Conduction, Convection and Radiation in the Field

Why this matters

Naming the three modes is a classroom exercise. Knowing which one is carrying most of the heat in front of you is a field skill, and it decides whether the thing you are about to change will do anything at all. Add a fan where the loss is radiant and you will spend a callback explaining why nothing improved. Wrap a line where the loss is running out through a bolted steel bracket and you will re-wrap the same line next year. Each mode answers to a different knob, and turning the wrong knob is the most common way a correct diagnosis produces a useless repair.

This card is organized around what each mode cannot explain, because that is what actually narrows a call. A sibling article covers the definitions and the underlying rate rule; the value here is the exclusions.

The knob each mode answers to

Read the right-hand column carefully. It is the column that tells you a proposed fix will not work.

What conduction cannot explain

Conduction needs contact. Heat crossing an air gap where nothing touches is not conduction in any meaningful amount, because still air conducts about as badly as the insulation people buy on purpose.

So conduction cannot explain a hot spot on a surface that is mechanically isolated from the source, and it cannot explain heat that appears on the far side of a genuine gap. What it does explain, far more often than techs expect, is a small metal part carrying an outsized share of the total. Metals conduct on the order of a thousand times better than fibrous insulation. Mild steel runs near 26 Btu per hr per ft per F, stainless roughly a third of that, and aluminum roughly four to five times mild steel; confirm against material data when the number is load bearing, because alloy and temper move it. Against that, a bolt or a bracket with a tiny cross-section competes with square feet of insulated surface.

Before probing any conductive path on operating equipment: read the surface with a non-contact instrument at a standoff first, because metal at burn temperature looks exactly like metal at room temperature, and pick hand protection rated for the temperature you just read under the hazard assessment at 29 CFR 1910.132(d)(1).

What convection cannot explain

Convection needs a fluid that can move. It cannot explain heat transfer across a sealed dead-air space, through a vacuum, or through a solid. It also cannot explain a surface that keeps heating when the air around it is already at the surface temperature, since there is no difference left to drive it.

The field version of this exclusion: if adding air movement does not change the reading, convection was not the dominant path. That is a test you can run in ten seconds with a portable fan before you quote a permanent fan, a louver or a duct modification.

Convection also has an asymmetry worth carrying. Natural convection, the drift caused by warm fluid rising on its own, is weak. Forced convection, fluid pushed by a fan or a pump, is far stronger and scales with velocity. That is why a coil with no airflow behaves almost like a coil with no load: the metal is fine, the difference is fine, and nothing is moving the heat away.

What radiation cannot explain

Radiation needs line of sight. Two surfaces exchange radiant heat in proportion to how much of each other's view they occupy, so a surface tucked behind an obstruction is out of the exchange no matter how hot the source is. Radiation also cannot be defeated by a shiny surface that is pressed flat against another surface, because at that point the two are in contact and you have converted the problem into conduction. A radiant barrier only works facing an air space.

Two properties decide radiant behaviour, and techs mix them up:

• Emissivity is how readily a surface radiates. Ordinary paint, oxidized metal, rubber, plastic and dirt all sit high, roughly 0.85 to 0.95. Bare polished metal sits very low, often under 0.1.
• View factor is the geometric share of one surface's view that the other occupies.

That low emissivity on bare polished metal has a direct instrument consequence: an infrared thermometer aimed at bright metal reads badly wrong, usually far too cold, because most of what it sees is a reflection of the room. Read a matte spot, a strip of electrical tape applied and allowed to reach temperature, or take a contact reading instead. This is not a fussy detail. It is the single most common reason a thermal reading and a contact reading disagree on the same part.

Radiation is also the only mode that gets dramatically stronger as things get hot, because it follows absolute temperature to the fourth power. Near room temperature it is a co-equal contributor. At flue and firebox temperatures it dominates everything else, which is why standing beside a hot surface is uncomfortable in a cool room.

Ranking the paths on one cabinet

A sealed control enclosure in a mechanical room. Skin reads 95 F on a matte painted panel with an infrared thermometer at roughly one foot; room air reads 70 F at the same moment, both recorded on the same notes line. Exposed skin is about 12 ft2. The cabinet sits on four steel feet bolted to a structural channel that reads room temperature.

Path 1 and 2, the surface. The combined still-air surface film for an indoor vertical surface is about R-0.68 in the ASHRAE Handbook Fundamentals, which is a total surface coefficient of 1 / 0.68 = 1.47 Btu per hr per ft2 per F. That published film is a combined number: it already contains both radiation and natural convection. Splitting it takes one more step. For a high-emissivity painted surface near room temperature, the radiant coefficient works out close to 1.0 Btu per hr per ft2 per F, which leaves roughly 0.47 for natural convection.

• Radiant share: 1.0 / 1.47 = about 68 percent
• Natural convection share: 0.47 / 1.47 = about 32 percent

Total surface loss: 1.47 x 12 ft2 x 25 F = about 440 Btu per hr.

Path 3, the feet. Four mild-steel feet, each roughly 1 in2 in cross-section, so 4 in2 total, which is 0.0278 ft2. Path length through the foot is about 2 in, or 0.167 ft. Using 26 Btu per hr per ft per F:

26 x 0.0278 x 25 / 0.167 = about 108 Btu per hr

What the ranking says. Four little feet with a combined cross-section the size of a coaster are moving about 108 Btu per hr, roughly a quarter of the 440 Btu per hr leaving 12 ft2 of skin. Radiation off the skin, at about 300 Btu per hr, is the largest single contributor and it is more than double the convective share.

Now check each proposed fix against the ranking. A small fan aimed at the cabinet attacks the 32 percent convective share, and it can raise that share meaningfully because forced convection beats natural convection, so it is a live option. Painting the cabinet a different color does nothing, because visible color and thermal emissivity are unrelated for ordinary paints: matte white and matte black both sit near 0.9. Wrapping the cabinet in insulation would cut the surface loss badly and make the inside hotter, which is the opposite of the goal for an enclosure that needs to reject heat. And isolating the feet, which looks like the tidy engineering answer, would remove a quarter of the total heat rejection from a cabinet that is currently coping.

The failure mode this ranking prevents. A tech who does not split the surface film assumes "warm cabinet, add airflow" and sizes a fan against the whole 440 Btu per hr. The fan influences the convective path only, delivers a fraction of the expected improvement, and the cabinet still runs hot. The number that would have caught it in advance is the 68 to 32 split, and it took one published film value and one coefficient to get there.

Where the ranking flips

The 68 to 32 split is a near-room-temperature result, not a law. Name the condition that moves it:

• Higher temperature moves it toward radiation, hard. Because radiation follows absolute temperature to the fourth power while convection is close to linear, a surface at flue temperature is overwhelmingly radiant, and the same insulation decision comes out differently.
• Any forced air movement moves it toward convection. In an airstream the convective coefficient can exceed the radiant one by a wide margin, which is why the same component behaves differently on a bench and in a duct.
• A low-emissivity surface removes radiation from the ranking almost entirely. Bare polished metal or a clean foil facing drops the radiant coefficient by roughly an order of magnitude. It also has to stay clean: dust and oxidation raise emissivity, and a dusty foil facing has quietly stopped being a radiant barrier.
• A small high-conductivity bridge can outrank both. The feet example is the mild case. A continuous metal support, a run of pipe leaving a jacket, or a metal fastener pattern through an assembly can carry more than the entire insulated area around it.

Checking you named the right mode

Each mode has a cheap confirming test that does not require disassembly:

• Suspect convection? Add or block air movement temporarily and re-read. If the surface temperature does not move, convection was not dominant.
• Suspect radiation? Put a piece of cardboard or a non-reflective shield between the hot surface and the surface you are measuring, without touching either, and re-read after a minute. A drop means radiant coupling; no change means the path was not line of sight. Never place a shield where it can contact a live conductor, a moving part, or a combustion path; if the space is inside an electrical enclosure, do this only after the circuit is de-energized, locked out under 29 CFR 1910.333(b)(2) and proved dead with the live-dead-live sequence in NFPA 70E-2021, 120.5.
• Suspect conduction? Scan along the suspected metal path away from the source. A conductive bridge shows a smooth temperature gradient with distance. A radiant or convective effect does not, it shows up wherever the geometry aims it.

If the reading disagrees with the mode you named, believe the reading. The commonest mislabel in the field is calling a conductive bridge an insulation failure, because the symptom sits on the insulated surface and the cause is the metal running under it.

References

• ASHRAE Handbook Fundamentals, surface film coefficients, emissivity tables and thermal conductivity data
• 29 CFR 1910.132(d)(1), PPE hazard assessment and selection for hot-surface contact
• 29 CFR 1910.333(b)(2), de-energized electrical work; NFPA 70E-2021, 120.5, live-dead-live verification
• See related: How Heat Actually Moves and Why It Matters on a Call; Why a Surface Feels Hot When the Air Does Not