What is a hot spot? What is a cold spot?
A hot spot is the structurally warmest location in a conditioned room or piece of equipment. A cold spot is the structurally coldest location. "Structural" is the key word here: it is not about a one-off peak, but about a location that is consistently warmer or colder than the rest throughout the measurement period.
They arise because air, cooling and heat sources are never perfectly evenly distributed. The average of a room therefore says little: it is precisely the extremes that determine whether your storage is reliable.
Why do hot spots and cold spots occur?
The temperature distribution in a room is the result of cooling or heating, airflow, insulation, loading and external influences. A few common causes:
- Cooling and air outlet. Right by the evaporator or air outlet the air is coldest, a typical cold spot.
- Doors and openings. When opened, warm air flows in; just behind the door a hot spot often forms.
- Heat sources. Lighting, motors, equipment or an exterior wall in the sun warm up locally.
- Stratification. Warm air rises. In tall rooms and racking warehouses it is warmer at the top than at the bottom.
- Poor air circulation. In corners, behind pallets or in dense racking, air can stagnate, causing deviating temperatures.
Where are hot and cold spots usually located?
| Location | Often found at |
|---|---|
| Hot spot (warmest) | Doors and openings, the top (stratification), near heat sources, against exterior walls or windows, where air flows back. |
| Cold spot (coldest) | Right by the cooling, the air outlet or the evaporator, and in cold rooms sometimes at the bottom or in an enclosed corner. |
These are rules of thumb, not guarantees: the actual location depends on your specific room, layout and loading. That is why you measure them rather than estimate them.
Why hot and cold spots are decisive
The average can sit nicely within the margin while a hot or cold spot falls just outside it. Two examples:
- Cold spot at 2–8 °C. A cold spot dropping towards freezing can freeze products. For many vaccines and biologicals that is irreversible damage, even if the average is correct.
- Hot spot at an upper limit. A hot spot can exceed the maximum storage temperature while the rest of the room is fine.
For audits under GDP, GMP, WHO or ISPE you must be able to demonstrate that every storage location in use stays within the limits, not just on average. Hot- and cold-spot analysis is the evidence for that.
How do you find hot and cold spots?
You find hot and cold spots with a temperature mapping: you place several calibrated data loggers at predefined measurement points and record the temperature continuously over an agreed period. By overlaying the curves, you see which positions are consistently the warmest and coldest.
The number and location of the loggers is justified risk-based. For a small fridge, about 9 measurement points is a logical starting point; for larger rooms you work with a grid at several heights. Read how many measurement points you need.
From hot and cold spot to monitoring position
A mapping is a one-off or periodic study; afterwards you monitor the room continuously with fixed sensors. The hot and cold spots found determine where those permanent sensors belong: ideally at or near the worst-case location for your product. A deviation becomes visible there first.
If the monitoring sensor is placed in a too-favourable zone, it gives a false sense of control. Read the difference between monitoring and mapping.
Common misconceptions
- "The average is fine, so everything is fine." The average hides precisely the extremes that matter.
- "The hot spot is always at the top." Often it is, but doors, heat sources and loading can move the actual hot spot.
- "One measurement is enough forever." Loading, season and changes can shift the spots; repetition is risk-based.
- "The existing sensor is surely placed well." Only a mapping shows whether that position is representative of the hot or cold spot.
Sources used
- EU GDP 2013/C 343/01, chapter 3.2 (in particular 3.2.1) on temperature mapping and placement of monitoring equipment at the locations with the greatest fluctuations.
- WHO TRS 961 Annex 9 Supplement 8, Temperature mapping of storage areas.
- ISPE Good Practice Guide: Controlled Temperature Chambers, 2nd edition.
- IGJ, Questions on the EU guideline on Good Distribution Practice (GDP), version 9.
Preparing for an audit? Also read what auditors want to see in a temperature mapping.