Who This Is For (And Why I'm Writing It)
If you're a facility manager or a commercial HVAC contractor who has to clean evaporator coils on McQuay chillers, this is for you. Specifically, the coils on water-cooled and air-cooled units—the ones that always look fine until they're not.
I'm not going to sell you on why cleaning matters. You already know. What I am going to do is give you the exact checklist I use now after a particularly expensive lesson involving a McQuay chiller, a small freezer that wasn't so small, and a Bendix air dryer that was a total red herring.
This checklist has 5 steps. Step 4 is the one I missed. I'll tell you which one.
The 5-Step Evaporator Coil Cleaning Checklist
Step 1: Isolate the Unit and Verify Power Off
First, you need to lock out/tag out the McQuay unit. This sounds obvious, but here's the thing: I've seen guys skip verifying voltage at the disconnect because they "knew" the switch was off. Don't. Pull the disconnect. Use a meter. If it's a water-cooled chiller (like a McQuay WSC or WDC series), also close the isolation valves on the water circuit.
Checkpoint: Is the unit isolated from both electrical and water sources? Yes/No.
Step 2: Access the Coil and Perform a Dry Inspection
Remove the access panels. On a McQuay air handler or rooftop unit, this usually means a dozen or so sheet metal screws (keep them in a cup, not on the ground). Now, look at the coil before you touch it. Take a photo. Check for:
- Bent fins (from pressure washing or clumsy tool handling)
- Oily residue (compressor oil leak indicator)
- Corrosion (especially on copper tubes in coastal environments)
Don't start spraying yet. A dry inspection tells you where the trouble spots are.
Step 3: Remove Loose Debris (The Right Way)
Use a vacuum with a soft brush attachment or compressed air (blow from the exit side inward). Do NOT use a wire brush. Do NOT use a pressure washer at full bore. I've seen a pressure washer fold fins on a McQuay condensing coil so badly it looked like a crumpled beer can.
If you have a small freezer unit (like a freezer room condenser) with lint or dust accumulation, this step is 90% of the job. You might not need chemicals at all after this.
Checkpoint: Is the coil visually free of dust, leaves, and lint? Yes/No.
Step 4: Apply the Cleaning Solution and Let It Dwell (This Is My Mistake)
This is the step I screwed up.
When I first started doing this work, I assumed that the cleaning solution did its job immediately. Spray on, scrub off, rinse. That's wrong. The dwell time—the time the chemical sits on the coil—is what breaks down the biofilm and grease. For a commercial kitchen or a restaurant where the coil has years of airborne grease baked on, the solution needs to sit for at least 10-15 minutes. I was rinsing it off in 2 minutes.
Use a pH-neutral coil cleaner (or a mild alkaline cleaner for heavy grease). Apply with a low-pressure sprayer. Wait. Follow the label, not my old bad habit. The coil on my McQuay chiller looked clean superficially, but the biofilm underneath was trapping heat. The chiller ran inefficiently for months. It affected a $3,200 repair order and cost us a 1-week delay while we re-did the job. A lesson learned the hard way.
Checkpoint: Did you let the chemical dwell for the manufacturer's recommended time? Yes/No.
Step 5: Rinse Thoroughly (Low Pressure, Pure Water)
Rinse from the inside out (the opposite side of the airflow). Use low pressure—think garden hose with a nozzle, not a pressure washer at 2000 PSI. High pressure can bend the fins and drive dirt deeper into the coil core.
Check the condensate drain pan. Make sure it's clear. You have just introduced a lot of water into the system. If the drain is clogged, you'll create a flood inside the unit. Trust me on this one—I did that too. On a McQuay air handler, standing water in the pan leads to rust and mold in about 30 days.
Checkpoint: Is the drain pan dry and the drain line flowing freely? Yes/No.
A Quick Note on Bendix Air Dryers (The Red Herring)
In my story above, I mentioned a Bendix air dryer. Here's why: when my McQuay unit had reduced cooling capacity, we first blamed the pneumatic controls. We thought the Bendix air dryer (which conditions the compressed air for actuators) was sending wet air into the controls. We overhauled it—new desiccant cartridge, new purge valve. Total waste of time. The real culprit was the dirty evaporator coil. The Bendix was fine.
What most people don't realize is that air dryers for HVAC controls are often blamed incorrectly. The Bendix AD-IS (a common model) has a maintenance schedule: replace the cartridge every year or 2,000 operating hours. If your McQuay unit has pneumatic controls, the air dryer is worth checking—but it's rarely the root cause of capacity loss. Start with the coil.
Common Mistakes to Avoid
- Skipping the dwell time. I already beat this one to death. Don't be like me.
- Mixing chemicals. Don't mix coil cleaner with bleach or ammonia. Bad things (toxic gas) happen.
- Cleaning a hot coil. The coil should be at ambient temperature. Hot coils evaporate the cleaner too fast, reducing effectiveness and leaving streaks.
- Not covering electrical components. The motor, starter, and control box need to be covered with plastic. You will spray them. Cover them first.
- Forgetting the Bendix air dryer. Not because it's the primary issue, but because it can fail. I check the purge valve now during every PM to rule it out early. The few dollars for a rebuild kit is worth the peace of mind.
That's the checklist. Five steps. I'd print it out for your next McQuay chiller service visit, but I know you'll keep it in your phone like the rest of us. Save the money you would have wasted on my mistakes.
