy between two fan control approach such as VAV compared to constant flow control or even to staged control. To make this calculation we need to do this in two steps or two runs of the model. Basically, we need to first run the model with one exhaust fan control approach, and then run it again a second time with the other fan control approach. By looking at the difference between the exhaust fan energy use between these two runs we can simply calculate the savings in energy by using a VAV exhaust fan control approach. How to do all this is shown below:


The Method For Determining VAV Exhaust Fan Control Savings:

1.Open up the Lab ROI Tool and fill in the assumptions appropriate to your situation. The tool can be used to determine the exhaust fan control savings in either the Base or the Proposed Design case. For this example, we assume the interest is in the savings in the Proposed Design case.

2. In the first step or run of the ROI tool model, set the Exhaust Fan Control Strategy assumption to the approach you want to compare the VAV approach to, either the Constant or Staged Fan approach. This assumption is set on the “Assumptions” Tab, Cell G51:

Open the drop down box and select either “CV w/ Bypass Damper” or “Staged Fans w/ Bypass Damper”

3. Next, open up the Metrics tab of the Lab ROI Tool:

4. Assuming we are looking for the savings in the Proposed design using demand based control, record the value listed for the Exhaust Fan energy costs in cell O41 or in this example, $29,318:Assuming we are looking for the savings in the Proposed design using demand based control, record the value listed for the Exhaust Fan energy costs in cell O41 or in this example, $29,318:

This is the exhaust fan energy use when using a staged fan control approach when also using demand based control. (If we wanted to calculate the savings for the base case we would instead record the base case exhaust fan energy shown in cell F41 in the Teal color section.)

5. Next go back to the assumptions tab and set the Exhaust Fan Control Strategy assumption to the VAV approach which is listed as “VFD (Variable Velocity)”:

6. Now that the ROI tool has calculated the new energy use for the VAV mode go back to the Metrics tab and record the new value for the exhaust fan energy use of the Proposed case similar to before in Cell O41. In this case the value is $14,318:

7. The final step in the method is to simply subtract the exhaust fan energy use for the VAV approach from the value previously recorded for the staged approach. For this example, we subtract $14,318 (VAV) from $29,318 (staged) for an annual VAV exhaust fan control approach savings of exactly $15,000.


That is all there is to calculating the savings! Since this result does not print out as part of the standard report you may want to create your own page or a separate report to note the two annual energy use values as well as the net saved energy. You might also want to state the simple payback of the control approach based on the incremental cost of the VAV exhaust fan control system divided by the annual control approach energy savings.