Why Most Engineers Get McQuay Fan Coil Units Wrong (And I Did Too)

I’ll say it bluntly: most engineers I meet are speccing McQuay fan coil units wrong.

Not because the units are bad — they’re not. McQuay makes solid commercial FCUs, especially for mid-range sensible cooling loads. But I keep seeing the same mistake: picking a model purely from a catalog, without thinking about the actual building conditions. And yeah, I made that mistake myself.

In 2019, I was the service coordinator for a mid-sized MEP firm handling McQuay HVAC service orders. My first major screw-up was a 10-ton chiller-linked FCU selection for a Boston office lobby. On paper, the model matched the heat load. But I didn’t account for the ventilation load from the dedicated outdoor air system. Six months later, the unit couldn’t keep the space cool, the client was furious, and we had to swap a coil on our dime. That lesson stuck. Here’s what I’ve learned since.

Argument 1: Spec sheets are designed for ideal conditions, not your building

Every McQuay fan coil unit catalog lists cooling capacity at standard conditions — typically 80°F DB / 67°F WB entering air, and 45°F entering water. If your project is in Boston, that’s a fantasy. Actual outdoor air can hit 95°F with 75% humidity. The water temperature from the chiller might be 42°F after distribution losses. Once you factor in the fresh air load from the DOAS, the coil capacity drops 15-25% from the catalog number.

What most people don’t realize is that the AHRI 410 rating for FCUs tells you the sensible capacity under one specific test condition. It’s a starting point, not a guarantee. If you don’t apply a correction factor for your actual entering air and water temps, you’ll undersize the coil. I did this on a Houston project in 2022 — a Milwaukee leaf blower would’ve moved more air than that unit could cool. We had to add a second unit, which ate up all the ceiling space we’d reserved for the lighting.

Argument 2: The “bigger is better” instinct will kill your project, too

On the flip side, I’ve seen oversizing cause just as many headaches. One client insisted on a 600 CFM unit for a 400 CFM load just to be safe. The result? Short cycling, poor humidity control, and constant condensation on the supply air grilles. The building manager said they “needed an outdoor fan to dry the stains off the ceiling tiles.”

McQuay’s product line includes multiple cabinet sizes and coil selections within the same CFM class. A 500 CFM unit with a 4-row deep coil can handle a higher sensible load than a 600 CFM unit with a 3-row coil, and it’ll fit in the plenum space you already have. Don’t just look at the air volume — look at the coil depth, the fin density, and the water flow path. I learned this the hard way after approving a $3,200 order for the wrong model. The reorder cost an extra $890 and a 1-week delay.

Argument 3: The “standard” McQuay model is rarely the right one for your load profile

Here’s something vendors won’t tell you: McQuay offers multiple motor and drive options on the same FCU chassis. A standard PSC motor draws fixed RPM, so if the duct static pressure is different from the design assumption, the airflow changes. An ECM motor (Electronically Commutated Motor) can adjust its speed automatically to maintain constant CFM. If you spec the PSC version for a variable-air-volume application, you’re asking for trouble.

In 2023, I had a project where the architect wanted all FCUs to fit into a 12-inch ceiling plenum. The standard McQuay cabinet height was 14 inches. We had to go with a custom coil configuration (the manufacturer’s own engineering manual, circa 2023, says 12-inch is possible with a 2-row coil and a shallow drain pan). But the lead time jumped from 4 weeks to 10. If I’d caught the ceiling constraint at the spec stage, I could’ve designed around it. Instead, we cost the project two months of schedule float.

Argument 4: Maintenance access is the silent killer of good FCU performance

I’ve serviced McQuay FCUs in dozens of buildings. The ones that perform best have one thing in common: the filter access is on the side facing the corridor, not the interior zone. When filters are hard to reach, maintenance staff either don’t change them or they install cheaper ones that collapse under the fan pressure. A clogged filter drops airflow by 20% in the first month.

I always specify filter sections with a minimum MERV 8 and a permanently installed pressure drop gauge. That way, the building manager knows when to change the filter — not by guessing, but by a number. This sounds basic, but 3 out of 10 FCU failures I’ve investigated were caused by dirty filters. The other 7 were bad control wiring or wrong settings. The point is, don’t design an FCU that’s a pain to maintain. Your spec will fail in real-world service.

Counterpoint you might be thinking: “But the spec sheet says it works.”

I get it. The AHRI directory lists the model as valid. The McQuay sales rep says it’s a proven design. But the spec sheet assumes a perfect world where air density is 0.075 lb/ft³, water flow is exactly 3 GPM per ton, and the duct system has zero leakage. Real buildings have leaky ducts, high-lift condensate pumps, and chiller plants that aren’t running at design delta T. Relying on the catalog without applying a 15% safety factor is a gamble I’ve lost before.

When someone tells me, “The catalog says it handles 20,000 BTU/h,” I ask them: “At what entering water temp, what entering air temp, and what CFM?” If they can’t answer those three parameters, they’re buying the spec sheet, not the real-world building. I keep a copy of the AHRI 410 standard on my phone — it’s free to download from their website (as of January 2025, at least). I reference it whenever a vendor quote looks too optimistic.

So here’s my honest rule of thumb

For any McQuay fan coil unit order, do these three things before you click “purchase”:

• Run the coil selection with your actual entering water and air conditions. McQuay’s free selection software (available from any McQuay HVAC service provider) lets you input project-specific data. Use it.
• Check the plenum space. A 14-inch cabinet won’t fit in a 14-inch ceiling with a fire damper and a duct takeoff. Leave at least 4 inches of clearance above the unit for coil pull-out.
• Call a local service rep and ask: “What model McQuay FCU do you see failing the most in our climate zone?” They’ll tell you the truth if you listen. For Boston (where I work), the 300 CFM model with the plastic drain pan is a common problem because the pan cracks under freeze-thaw cycling. Swap it to the metal pan option for $35 more.

I used to think I could spec everything from my desk. Now I mock up the unit in the ceiling grid? Not literally, but I make a point to visit the job site during the rough-in phase. If the mechanical contractor is struggling to fit the FCU between a steel beam and a sprinkler header, I get to redesign on the spot. That’s happened four times in the last two years. Each time, I caught the problem before it became warranty work.

The real value of a McQuay fan coil unit isn’t the catalog number. It’s how well it integrates with the building’s actual construction, its maintenance reality, and its load profile. If you treat the spec as a conversation, not a decree, you’ll end up with fewer callbacks and happier clients. Trust me, I’ve been on both sides of the invoice.