The Call That Made My Stomach Drop
In March 2024, a customer called with a report that their walk-in freezer was running warm. The display said 38°F. By the time I drove across town, it was 42°F.
I'm a service manager at a commercial refrigeration company — I've been in this trade for eight years, and I've personally made 13 expensive mistakes totaling roughly $19,000 in wasted budget. I log all of them so new techs don't repeat my errors. But this call was especially bad, because the mistake was mine, and it was the second time I'd made it.
What The Bohn Condensing Unit Showed Me
The customer had bought a Bohn condensing unit and matching evaporator 14 months earlier. My crew installed it. When I popped the access panel, the first thing I saw was a thick gray blanket of dust and lint covering the condenser coil. The Bohn refrigeration unit was barely a year old, but the coil looked like it had been running in a cotton mill for five.
I checked the refrigerant pressures, the fan motor, the sight glass. All fine. The high-pressure limit switch had tripped, which saved the compressor — but just barely. The coil was so plugged that the condenser fan was pushing air against a wall of packed dust, not fin gaps.
Then I saw the filter.
It was a 1-inch fiberglass filter, the $3.50 kind. And the arrow on the frame was pointing away from the coil — up, toward the fan. Backwards. I had personally installed it during a routine service visit months earlier and said to the customer, "This filter will keep your coil clean." It did the opposite.
Flashback: The First Time I Learned About Filter Arrows
In 2022, I was still new to commercial refrigeration. I installed a Bohn low-temperature unit at a bakery and read the manual during the install. The instruction was: "Install the air filter with the directional arrow pointing the direction of airflow." Simple enough. The airflow went from the return duct, through the filter, into the evaporator coil. The arrow should point at the coil.
I put it in backwards anyway, because the filter frame had a small lip that made it easier to grip that way. My reasoning was: it's just a filter, it catches dust either way.
That mistake cost us $890 in redo labor and two service visits to chase a low-cooling complaint. Not as catastrophic as today, but enough to sting.
Here's the thing about filter direction that most people miss: a filter isn't just a dust catcher. The frame, the pleats, and the support grid are all directional. When you install a pleated filter backwards, the airflow can press the pleats flat, reducing the surface area and causing the filter to clog faster. On a fiberglass filter, the mesh backing is designed to catch dust from one side. The arrow is there because the manufacturer knows which way the media holds up best. Ignoring it is like thinking a one-way valve is just a weird fitting.
This is also the answer to the classic home furnace question: which way to put air filter in furnace — up or down? The answer is: point the arrow in the direction of the airflow. Not "always up." Not "always down." On an upflow furnace, the arrow points up, toward the blower. Downflow? Point it down. Horizontal? Sideways. The orientation of the equipment doesn't matter — the arrow always points with the air.
The Expensive Recovery
Back to the walk-in freezer. The coil was plugged, the unit was locked on high-pressure limit, and the refrigerant charge was out of spec because of the pressure limit cycling. I had to:
- Clean the condenser coil — carefully.
- Replace the backwards filter.
- Reclaim and weigh in the correct refrigerant charge.
- Write "ARROW FACES COIL" on the access door with a Sharpie.
Total bill: $3,200. That covered labor, refrigerant, coil cleaner, a new filter, and a new time switch that had fried when the fan cycled oddly. The customer didn't pay a cent — my company did, because I caused the problem. My boss just said, "Add it to the log."
Cleaning Coils: Milwaukee Leaf Blower Edition
If you've watched any commercial refrigeration tech clean a condenser coil, there's a good chance they used a cordless leaf blower. It's fast, effective, and it almost works too well. I use a Milwaukee blower myself. It's great for clearing light surface dust off coils between scheduled maintenances.
But leaf blowers have a dark side. On that backwards-filter coil, the dust had packed into the aluminum fins like felt. When I hit it with the Milwaukee on full power, the blower pushed the dust deeper and bent several fins flat. I had to pull out a fin comb and spend two hours reshaping the coil before I could even apply chemical cleaner. Not my proudest day.
So if you're maintaining a Bohn refrigeration unit or any commercial condenser: use a leaf blower or compressed air, but keep the pressure low and blow from the opposite side of the coil — from the inside out. Spray coil cleaner first, let it soak, then rinse with low-pressure water. The fins on a Bohn condenser coil are thin; treat them like aluminum foil with an attitude.
What About Bladeless Fans?
A customer once asked me if they could use a bladeless fan to blow dust off their condenser coil. I told them it wouldn't do much. Bladeless fans move a lot of air, but at low pressure and with a wide, diffuse stream. To clean a coil, you need concentrated, high-velocity airflow to dislodge dust from between the fins. A bladeless fan might keep people comfortable in a lobby, but it's just rearranging dust when pointed at a condensing unit.
The same logic applies to cooling a compressor room. A bladeless fan can circulate air, but it can't push replacement air through a bank of condensing coils the way a legit ventilation fan can. Use the right tool for the job.
What I Do Now
Since the $3,200 mistake, I created a one-page checklist for every Bohn evaporator and condensing unit that goes through our shop:
- Filter? New, and installed with the arrow pointing at the coil.
- Condenser fins? Straight, no bent sections, no dust blanket.
- Fan blades? Balanced, clean, no cracks.
- Airflow direction? Marked with a Sharpie on the access door.
In the past 18 months, my team caught 47 potential issues during pre-season inspections. Most were dirt-related: old filters, backwards filters, coils that looked like someone had leaf-blown a bonfire into them. Fixing those before they cause a compressor failure is way cheaper than replacing a condensing unit.
So, Filter Up or Down?
Let's settle the searching once and for all. Someone who types "which way to put air filter in furnace up or down" into Google is usually standing in front of a dirty filter, trying to decide which side faces out. The answer is printed right on the filter frame: there's an arrow, and it must point in the direction of the airflow. For a furnace, that direction is toward the blower and the heat exchanger. For a Bohn evaporator, that direction is toward the coil.
If you can see the coil fins when you look through the empty filter slot, you're looking in the direction of the airflow — so the arrow should point toward the coil. If all you see are the stiff support wires of the filter frame, the filter is backwards.
Honestly, it's a $5 fix that protects $5,000 of equipment. I made the mistake once in my first year of commercial refrigeration, then again in 2024 because I got careless and assumed I knew better. The second time, the equipment had a different owner, but the failure was identical. I'm writing this so maybe some tech somewhere can avoid adding a $3,200 line item to their own mistake log.
Check the arrow. Point it the right way. Keep your Bohn refrigeration units and your furnace filters honest. And if you really need to use a leaf blower on a coil, use low speed — your fin comb will thank you.
