The Day My McQuay Unit Shut Down
I legitimately believed our McQuay chiller would run forever. It was a robust model, always started up, and the discharge temperatures were rock solid. Then, on a Tuesday in July 2022, it gave me the middle finger. The unit went into high-pressure lockout. We had a critical server room cooling load on the line, and the building's management was screaming. (Real talk: I think the plant manager was sweating more than the equipment.)
My first instinct was the compressor or a failing valve. The error code was vague. So, naturally, I started diagnosing the expensive stuff. I even called our parts supplier to price out a replacement compressor for a McQuay package unit from 2016. (Should mention: the unit wasn't even that old. That’s the part that made the whole thing worse.)
What I Thought Was The Problem (And What Actually Was)
If you're seeing high head pressure on a McQuay HVAC unit, you immediately think of a handful of issues: non-condensables in the system, a failed expansion valve, or a worn-out compressor. I spent two hours checking the refrigerant charge and pressures. The subcooling and superheat readings were, if I remember correctly, way off from the manufacturer's chart. This pointed to a restriction. I was ready to cut into the line set.
But I stepped back. A senior tech years ago told me: "Check the basics before you go hunting for the monster." I hate that advice because it always makes me feel stupid when it works. So, I took a look at the condenser coil. I should have done this first.
It wasn't just dirty. It was caked. The fin pack on the air-cooled condenser looked like a felt pad. There was a layer of construction dust and cottonwood seeds about a quarter-inch thick. The heat exchanger was essentially insulated from the ambient air. The unit couldn't reject heat. That was the entire problem. No compressor replacement needed. No expensive refrigerant leak search. Just a cleaning.
(Oh, and that 'restriction' I thought I saw in the refrigerant circuit? It was a classic symptom of an overcharged system due to poor heat rejection. The system was trying to condense, but couldn't, so it looked like there was too much liquid. A textbook misdiagnosis.)
The Real Cost Of Dirty Coils
So, what was the damage from my little adventure? Time and money.
- The Misdiagnosis: 3 hours of diagnostic labor at $150/hour. That’s $450 down the drain.
- The Parts Scare: We had a rush order placed for a contactor and a pressure switch we didn't need. That was another $200 in parts we couldn't return because they were 'special order.'
- The Downtime: The server room temp spiked. That cost us about $1,200 in lost productivity for the building's tenants.
- The Actual Fix: 2 hours of coil cleaning with a chemical foam and a garden hose. Total cost of labor and supplies: $250.
I still kick myself for that one. If I’d just spent ten minutes looking at the coil first, I would have saved $1,850 and the embarrassment of telling the building owner I 'fixed' a unit that was just dirty.
The Deep Lesson: 'Clean Enough' Isn't Clean
The big issue here isn't just about seeing dirt. It's about understanding how it impacts the system dynamics. Most techs I know (myself included, up until that point) think, "Oh, the coil looks a bit grey, but air is still moving through it." That's a dangerous assumption.
According to a paper from ASHRAE (Standard 62.1, for those keeping score), fouled coils can reduce chiller efficiency by 30–40% before you even see a pressure alarm. The unit was probably running inefficiently for months. That wasted energy cost the building owner hundreds of dollars in avoidable electricity bills.
For McQuay equipment specifically, this is critical. McQuay chiller designs often rely on tight approach temperatures. That means the coil has to be damn near perfect for the unit to hit its rated efficiency. I learned the hard way that 'clean enough' is not a real thing. It's either clean, or its costing you money.
The Simple Fix: A Proper Cleaning Protocol
Here's the boring answer, because the interesting answer (what I actually did) was incredibly expensive. Clean your McQuay HVAC unit's condenser coils at least once a year. For units near construction or cottonwood trees, do it twice a year or even quarterly.
How? Don't just hit it with a garden hose. That doesn't work. Get a low-pressure coil cleaner. Something that foams. Spray it on from the inside out. Let it sit for 10 minutes. Rinse thoroughly from the inside out.
And here's the trick: measure the approach temperature before and after cleaning. If the approach temperature drops by more than 5°F, you were losing efficiency. You’ll instantly validate the cost of the service. That data point is worth more than a thousand guesses.
Since that disaster in 2022, I’ve implemented a strict cleaning schedule for every McQuay rental unit and permanent install in my fleet. We've caught four potential failures just by observing coil conditions during PMs. Looking back, I should have made that change five years sooner. At the time, I thought preventative maintenance was just changing filters. I was wrong.
