I don't normally write articles. I'm the guy who keeps the maintenance spreadsheet. But after enough expensive repair calls, I started documenting the pattern. It wasn't what I expected.
I've handled HVAC maintenance orders for about 11 years. I've made a few significant mistakes—maybe five, not counting the small stuff—and they've cost roughly $28,400 in wasted budget. I keep the list because it keeps me honest.
The Problem You're Probably Chasing
Most calls start with the same complaint: “The McQuay HVAC unit is losing capacity.” When I ask what the last tech did, the answer is usually one of three things. They changed a sensor. They added refrigerant. Or they “checked everything.” Then the problem comes back in a few weeks.
From the outside, it looks like the equipment is unreliable. The reality is the maintenance process is unreliable.
Here's a specific example. A 60-ton McQuay chiller in a medical office building started tripping on high head pressure. First visit: condenser coils were dirty. We cleaned them, checked the charge, moved on. Three weeks later, same alarm. The property manager started asking for new compressor quotes.
It's easy to sell a new compressor. It's harder to ask why the compressor is overheating when the coil is clean.
What's Actually Going On
That machine was not the real problem. Three layers were behind it.
First, the condenser fan speed wasn't ramping up. The outdoor air sensor had failed open. The controller thought it was -22°F, so it kept the fan at minimum speed. The coil was clean, but with low airflow, the high head pressure was inevitable. The sensor cost about $70. It caused four service calls before we found it.
Second, the chilled water loop had no chemical treatment. The tubes had scaled up. Heat transfer needs water flow. Scale acts like insulation. When we finally cleaned the tubes, the pressure dropped back to normal. The refrigerant charge was never the issue.
Third, a previous tech overcharged the system because subcooling looked low. Low subcooling can mean low refrigerant. It can also mean poor water flow. He added gas until the sight glass looked right. Then we fixed the water flow, and too much refrigerant made the problem worse. Not the machine's fault. Ours.
According to AHRI Standard 550/590, rated chiller performance assumes clean coils, design water flow, and proper airflow. If those conditions are missing, the rated tons don't exist.
The Filter Problem You Don't Think About
Then there's the air filter. People assume a filter is just a dust catcher. It's not. It's a system component.
A 2-inch pleated filter with a high pressure drop starves the fan coil unit. Airflow drops. Coil temperature drops. The compressor works harder. On a McQuay fan coil, a clogged filter can produce freeze-up symptoms that get misdiagnosed as a refrigerant problem. I used to think filters were too cheap for anyone to skip. Wrong.
I replaced a fan coil motor once because the unit ran hot with no airflow. The filter looked fine from the front. From the downstream side, it was solid. A lesson learned the hard way.
Same logic applies to something like a Can-Am air filter. It looks small, but if the engine can't breathe, it loses power and wears faster. HVAC filters work the same way. If the air path isn't open, the machine can't do the work it was rated for.
The Heat Source That Makes No Sense—Until It Does
Another missed layer is the space itself. I once worked with a facility manager trying to fix cold spots in a warehouse. They bought a 60,000 BTU propane heater for the loading dock. It warmed people nicely. But the rooftop McQuay units were designed for recirculation with a certain amount of ventilation. The propane heater added moisture and combustion gases to the return air. The controls started doing strange things because the return-air conditions no longer matched the design assumptions.
The heater wasn't bad. The integration was missing. This is why the physical space has to be part of any serious McQuay HVAC maintenance process.
What Those Mistakes Cost Me
Let me be specific about the money.
In 2019, I signed off on a compressor replacement for a chiller that was still short-cycling. The diagnosis looked clear: high winding temperature, oil smell, compressor failing. I replaced the compressor, dried the system, installed a new filter dryer. Six weeks later, the new compressor started dying too.
Then we found a condenser fan relay that was failing under load. Voltage looked fine at rest. As soon as the fan drew current, the relay dropped out. The compressor overheated. New compressor, new relay, labor, refrigerant, embarrassment. If I remember correctly, it ended up around $9,700.
Another one: I saved $400 by using a universal controller instead of the OEM board. It powered on. Setpoints worked. But it didn't communicate with the building automation system correctly. The unit ignored the staging schedule, ran during peak demand, and spiked the utility bill. We replaced it with the correct board later. Waste: $2,150.
Then there was the back-ordered damper actuator. The lab needed airflow, so I said, “make it work.” One of the techs rerouted a flex duct to bypass the damper. It didn't balance the room. It reduced airflow to a lab, the cooling coil froze, and water ended up on the floor. The actuator was $260. The cleanup was $3,200. Penny wise, pound foolish.
Total: around $28,400, give or take. I don't need the spreadsheet anymore.
The Checklist I Use Now
After those mistakes, I changed my process. It's not complicated, but it requires patience.
Before any scheduled McQuay HVAC maintenance, I want five things:
- Baseline data from the last 30 days. Supply air temp, return temp, water temps, pressure differentials, current draw, superheat, subcooling, ambient conditions. If a tech can't show me the baseline, I don't trust the diagnosis.
- Air path condition. Check filters before and after. Check coils, fan blades, and static pressure. A dirty coil isn't a coincidence. It's a trend.
- Water side condition. Flow rates, strainer pressure drops, treatment levels. A tube with scale can't transfer heat. Refrigerant can't fix that.
- Control sequence testing. Sensors fail. Actuators drift. Setpoints get changed. Verify the controller is doing what it thinks it's doing.
- The physical space. What changed since the last service? New equipment? A new partition wall? A propane heater on the loading dock? It matters.
Portable fans are part of this too. A Woozoo fan is great for keeping people comfortable and moving air out of a dead spot. I've used them myself. But a fan doesn't improve airflow through the filter. It doesn't clean the coil. It doesn't fix the compressor. If the system needs maintenance, circulation fans just keep people comfortable while the problem keeps getting worse.
The industry has changed. Best practice in 2020 is not enough in 2025. Equipment is more controlled, refrigerants are more sensitive, and maintenance diagnosis is more about data than feel. But the fundamentals haven't changed: heat transfer, airflow, water flow.
The equipment isn't always the problem. Sometimes the process is. At least, that's been my experience in commercial HVAC maintenance. It cost me about $28,400 to learn it. You can have the lesson for free.
