Should You be Adding a VFD on Every Packaged Rooftop Unit?

Switching to Geico may or may not save you 15% on your car insurance, BUT, adding a variable frequency drive (VFD) to your multistage equipment will do the trick! Okay, you caught me speeding- adding a VFD won’t save you anything on your car insurance, but it WILL save you or your customer money on their electric bill. In fact, after taking a deep dive into the numbers, adding a VFD doesn’t just save a bunch of money on utilities, it could be an AMAZING investment for a building owner or tenant. 

Since we are very conscience of your time, we will investigate just three of the most common 2-stage packaged rooftop sizes- 7.5 ton, 10 ton and 15 ton units. There are 10-15 more combinations that we could run numbers on, but this should be a great start.

This is also a great time to tell you that our analysis is based on factory installed VFDs in York equipment, however if you have a different brand of choice, don’t fret. The numbers should still be very close. 

So without further ado, here are the results:

Table 1: 

*Payback and return on investment when adding a VFD. Figures are based on 1,500 runtime hours at $0.14 kWh. Payback and return on investment will vary due to construction, individual usage, setpoint, electricity cost and other variables. 

Could it really be? 30%-60% return on investment (ROI)? It doesn’t seem right! I have to admit, when I first saw this I was shocked! Not nearly as shocked as when Donald Trump wanted to build that wall…But, now it makes complete sense why ASHRAE and IECC are soon to be requiring two stage fan on anything larger than 5 tons. 

Other considerations

The crazy thing is the actual ROI might be even BETTER than shown above. We kept everything relatively conservative, and we EXCLUDED the following:

  • Rebates: The building owner might be eligible for a KCP&L (or other utility company) rebate, which could pay for a portion, or in some cases the ENTIRE price of the VFD. 
  • We did NOT include an escalating price for electricity. And we all know which direction those prices are going in the near future. 
  • Benefits of 2 stage fan for heating: Our calculations only included the increased efficiency and payback during the cooling season.
  • Benefits for running fan in low speed during occupied mode when there is no call for heating or cooling.

Wait, there’s more?

If we assumed the only benefit for adding a VFD was an off-the-charts ROI, it would still be a no-brainer to add one. But there is more. Adding a VFD to multistage equipment is a game-changer when it comes to comfort. Since the fan speed matches the cooling call, we get MUCH better temperature and humidity control. And if you run your machine in Single Zone-VAV mode, the unit will modulate fan speed to maintain space temperature and cycles compressors to maintain an adjustable leaving air temperature. Now we’re talking!

You want more benefits? Okay, one more, but first let me ask you a question. When you get in your car, do you immediately start the engine and then slam on your gas pedal 100% to get to your cruising speed? No! That would be crazy, and very hard on your engine. Well that’s what happens with your commercial HVAC equipment when you don’t have a VFD. When you get a call for fan, cooling or heating, your motor goes from 0 to full blast immediately. This causes undue stress on your whole fan assembly. Adding a VFD will soften that start which will help lengthen the life of your belts, bearings and entire fan assembly. 

Tenant upgrade 

One last thing- you might be thinking, “Who really cares? I deal with mostly property management companies, and building owners that don’t care about saving on their utilities”. No problem! Ask your customer to offer the VFD as a “tenant upgrade” and now your property manager or building owner can recoup some of their investment. Now EVERYONE wins! 

What are your thoughts on adding VFD’s to packaged rooftop units? If you are not offering them on every job, why not? We’d LOVE to hear from you!

Brad Telker
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