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BTU & UA Explained: How to Spec a Reefer Unit
From the basics any driver can follow to the numbers behind every unit selection — including a full interactive load calculator built on Carrier Transicold methodology.
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01
Level 1 — The basics
What Does a Reefer Unit Actually Do?
A reefer unit is a heat pump — not a cold generator. It doesn't create cold. It removes heat from inside the box and dumps it outside. The faster it can move heat, the better it protects your load from temperature excursions.
Think of it this way
Your reefer unit is a bouncer at the door. Heat keeps trying to sneak into the box. The bouncer's job is to throw it back out faster than it gets in. Two numbers control whether the bouncer wins: how strong they are (BTU), and how many cracks in the door (UA).
Everything else in reefer spec — unit selection, trailer insulation grade, door seal maintenance — comes back to these two numbers and how they relate to each other.
02
Level 1 — The basics
BTU — The Unit's Muscle
BTU stands for British Thermal Unit. For cold chain purposes: a reefer unit rated at a higher BTU/hr can remove more heat per hour. More BTU = more muscle. Ratings are always stated per hour (BTUH).
7,000
BTU/hr
Small truck body
Small truck body
9,500
BTU/hr
Standard trailer unit
Standard trailer unit
13,500
BTU/hr
High-capacity unit
High-capacity unit
12,000
BTU/hr
= 1 ton refrigeration
= 1 ton refrigeration
Key rule — drivers & dispatchers
Hotter outside = less BTU from your unit. A unit rated at 9,500 BTU/hr may only deliver 7,200 BTU/hr on a 38°C (100°F) summer day. Always pre-cool your box before loading — trying to pull down a hot box cuts your effective capacity dramatically.
Pulldown vs. Maintain Capacity
Manufacturers publish two figures: pulldown capacity (BTU needed to bring a warm box down to setpoint) and maintain capacity (BTU needed to hold it once there). Pulldown demands far more energy. Drivers loading warm product or making frequent door opens create pulldown conditions even mid-run.
03
Level 1 — The basics
UA — How Leaky Is the Box?
UA measures how fast heat seeps through the walls, floor, ceiling, and doors of the box. A low UA means heat leaks slowly. A high UA means the box bleeds cold fast — and your unit has to work harder just to hold temperature.
100°F
Ambient
Insulation (UA value)
››
35°F
Setpoint
Think of it this way
UA is the number of cracks in your box. A new, well-sealed trailer has few cracks (low UA ≈ 130). An old trailer with worn door seals and moisture-absorbed foam has many cracks (high UA ≈ 250+). More cracks = more work for your unit.
04
Level 2 — How they connect
The Temperature Difference Multiplier
Heat doesn't leak at a flat rate — it leaks faster the bigger the gap between inside and outside temperature. A box running at 35°F on a 100°F day is fighting a 65-degree battle. The same box in 50°F weather only fights a 15-degree battle — over four times less heat pressure.
Heat Leak (BTUH) = UA × ΔT
UA = box leakage rate (BTU/hr·°F) · ΔT = ambient temp − setpoint temp (°F)
Worked example — Manitoba summer
UA = 78.8 (like the milk truck in this calculator) · Ambient = 89°F · Setpoint = 36°F
Heat leak = 78.8 × (89 − 36) = 78.8 × 53 = 4,176 BTUH base load
Add solar gain, degradation, and door recovery — total reaches ~7,735 BTUH. A unit at 9,500 BTUH has workable headroom.
What Makes Up the Total Heat Load?
Transmission (UA × ΔT) is the baseline, but not the only load. On a real multi-drop distribution route:
55%
Transmission (UA × ΔT)
25%
Door opens / infiltration
12%
Product & respiration heat
8%
Solar radiation on roof/sides
Multi-drop alert
On high-frequency delivery routes (12+ door opens, 5+ minutes each), infiltration can jump to 35–40% of total load. Spec your unit accordingly or install door curtains. The load calculator below includes door opening recovery in its calculation.
05
Level 3 — Spec engineering
UA in Detail: U × A Unpacked
UA is a product of two things. U is insulation quality per square foot — how easily heat moves through the material (lower = better). A is the total surface area of the box. A 53-ft trailer has more surface for heat to enter than a 24-ft truck body, even with identical insulation.
UA = U (BTU/hr·ft²·°F) × A (ft²)
U improves with insulation thickness (polyurethane k=0.134) · A is set by body dimensions · Efficiency factors apply per surface
Carrier Transicold's load calculation methodology — the same used in the calculator below — computes UA surface by surface: nose, roof, road-side wall, curb-side wall, floor, and rear doors. Each gets its own insulation type, thickness, and efficiency factor.
UA Reference Values by Equipment Type
| Equipment | Typical UA | Rating | Notes |
|---|---|---|---|
| New 53-ft reefer trailer | 130 – 180 | Good | Standard foam panel |
| High-spec insulated trailer | 80 – 130 | Excellent | Multi-point door seals |
| 24-ft truck body (polyurethane) | 70 – 100 | Good | Smaller surface area |
| Aging trailer (>8 yrs) | 180 – 320+ | Degraded | Seal wear, wet foam |
| ISO refrigerated container | 60 – 100 | Excellent | Steel, tight tolerances |
| Dry van conversion | 200 – 280+ | Poor | Retrofit insulation only |
Frozen vs. Fresh — Why ΔT Changes Everything
| Load Type | Setpoint | ΔT at 100°F | Leak (UA=150) |
|---|---|---|---|
| Fresh produce / dairy | 35°F | 65°F | 9,750 BTUH |
| Deli / fresh meat | 34°F | 66°F | 9,900 BTUH |
| Ice cream | 0°F | 100°F | 15,000 BTUH |
| Deep frozen | -10°F | 110°F | 16,500 BTUH |
A 9,500 BTUH unit comfortably handles fresh loads. The same unit fails completely on frozen. Frozen specs require UA below 120 and units above 13,000 BTUH at worst-case ambient — plus confirmed low-temp capacity, which drops significantly at extreme ΔT.
"Track your trailer UA every year. A box at 150 UA when new can quietly degrade to 240+ by year eight — pushing it past the original unit spec. Most product loss events trace back to UA drift nobody measured."
AVRO Parts — Winnipeg, MB
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Asset Management Tips
Annual UA testing
Run a temperature differential test every 12 months. Log UA per asset. A rising trend is an early warning before product loss claims start.
Moisture flag
Unexpected high UA on a young trailer usually signals moisture in the foam — from a wall breach, flood, or pressure-wash damage. Repair early before full saturation.
Reassignment rule
When UA degrades past spec for your unit, move that trailer to shorter routes with lower ambient exposure. Don't leave it on summer long-haul lanes.
06
Level 3 — Spec tool
Refrigeration Load Calculator
Based on the Carrier Transicold load estimation methodology. Enter your body dimensions, insulation, operating territory, and product — and get a full cooling and heating load estimate including door opening recovery, solar gain, and product respiration.
Load Estimation Tool
Carrier Transicold Methodology · v2.0 · Avro Parts Knowledge
Body Information
ft length
ft width
ft height
Insulation
Operational Data
Design High: 89°F · Design Low: -39°F · Mean Daily: 78°F
Product Information
°F
lb
Door Openings
Estimated Cooling & Heating Load
Calculated UA
—
BTU/hr·°F
Max Cooling Load
—
BTUH (summer peak)
Mean Daily Load
—
BTUH (average)
Heating Load
—
BTUH (winter peak)
Load Breakdown (max cooling)
Base (UA×ΔT)
—
Solar Gain
—
Degradation
—
Door Recovery
—
Respiration
—
Min recommended airflow (truck units)
—
cfm