How Many Fans Does an AHU Need?

More is better, right?

I have been in the HVAC industry for 10 years. Not as long as many of you, but long enough to know there is a difference between the engineering truth, the marketing claims, and the generally held conceptions many of us hold.

In my business, one of our core competencies is custom air handlers: new factory built and replacing old, hard to reach air handlers in very tight spaces.

I have seen new technologies come and go. I've seen my fair share of buzzwords and catch phrases that "are going to change the world!", yet, the reality is often something different.

I could write all year long on some of those, but I wanted to focus on one aspect of HVAC design in particular: Fan and motor selection.

I feel this is a great example of the wide gap between the engineering truth, the marketing claims, and our opinions as HVAC professionals.

Just the Facts, Ma'am

My intent is not to call out any particular brands or anyone in particular. Actually, I hope to learn a lot myself in this process; I'll be the first to admit the more I see in this industry the grayer the lines become, and I could use more education.

So, may years ago, most large custom air handlers were designed with a single fan. I've seen plenty of 30-40 year old units serving spaces like hospitals, dorm rooms, labs and manufacturing facilities. The units have been patched and repaired and duct-taped as much as possible, but they're still chugging along with one, giant fan.

We are usually called in when the customer realizes the payback period on a new, custom AHU could very well be only a couple years, compared to the old, maintenance-intensive, inefficient unit they currently have.

I have also seen plenty of units with 2 fans. The thought is that if one fan/motor fails, the second fan ramps up with a VFD and you can get close to or even match the normal operating CFM and static pressure on a single fan. When we are doing our custom AHU selections, we can usually hit full CFM at design static pressure with a 2 fan design, even if we lose 1 fan.

Tried and True

It seems very simple so far, and apparently, that's how engineers designed units for decades. I know things *can* always be improved, but it sure seems to me it has worked quite well for a long time.

Well, about 5-10 years ago, there was a major push in the industry to begin designing air handlers with multiple fans. And I'm not talking about 2 fans, or even 4 fans. I'm talking about 9, 12, 18, 27 or more fans!

There was a lot of marketing behind this. Better efficiencies, smaller fans that are easier to handle, lower horsepower, better airflow, etc., were some of the reasons being given. Some of these reasons hold water, some I'm not so sure of.

Now, this is the part where I need YOUR help!

I understand basic fan laws and physics. All things being equal, fewer, larger fans will be more efficient every day of the week and twice on Sundays.

Keep Your Eye on The Cup with the Ball

Yet, there seems to be some engineering, or perhaps pseudo-engineering behind the push for extreme numbers of fans.

When I work with customers and engineers, the responses I've gotten are all over the board:

• Yeah, fewer fans are more efficient
• The more fans you have, the less connected load
• The more fans you have, you get better airflow
• If you use more fans, you can have a lower connected load and lower nameplate horsepower
• You don't need a bunch of fans, 2-4 is just fine and draw less Amps when it's operating

And those comments are coming from PEs.

And this is where my knowledge begins to fail me. I know what nameplate data is. I know what connected load and BHP are. It seems that on a theoretical level at least, there can only be one "most efficient" design from an energy consumption standpoint. I've always assumed that would be fewer, larger fans.

But, with all the different opinions out there, I am beginning to see the picture isn't so cut and dry. Is this a bunch of marketing mumbo-jumbo? Is there some truth to it? Does it all "just depend on the specific application"?

Calling All Stations

I would absolutely love to hear from those of you who have experience designing or operating custom air handlers, especially if you have designed units with 2-4 fans as well as units with 6 or more fans.

What are the benefits of each way of thinking? Does it depend on the application?

We have a very good client of ours that is maxed out at ALL of their facilities when it comes to power. Normal and Emergency power are simply maxed out. So, a major concern of theirs when replacing old air handlers is minimizing HP/Amp draw in any possible scenario (normal vs. losing a fan motor) and overall fan efficiency is less of a concern. Maybe this is a good reason for multiple fans?

Thank you in advance. I'd love to get as many facts, experiences and opinions on this. I hope I am not revealing any shortcomings in my own experience, but the fact that I have gotten different opinions from PEs as well as Facilities Managers, makes it seems to me that this is a topic that is not fully understood. Or, at the least, there may be a clear answer in perfect world conditions, but the perfect fan/motor selection for any given AHU depends on the design conditions, customer needs and site capability.

Thanks in advance!