The Stupidest Thing About Small Truck Refrigeration That Nobody Talks About

We ask the question – if hot air rises, and cold air falls, why does the roof-mounted evaporator pull air from below it – from the coldest zone in the cargo box – when it’s literally sitting in a pool of warm air at ceiling level?

If you run a refrigerated courier truck, here’s a question that should keep you up at night: Why does your roof-mounted evaporator pull air from below it – from the coldest zone in your cargo box – when it’s literally sitting in a pool of warm air at ceiling level?

Let me be more specific. Walk to your truck right now, open the cargo doors, and look up at your Thermo King, Carrier, or other roof-mounted evaporator unit. See that unit mounted on your ceiling? Now look at the bottom of that unit – see that intake opening where air gets sucked into the evaporator? It’s facing downward, pulling air from the space below it.

Here’s what’s happening: Your evaporator is mounted at the highest point in your cargo box, sitting in the warmest air (because hot air rises). But instead of pulling that warm air directly into the unit to be cooled, the intake is positioned to pull air from below the unit – from the colder zone where cold air has naturally sunk.

That relatively cold air from below gets pulled up into the unit, gets cooled even more (unnecessarily), gets blown out horizontally toward your doors, and immediately sinks back down because cold air is denser than warm air.

Meanwhile, the warm air that naturally accumulates at ceiling level – literally surrounding the top and sides of your evaporator unit – just stays there, warm and ignored.

You’ve got an evaporator that’s physically located in the warmest zone but configured to pull air from the colder zone below it. It’s thermodynamically insane. And it’s been standard practice since the first refrigerated trucks appeared around 1913.

112 years. Nobody’s questioned it. Nobody’s fixed it. And you’re burning fuel every single day to compensate for this fundamental design flaw.

How Current Roof-Mounted Evaporators Actually Work (Or Don’t)

Let’s trace the air path in a typical small truck refrigeration system to understand exactly how absurd this is.

The Standard Configuration:

Your evaporator unit is roof-mounted at the front of your cargo box. The unit has an intake opening at its bottom surface, facing downward into the cargo space. The discharge openings (where cold air blows out) face horizontally toward your rear doors.

The Air Flow Cycle:

1. Evaporator pulls from below: The evaporator fans draw air upward through the bottom intake opening. This air comes from the zone below the evaporator – which is relatively cooler because cold air naturally sinks. You’re pulling air that’s already cooled from previous cycles.
2. Already-cooled air gets cooled more: That air from below (maybe already at -18°C to -20°C from previous cooling cycles) gets pulled into the evaporator, run through the coils, and cooled even further (to -25°C or -30°C). You’re spending compressor power to super-cool air that’s already reasonably cold.
3. Cold air discharged horizontally: The evaporator blows this super-cold air out horizontally toward your cargo doors at ceiling height. It travels maybe 2-3 meters toward the doors before physics takes over.
4. Cold air immediately sinks: Because cold air is denser than warm air, that freshly-discharged air drops down toward the floor within a few meters of discharge distance. Gravity wins. Every single time.
5. Cold air pools below the evaporator: The coldest air in your entire cargo space sinks and forms a layer in the lower portion of your box. Eventually, some of this cold air gets pulled back up into the bottom intake of the evaporator, and the cycle repeats.
6. Warm air completely ignored: Here’s the insane part – your evaporator is mounted at ceiling level, literally surrounded on top and sides by the warmest air in your cargo box. Heat infiltration through your roof insulation warms the upper air around the unit. Warm air from door openings rises straight to the ceiling. Product respiration heat rises. Every source of heat in your cargo space naturally accumulates at ceiling level – right where your evaporator is located. But your evaporator’s intake faces downward, pulling from below, completely ignoring all that warm air surrounding it.
7. Temperature stratification: You end up with a stable temperature gradient: relatively cold in the lower zone (where your intake pulls from), progressively warmer as you approach ceiling level, and warmest at the very top – right above and around your evaporator unit. Your temperature sensor (usually mounted mid-height on a wall) averages this out and keeps running your compressor to maintain setpoint.

The Result:

Your evaporator is sitting in a pool of warm air but is configured to pull cooler air from below it. Your system works harder than necessary, runs longer cycles, consumes more fuel, and still creates uneven temperature distribution. Products in the lower portion of your load might be at -22°C while products near the ceiling (right next to your evaporator) are at -16°C.

All because you’re pulling air from below (where it’s already cooler) while ignoring the warmest air that’s literally surrounding your evaporator unit.

Why This Design Persists: The World War I Legacy Nobody Questions

Here’s the truly maddening part: this bottom-intake design made sense in 1913 when the first mechanically refrigerated trucks appeared. Let me explain why, and then why it no longer makes sense but nobody’s changed it.

The Original Logic (1913-1960s):

Early refrigerated trucks had massive evaporators with enormous fans and minimal insulation by modern standards. The design philosophy was simple: overwhelm the problem with brute force. Discharge so much cold air with such velocity that it circulated throughout the entire cargo space regardless of natural convection patterns.

Intake placement wasn’t particularly important because the sheer volume of air movement overpowered any stratification effects. You had fans moving 1,500-2,000 cubic meters per hour in a 10 cubic meter cargo box – complete air changes every 15-20 seconds. Temperature stratification couldn’t form because air was being violently mixed constantly.

Bottom-facing intakes made practical sense: they were protected from direct sunlight and road debris hitting the unit from above, and the design kept the evaporator compact.

What Changed (1970s-2020s):

Several factors converged that should have forced a redesign, but didn’t:

1. Insulation improved dramatically: Modern polyurethane foam insulation with 80-100mm thickness means far less heat infiltration. You don’t need massive airflow to compensate for poor insulation anymore.
2. Energy efficiency became important: Fuel crisis of the 1970s, then environmental concerns, then profit margins drove manufacturers toward smaller, more efficient systems. Evaporator fans got smaller and consumed less power. Air velocity decreased from 4-5 m/s down to 2-3 m/s.
3. Cargo box sizes standardized smaller: Small truck refrigeration evolved toward 1-4 ton vehicles with 3-8 cubic meter cargo spaces. Proportionally smaller boxes but with the same bottom-intake design inherited from larger trucks.
4. Variable-speed technology emerged: Modern systems can modulate capacity, but they still use the same bottom-intake air distribution design from the fixed-speed era.

The Result:

You’ve got modern, efficient, small-capacity systems that can’t generate the brute-force airflow of their predecessors, but they’re using the same bottom-intake design. Natural convection now dominates the airflow pattern because fan velocity is no longer strong enough to overcome it.

The absurdity of pulling air from below (the cooler zone) while sitting in warm air (at ceiling level) is now fully exposed.

And nobody’s redesigned the fundamental layout because:

“This is how we’ve always done it.”

The evaporator unit comes from the factory with bottom intake and horizontal discharge. Body builders mount it on the roof following the installation manual. Nobody questions whether the intake should face a different direction. The template from 2023 looks like the template from 1973, which looked like the design from 1923.

The Physics You’re Fighting Every Single Day

Let’s put numbers to this. Because understanding the scale of the problem makes it clear just how stupid the current design really is.

Natural Convection is Powerful:

In a poorly-designed refrigerated space, natural convection (the movement of air due to density differences from temperature) can be ten times stronger than forced convection from undersized fans.

Here’s what that means practically:

• Your evaporator fan might move 400 cubic meters per hour
• Your cargo box is 5 cubic meters
• That’s nominally 80 air changes per hour – sounds like plenty, right?

But natural convection in a 1.4m high cargo box with an 8-10°C temperature difference between lower zone and ceiling creates a stable stratified layer. Your fan pulls air upward from below (fighting the natural tendency of cold air to stay down), cools it more, blows it horizontally, and watches it sink immediately back down.

You’re working against natural convection instead of with it.

The Temperature Stratification Problem:

Studies in refrigerated storage show that poorly-designed air distribution can create temperature gradients of 1-2°C per meter of height. In your 1.4m high cargo box, that could mean:

• Lower zone (0.3-0.5m height): -21°C (where your intake pulls from)
• Mid-height (sensor location): -20°C (setpoint)
• Upper zone/ceiling level: -17°C to -15°C (warmest, right around your evaporator)

That 6°C gradient means:

1. Products stored at different heights experience different conditions. Your frozen goods in the lower portion might be rock-solid while products near the ceiling are softer than they should be.
2. Your compressor runs longer. To bring ceiling-level air down to setpoint, you keep cooling the lower zone air, wasting energy and compressor run time.
3. Your evaporator is sitting in the warmest zone but can’t effectively cool it. The absurdity here is remarkable – the evaporator is physically located in the warmest air but its intake faces away from that warm air, pulling from the cooler zone below instead.

The Door Opening Death Spiral:

This is where the bottom-intake design really shows its weakness. Open your cargo doors during a Gauteng summer day (35°C ambient), and here’s what happens:

1. Heavy cold air pours out the bottom of the door opening (you can literally see the fog)
2. Light warm air rushes in at the top of the door opening
3. Warm air rises to the ceiling – right to where your evaporator is mounted
4. Warm air accumulates at ceiling level, surrounding your evaporator unit
5. Door closes
6. Your bottom-intake evaporator completely ignores that warm air sitting at ceiling level (literally above and around it!) and continues pulling cooler air from below

The warm air sits at the ceiling, slowly mixing downward through natural diffusion – which is slow. Your compressor runs longer trying to cool the overall space while that warm air pocket sits there at ceiling level, surrounding your evaporator unit, completely ignored by the bottom-facing intake.

If your evaporator simply had top-mounted or side-mounted intakes pulling from ceiling level (where it’s already mounted!), it would capture that warm infiltration air immediately and cool it within seconds. Bottom-intake design might take 5-10 minutes to fully recover. Multiply that by 15-30 stops per route, and you’re burning significant extra fuel every single day.

The Alternative Nobody’s Implementing: Work With Physics, Not Against It

Here’s what actually makes thermodynamic sense for roof-mounted small truck refrigeration, and it’s so obvious that the fact nobody’s doing it borders on malpractice.

Top/Side-Intake, Downward-Discharge Design:

Keep your evaporator roof-mounted – that position is fine. But reconfigure the intake to pull air from the top and sides of the unit (at ceiling level, where warm air accumulates). Redirect the discharge to blow downward toward the floor.

Why This Works:

1. Capture warm air where it accumulates AND where your evaporator already is: Heat rises to ceiling level. Your evaporator is at ceiling level. Why would you pull air from below when the air you need to cool is literally surrounding your unit?
2. Work with natural convection instead of against it: Warm air naturally wants to rise to the ceiling. Your intake at ceiling level captures it without fighting physics. Cold air naturally wants to sink. Discharge it downward and let gravity do the distribution work.
3. Create a natural circulation loop:
   • Warm air rises naturally to ceiling level (where your evaporator is)
   • Gets pulled directly into top/side-mounted intakes on the evaporator unit
   • Gets cooled
   • Gets discharged downward
   • Sinks naturally throughout the cargo space
   • Eventually warms slightly and rises back to ceiling level
   • Continuous natural circulation without fighting convection
4. Faster door opening recovery: Warm air from door openings rises straight to ceiling level where your intakes are located. Gets pulled into the evaporator immediately. Gets cooled and redistributed. Recovery time cut by 60-70%.
5. More even temperature distribution: By pulling the warmest air (at ceiling level where your unit already is) and discharging downward throughout the space, you minimize stratification. Top-to-bottom temperature difference might drop to 1-2°C instead of 6°C.
6. Lower fan power requirements: You’re no longer fighting natural convection. Warm air wants to flow into a ceiling-level intake. Cold air wants to sink from a downward discharge. Fan power can be reduced because you’re working with physics instead of against it.

The Practical Configuration:

• Evaporator remains roof-mounted (no change to mounting position)
• Modify intake configuration: Add intake grilles/openings on top and sides of evaporator housing to pull air from ceiling level
• Block or minimize bottom intake opening
• Redirect discharge downward at 45-60° angle toward floor (may require internal baffles or external deflectors)
• Potentially reduce fan speed/power since you’re working with natural convection

This isn’t exotic or unproven. Industrial cold storage facilities figured this out decades ago. Large refrigerated warehouses use overhead evaporators that pull from ceiling level precisely because it’s the most efficient configuration for temperature uniformity and energy efficiency.

But small truck refrigeration? Still using bottom intakes like it’s 1913.

Real-World Examples That Prove This Works

This isn’t theoretical. There are applications where ceiling-level intakes are standard with roof/ceiling-mounted evaporators, and they dramatically outperform bottom-intake designs.

Industrial Cold Rooms:

Walk-in cold rooms and freezers in restaurants, butcheries, and food processing facilities? Ceiling-mounted evaporators with ceiling-level air returns are standard. The units are designed to pull air from around the evaporator at ceiling level – often with intake grilles on the sides or top of the unit housing.

Every refrigeration contractor knows this is the correct design for cold spaces. Nobody would design a ceiling-mounted evaporator with a bottom-facing intake that ignores all the warm air at ceiling level.

But mount that same cold space on a truck chassis, and suddenly everyone accepts bottom-intake units. It’s the same physics, the same principles, but the industry standard is backwards on wheels.

Commercial Refrigerated Display Cases:

Vertical refrigerated display cases (like in supermarkets) use overhead evaporators that pull air from the top of the case – where warm infiltration enters and where the evaporator is located. The intake is positioned to capture the warmest air, not pull from the coldest zone.

The refrigeration industry already knows that capturing warm air where it accumulates (and where the evaporator is located) works better. They just haven’t applied this knowledge to small truck refrigeration.

Large Truck Multi-Temperature Systems:

High-end large truck refrigeration systems (8+ tons, multi-zone setups) often use more sophisticated air distribution with multiple intake points positioned strategically to capture warm infiltration zones.

The technology and understanding exist in the large truck segment. It just hasn’t trickled down to small trucks because nobody’s demanding it.

Why Hasn’t Anyone Fixed This? The Manufacturing Inertia Problem

Here’s the uncomfortable truth about why small truck refrigeration design is stuck in the past: evaporator units are manufactured as complete assemblies with bottom intakes, and nobody questions the design.

The Current Process:

1. Manufacturer (Thermo King, Carrier, etc.) designs evaporator unit as complete assembly
2. Unit comes from factory with bottom intake, horizontal discharge built-in
3. Body builder purchases complete unit
4. Body builder mounts unit on roof following installation manual
5. Unit installed as-is – no customization of intake/discharge configuration
6. Truck delivered to customer

Nobody in this chain has any ability or incentive to modify the intake configuration. The unit arrives as a sealed assembly. Body builders aren’t refrigeration engineers – they’re installers following a mounting template. The customer doesn’t know enough about thermodynamics to question intake placement.

The Feedback Loop Is Broken:

You, the operator, experience the consequences: higher fuel consumption, longer recovery times after door openings, temperature variation between lower and upper cargo zones, harder compressor cycling. But you have no idea these problems stem from bottom-intake configuration.

You assume it’s just how refrigeration works. You don’t complain to the body builder because they just mounted what the manufacturer supplied. You don’t complain to the manufacturer because you assume they’re the experts. The manufacturer never hears that there’s a problem. The unit design never changes.

The Historical Inertia:

Bottom intakes made sense when:

• Fan power was enormous and could overcome natural convection (1920s-1980s)
• Cargo boxes were poorly insulated and needed brute-force air circulation (pre-1990s)
• Units were designed for large trucks where the intake-to-ceiling distance was significant

None of these conditions exist for modern small truck operations. Fan power decreased for efficiency. Insulation improved dramatically. Small truck cargo boxes are only 1.2-1.4m tall – there’s minimal space between the bottom intake and the ceiling where warm air accumulates. But the bottom-intake design persisted through every evolution because “this is how our units have always been designed.”

What Would It Take To Fix This? (And Why You Should Care)

Let’s talk specifics. What would actually need to change, and what would it cost?

For Manufacturers:

The evaporator unit itself would need redesign:

1. Add top/side intake grilles: Modify the evaporator housing to include intake openings on top and sides of the unit. This captures ceiling-level air where warm air accumulates.
2. Minimize or baffle bottom intake: Reduce the bottom intake opening or add internal baffles that preferentially pull air from top/side intakes.
3. Redirect discharge downward: Add internal baffles or modify discharge configuration to direct airflow downward at 45-60° instead of horizontally.

Development cost: This is a one-time engineering modification to the evaporator housing design. For a manufacturer producing thousands of units annually, the per-unit cost impact would be negligible – maybe R500-R800 additional manufacturing cost.

The Fuel Savings Math:

Let’s be conservative. Assume top/side-intake with downward discharge improves refrigeration efficiency by just 20% (actual improvements could be 25-35% based on working with natural convection instead of against it).

• Typical refrigerated 1-ton truck: 12 liters/100km fuel consumption
• Refrigeration penalty: ~3 liters/100km (25% of total consumption)
• 20% improvement: 0.60 liters/100km saved

Over 50,000km annually (typical courier operation):

• 300 liters diesel saved annually
• At R22/liter: R6,600 annual savings
• Additional unit cost: R500-R800

ROI for operator: Payback in 1-2 months. Then R6,600+ annual savings forever.

Scale that across a fleet of 5 trucks over 10 years: R330,000 saved. From redesigning the intake configuration.

The Retrofit Challenge: Can You Fix Your Existing Truck?

For operators with existing bottom-intake evaporators, retrofitting is challenging but not impossible.

What Would Be Required:

1. Create top/side intake openings: This requires modifying the evaporator unit housing to add intake grilles at the top or sides. Potentially cutting into the housing (which may void warranties and affect unit integrity).
2. Baffle or block bottom intake: Partially block the existing bottom intake to force more air to enter through new top/side openings. Complete blocking might cause airflow issues, so careful baffling is needed.
3. Add discharge deflectors: Install deflectors at discharge outlets to redirect airflow downward instead of horizontally.

The Problems:

• Modifying a sealed evaporator housing risks warranty violations
• Cutting into the housing could affect refrigerant line integrity or structural strength
• Proper baffling requires understanding internal airflow dynamics
• Labor cost could be R8,000-R15,000 for a quality retrofit by a skilled technician

The Reality Check:

Retrofitting existing units probably doesn’t make financial sense unless you’re already doing major evaporator replacement. The risk and cost are too high for existing equipment.

But this information is crucial for your next truck purchase. You now understand the fundamental design flaw. You can demand better from manufacturers.

How To Actually Challenge This Industry

If you’ve read this far and you’re thinking “this is so obvious, why hasn’t anyone done it?” – you’re right. It is obvious. Which makes the fact that nobody’s doing it even more frustrating.

The Design That Makes Sense:

• Your evaporator is on the roof (where warm air naturally accumulates)
• Pull air from the top and sides (where your evaporator is, and where heat accumulates)
• Discharge downward (working with gravity instead of against it)
• Work with natural convection, not against it

The Design We’re Actually Using:

• Your evaporator is on the roof (correct)
• Intake faces downward, pulling from below (ignoring warm air surrounding the unit)
• Pull cooler air that’s already sunk (wasting energy to cool it more)
• Discharge horizontally (so it sinks back down immediately)
• Watch the warm air at ceiling level stay warm (despite surrounding the evaporator)
• Fight natural convection constantly

It’s thermodynamic insanity.

Here’s how to actually force change:

For Individual Operators:

1. Demand answers from manufacturers. When purchasing a new truck, ask your dealer: “Why does this evaporator have a bottom intake when it’s mounted at ceiling level where warm air accumulates?” Make them explain the thermodynamics. Most can’t, because there is no good explanation.
2. Contact manufacturers directly. Write to Thermo King, Carrier, and other manufacturers. Ask them to explain why their small truck evaporators use bottom intakes that ignore ceiling-level warm air. Demand they develop top/side-intake models for small truck applications.
3. Document and share your temperature data. Use data loggers to document the temperature stratification in your cargo box. Show the 5-6°C difference between lower zone and ceiling level. Publish it. Make the problem visible.

For Manufacturers (Thermo King, Carrier, etc.):

You know this design is suboptimal. Your engineers understand thermodynamics. Your industrial cold room divisions already use ceiling-level intakes on ceiling-mounted evaporators.

Why are your small truck evaporator units still using bottom intakes that ignore ceiling-level warm air?

Develop a “high-efficiency” model with top/side intakes and downward discharge. Market it as “physics-based design” or “natural convection optimized.” Charge a R800-R1,000 premium. Operators will pay it gladly for R6,000+ annual fuel savings.

Be the first manufacturer to offer this. Own the innovation space in small truck refrigeration.

For Industry Associations and Fleet Operators:

Commission a simple comparison test: Two identical trucks, one with standard bottom-intake evaporator, one with a modified top-intake configuration. Same routes, same loads, same ambient conditions. Document the fuel consumption difference over 3-6 months.

When the top-intake truck shows 20-30% lower refrigeration fuel consumption and better temperature uniformity, publish it. Make it impossible for manufacturers to ignore.

For Innovative Body Builders or Refrigeration Contractors:

Is there a way to modify intake configuration as an aftermarket solution? Partner with a manufacturer or develop a retrofit kit that adds top/side intakes with proper baffling. You could create an entirely new product category: “efficiency upgrade kits for roof-mounted evaporators.”

First mover advantage is enormous in a market where nobody’s questioning the status quo.

The Challenge: Who’s Going To Be First?

The physics is undeniable. The economics are compelling. The problem is obvious once you understand it.

Your evaporator sits in warm air but pulls from below it. This is like having a water pump sitting in a pool of water but using a hose to pull water from 2 meters away. It makes no sense.

Someone will eventually fix this. The question is whether it’ll be a South African innovator, a local operator group demanding change, or whether we’ll wait another decade for international manufacturers to notice we exist.

The Current Situation Is Absurd:

We’re running temperature-controlled logistics in 2025 using evaporator units that:

• Sit in the warmest air zone but pull from the cooler zone below
• Ignore the warm air literally surrounding the unit
• Fight natural convection by pulling air upward from below
• Discharge horizontally so cold air immediately sinks back down
• Create 5-6°C temperature stratification unnecessarily
• Burn extra fuel daily to overcome these inefficiencies

All because evaporator units come from the factory with bottom intakes and nobody questions whether there’s a better way.

How much longer can you afford to run systems that ignore basic thermodynamics?

Your Move

If you’re an operator: Contact your refrigeration unit dealer and manufacturer. Demand they explain the bottom-intake design. Ask when they’ll offer top-intake models for small trucks. Make noise. Be the customer who questions the status quo.

If you’re a manufacturer: Be the first to develop a top/side-intake evaporator for small truck applications. The operator who saves R6,000+ annually in fuel will tell every other operator they know. Word spreads fast in this industry.

If you’re an industry association: Commission the comparative testing. Prove the fuel savings. Give operators the data to demand change.

And if you’re just someone who thinks “this is interesting but not my problem” – every frozen good you’ve ever ordered was probably transported in a truck where the evaporator sits in warm air but is configured to pull from below it, fighting physics and burning extra fuel for no reason except “this is how the unit was designed.”

The refrigerated courier industry needs manufacturers who understand thermodynamics as well as they understand manufacturing. Top/side-intake evaporators for roof-mounted small truck applications is the obvious first step.

Who’s ready to actually design it?

The Frozen Food Courier is a family-owned, specialized temperature-controlled last-mile courier operating in Gauteng and the Western Cape. We’re not refrigeration engineers, but we’re operators who pay attention to physics and economics. If you’re a manufacturer willing to develop top-intake evaporators for small trucks, or an operator group interested in collectively demanding better solutions, we’d love to hear from you. And if you’re as frustrated as we are about obvious problems that nobody’s fixing, you’re our kind of people.

Why Transparency Matters

Sharing this information doesn’t protect a competitive advantage. This information is shared because the cold chain logistics industry is plagued by technical complacency and acceptance of preventable waste and inefficiencies. Operators don’t challenge equipment manufacturers. Manufacturers don’t optimize for courier operations. Everyone accepts waste and inefficiencies because “that’s how it’s always been done.“

This is part of a broader mission: challenge industry complacency through confrontational, physics-based analysis that exposes the real costs of accepting inadequate equipment and obsolete logic.

• Because the cold chain logistics industry accepts too much stupidity as “industry standard.“
• Because operators don’t challenge manufacturers, and manufacturers don’t optimize for actual operational conditions.
• Because everyone treats waste and inefficiency as an inevitable cost rather than a solvable engineering problem.
• Because confrontational technical transparency is the path to industry improvement.

This isn’t rocket science. It’s applying industrial refrigeration principles that have existed for decades to transport refrigeration operations. The industry just hasn’t bothered because operators weren’t demanding it.

Demand it.

Copyright © 2025 The Frozen Food Courier. This article may be shared freely with attribution. The calculations, methodologies, and implementation approaches described are based on our operational experience and are offered to advance industry-wide engineering practices in transport refrigeration.