Design Review – Same Frame Size, Different Fan Sizes – Part 2

In my previous post, I looked at how for a given frame size in a modular rooftop air handling unit product line, there were several fan options available and that these different fan options had efficiency and first cost implications.  In this post, we will see how these factors translate into operating cost when you take a look at them from a life cycle cost perspective.

To understand the life cycle cost implications of the proposed fan selection versus the other options, I needed two pieces of information; the annual operating cost for each fan type and the cost differential associated with changing the fan selection to one of the other options.

Since the systems in question were constant volume and since I had a general idea of the number of hours per year the fans operated, it was not too difficult to estimate the annual operating cost for each option. I simply converted the brake horsepower to kW, divided it by the motor efficiency, and then multiplied it by the hours of operation for the ball room and the electric rate. The table below summarizes the results.


Since I didn’t have specific motor nameplate data, I picked my motor efficiency from a free DOE software package called MotorMaster. MotorMaster is an database created by a consotium of motor manufacturers that has fairly currnet product data, both performance and price. The package allows you not only to see what is available but also to compare different options on a first and life cycle cost basis and also can be used  to manage your motor inventory and track motor maintenance.


As you can see from my summary table, the more efficient fan options have some fairly significant energy savings associated with them, espeically when you consider that the proposal was for three units. If you look in the ASHRAE Applications Handbook in the Owning and Operating cost chapter, you will find that ASHRAE anticipates a service life of 15 years for rooftop air conditioners and of 25 years for centrifugal fans. Given the quality of the units that were the subject of the proposal, it is  likely that they will make the 15 year mark and they could easily last into 20 or more
years with good maintenance practices. These life cycles have two important implications in terms of the design review.

  • The savings delivered by the fan efficiency improvement are based in the physical arrangement of the fan. Thus the will be very persistent and will likely deliver the projected annual savings over the life of the system. This can really add up as it compounds over time as illustrated below.

  • If the improved fan is not selected and provided as a part of the unit when it is furnished, it is unlikely that it will be cost effective to modify the unit after the fact. Thus, the next opportunity to realize the improved efficiency and related savings lies approximately 15-25 years into the future.

At this point, it may seem like a “no brainer” to purchase the more efficient fan. And, in some ways, it probably is. But in the world Tracy operates in, he needs to demonstrate that the added cost of the new fan represents a good investment for the Owners of
the hotel. After all, they are in the hotel bussiness to make money, not to showcase efficient machinery. But, at the same time, Marriott International, the facilities management group that Tracy works for is very interested in efficiency and sustainability, right up to the top man on the totem pole. Thus, they are open to the possibility of investing some additonal money now to achieve a better life cycle cost over the life of the system.

The fastest way for Tracy to sell the improved fan efficiency to his general manager and accounting staff is to demonstrate that it has a fast simple payback. Towards that end, I needed to come up with some “off the cuff” way to understand the cost implications of the different fan wheel sizes. The obvious one is to simply go back to the manufacturer and ask how much more for the more efficient fan options?

But, at the time I was doing the review, that information wasn’t availalbe to me and I was curious about what the answer might be. So, I turned to my R.S. Means Cost Estimating information to see if they had any costs listed for different size fans. While they did not list pricing for different types of fan wheels in different sizes, they did list pricing for different size fans associated with different flow rates and motor sizes, as indicated below.


If one assumes that in general terms, a fan capable of moving more air has a larger fan wheel and a larger motor, then the Means information implies a cost premium of $1,800 to $2,850 up front to potentially save $1,360 to $1,500 or more per year for the life of the system or an implied simple payback of 2 years.

The bottom line is that by spending a little consulting budget to review a cost proposal that had to be developed with-in a limited time frame based on matching existing equipment, Tracy identified an opportunity that can be easily realized at this point in the acquisition process and will represent a good value for his Owner, an improved profit margin for the supplier, and a more sustainable approach to operating his facility, which ultimately benefits us all.

And, having worked with Tracy and other engineering directors like him, I suspect he will leverage what he learned by working through this review with me and share it with his staff and others and maybe even make the first pass at the review himself the next time. Those are all non-energy benefits that will ultimately improve the way we operate our buildings, all of which will benefit all of us.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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