I'm a facility manager who's been handling heating and cooling equipment orders for a little over seven years. I've personally made (and documented) a dozen significant mistakes, totaling roughly $12,000 in wasted budget. Now I keep a checklist to prevent others from repeating my errors.
If you're setting up a garage heater or a workshop cooling system, you've probably defaulted to assuming 'bigger is better' when spec'ing out an evaporator. Especially with a brand like Bohn, known for industrial-grade reliability. I did too. And I was wrong.
I now believe that over-specifying a Bohn evaporator for a standard residential or small commercial garage is a waste of money and can actually hurt the system's performance.
The 'Bohn is a Tank' Fallacy
The logic was simple, and at first, it felt bulletproof. Bohn evaporators, like their air cooled condensers, are built to last. They're robust. They're used in commercial walk-ins and blast freezers. So, if you use a big, heavy-duty model for your garage heater, you're future-proofing, right? More capacity means faster cooling, better performance, and a system that never struggles.
That's what I told myself back in September 2022. I was outfitting a 600 sq ft auto shop in Denver. The client wanted it comfortable for working in the summer and not freezing in the winter. I spec'ed a massive Bohn evaporator with a huge coil. I was proud of it. 'This thing is a tank,' I told the client.
The installation was a nightmare. The unit was too big for the mounting brackets we had. We had to fabricate new ones on-site. The airflow was so high it felt like a wind tunnel, which was great for a 3-second cool-down but terrible for even, comfortable temperatures. The unit would short-cycle in the shoulder seasons because it'd pull the space down to temp in about 4 minutes, then the compressor would kick on and off every 10 minutes. I felt like an idiot.
The worst part? The client was paying a premium for capacity they weren't using. The oversized evaporator cost almost $800 more than the correctly-sized model. And the short-cycling meant higher wear and tear on the compressor (a $1,500+ replacement down the road). My 'tank' was actually creating a more fragile system.
The Deeper Problem: Humidity, Air Filters, and the 'Right Tool'
This gets into a technical area that I really had to learn the hard way. The downside of an oversized evaporator for a small space isn't just about cost; it's about humidity and air filtration.
An oversized coil runs at a colder surface temperature for a shorter period. This means it doesn't properly dehumidify the space. The space gets cold, but it feels clammy. In a garage, that's a recipe for rust on tools and mold on cardboard boxes. I'm not an HVAC engineer, so I can't give you the exact psychrometric charts, but what I can tell you from a facilities management perspective is that a properly-sized unit will run longer cycles, which gives it time to wring moisture out of the air.
Then there's the air filter. A standard garage heater might use a throwaway fiberglass filter. But on a massive Bohn evaporator, you're dealing with a much higher static pressure drop. You need a thicker, more expensive filter (like a K&N or a high-MERV-rated can am air filter style panel). If the user doesn't upgrade their filter, the airflow is choked even more, making the short-cycling issue worse. I had one client who bought a high-quality K&N-style washable filter for it – which is smart for a large system, but it was a $200 part for a heater that originally needed a $5 filter.
'But What About Future Expansion?' – My Rebuttal to My Old Self
I can hear my past self arguing: 'What if they add onto the garage? What if they want to use it for a paint booth? Better to have the capacity now!'
Here's the problem with that logic for a small shop: it's a gamble you're paying for upfront. You're betting $800 of extra cost today on a possibility that might happen. And if it doesn't happen (which, let's be honest, 9 times out of 10, it doesn't), you've permanently paid a penalty in higher operating costs and worse comfort.
Also, if you do expand later, swapping out a correctly-sized evaporator for a larger one is often cheaper than the premium you paid oversized-running-irregularly for years. The upfront cost premium for that 'maybe' future expansion almost never pays off. This was accurate as of 2022. System design philosophies might have evolved, but the basic thermodynamics haven't.
The 'Small Order' Is Not a Problem
This is a common fear for small shops and DIYers. You think, 'I'm just a little guy, and the vendor won't take my $200 order seriously. I better buy a bigger, more expensive one so they don't think I'm a waste of time.'
When I was starting out, the vendors who treated my $200 orders seriously are the ones I still use for multi-thousand-dollar jobs. Bohn, for example, has a broad distribution network. Your local supply house can get a correctly-sized, lighter-duty Bohn evaporator without any trouble. The idea that you need to 'buy big to be a real customer' is nonsense. Small doesn't mean unimportant—it means potential.
So, what's my rule now? I don't look at the nameplate BTUs first. I look at the coil surface area. For a standard garage heater application, you want a coil that is just big enough to do the job at a standard 400-500 feet per minute face velocity, not one that's so big it chills the air instantly then goes idle. A smaller coil running for 15 minutes is infinitely better than a huge coil running for 4 minutes and then cycling for 8.
Look, I'm not saying Bohn makes bad equipment. Their bohn air cooled condenser series is, frankly, a benchmark for durability. But a 'tank' is for a battlefield, not a driveway. For a garage heater, spec the evaporator that fits the job, not one that fits your ego or a vague fear of being too small. A correctly-sized system works better, costs less, and runs more efficiently. That's the lesson that cost me $800 and a week of my life to learn.
