How Radiant Heat Behaves Differently

Why this matters

Radiant heat does not travel with the air, does not follow the ductwork, and does not care what a thermostat reads. It travels in straight lines between surfaces, and how much arrives is set by the temperature of the surface sending it, the temperature of the surface receiving it, and how much of one's view the other occupies. A tech who does not have that model in their head will keep adjusting air and keep getting told nothing changed. A tech who has it will walk a space, look at what sees what, and predict where the complaint is before anyone points at it.

Before you go near a radiant emitter

An operating radiant tube or panel is hot enough to burn through a glove on contact. Do not steady yourself on a tube, a hanger, or a reflector, and take surface temperatures with a non-contact instrument from the floor or a stable platform rather than reaching. If you need hands on the emitter, shut it off at the appliance control and let it cool; there is no glove rating that makes touching a live radiant surface a reasonable plan.

Gas-fired radiant equipment is a combustion appliance. If you smell gas anywhere in the bay, everyone leaves immediately, nobody touches a switch, nobody turns a light on or off, nobody uses a phone inside the building, and the call to the gas utility is made from outside.

Combustion products are an inhalation hazard and the route no one protects for. Before spending an hour under operating heaters, take an ambient carbon monoxide reading with a calibrated instrument at breathing height in the occupied part of the bay. If it is elevated, shut the appliances down, ventilate, and find the venting or make-up air problem before you troubleshoot anything about comfort. A glove and safety glasses do nothing for this one; the control is shutdown and ventilation.

Clearance to combustibles is a listed value on the equipment's own label and in its installation instructions. It is not a judgment call and it is not something to estimate by eye while you are standing on a ladder, which is itself the other hazard of this job: set the ladder on level footing at the right angle, tie or foot it, and keep your instrument in a holster so both hands are available on the way up.

The signal

A metal fabrication bay, gas-fired radiant tube heaters overhead, two work benches. Complaint is that bench B is cold and bench A is comfortable. The crew at B has been running a portable electric heater under the table for a month.

Air temperature measured at 5 ft: 58 degrees F at bench A, 58 degrees F at bench B. Same number, twice, taken ten minutes apart with the same instrument.

That reading is the first thing worth noticing. In a radiantly heated space, matching air temperatures do not mean matching conditions, because air temperature is not the quantity the heaters are primarily delivering.

Hypotheses that did not survive

Airflow or distribution. There is no distribution system here. The heaters do not blow. Nothing about air movement in this bay is under anyone's control except the doors, and both benches sit the same distance from the nearest overhead door. Dead end.

A failed heater. All three tubes were firing. Surface temperatures taken with a non-contact instrument, aimed at an oxidized section of tube so the emissivity assumption was defensible, read around 400 degrees F on the burner-end sections. The far ends of the tube runs read around 300 degrees F, which is not a fault: a radiant tube is hottest near the burner and cools progressively along its length as it gives up heat, so a temperature gradient along the tube is what a working one looks like. Recording that gradient turned out to be the most useful thing measured that day.

Infiltration at bench B. Checked with a smoke source at the nearest wall joint and door seal on a windy afternoon. Some movement, nothing that would explain a persistent complaint at one bench and not the other twenty feet away.

What the sight line showed

Bench A sits directly under a burner-end tube section, about 12 ft below it, with clear view straight up.

Bench B is 18 ft from the nearest tube section, that section is a far-end run, and a pallet rack installed the previous month blocks roughly half of the bench's upward view of it.

Nobody had moved a heater. Somebody had moved a rack.

The arithmetic, in three multipliers

Radiant exchange between two surfaces goes with the difference of the fourth powers of their absolute temperatures. Absolute means Rankine here, which is degrees F plus 459.67, and the fourth power is the reason radiant output is so sensitive to emitter temperature.

Multiplier one, emitter temperature. Take the surroundings at 50 degrees F, which is 509.67 R, and compare a burner-end section at 400 degrees F (859.67 R) against a far-end section at 300 degrees F (759.67 R).

• Burner end: 859.67 to the fourth is about 5.462e11; minus 509.67 to the fourth, about 6.748e10, leaves a driving term of about 4.787e11.
• Far end: 759.67 to the fourth is about 3.330e11; minus the same 6.748e10 leaves about 2.656e11.
• Ratio: 2.656e11 / 4.787e11 = 0.55.

A 100 degree F drop in emitter surface temperature, which is only about 12 percent in absolute terms, cuts net radiant output by about 45 percent. That is the fourth power doing its work, and it is why emitter surface temperature is the single highest-value measurement on a radiant system.

Multiplier two, distance. For a compact source, flux falls with the inverse square of distance. For a long tube viewed from close in relative to its length, it falls closer to the inverse first power, because as you back away you also see more of the tube's length. Bench B at 18 ft against bench A at 12 ft, treating this as the line-source case, gives 12 / 18 = 0.67. If these had been compact panel emitters instead, the same geometry would give (12 / 18) squared = 0.44, so the source shape genuinely changes the answer and is worth identifying before you use either rule.

Multiplier three, view. Half the bench's view of the emitter is blocked by the rack, so 0.5. View factor is linear: block half, lose half. This is the only one of the three multipliers that a person can change in an afternoon.

Together: 0.55 x 0.67 x 0.5 = 0.18. Bench B is receiving something on the order of 18 percent of the radiant flux bench A gets, in air that is the same temperature to the degree.

What the arithmetic predicted, and what it ruled out

The crew's first instinct was to move the rack. Run that through the same three multipliers with view restored to 1.0 and everything else unchanged: 0.55 x 0.67 x 1.0 = 0.37.

Clearing the rack roughly doubles bench B's radiant flux and still leaves it at about 37 percent of bench A. That prediction is the reason the rack move was not sold as the fix. Two of the three multipliers, the far-end tube temperature and the 18 ft distance, are geometry of the heater layout, and no amount of housekeeping touches them.

The options that actually move the other two multipliers: relocate the bench under a burner-end section, extend or re-hang the tube run so a hotter section covers the bench, or add a reflector above and behind the bench to put more of the emitter back into its view. A reflector works on the view multiplier, not the temperature one, so it has the same ceiling the rack move has unless it is aimed at a hotter section.

What would flip this analysis: if the far-end section had measured 400 degrees F like the burner end, the first multiplier goes to 1.0 and the combined figure becomes 0.67 x 0.5 = 0.33, so the rack becomes the dominant term rather than the whole story: clearing it takes bench B from 0.33 to 0.67 of bench A, and the 18 ft of distance still costs a third that no amount of clearing recovers. Measure the emitter before you promise anything about geometry.

Confirming it without a radiometer

Most shops do not own an instrument that reads radiant flux. There is a serviceable field substitute.

Leave two identical small steel plates, painted flat dark on both sides, one at each bench, positioned the way a person's torso would be. Give them 20 minutes to settle, then read each plate's surface temperature with a non-contact instrument and subtract the local air temperature.

The plate at bench A settled well above air temperature. The plate at bench B settled barely above it. Use that as a rank ordering and as a before-and-after on the same plate at the same bench, not as a ratio: the plate reaches equilibrium through a fourth-power radiant balance against a linear convective loss, so its temperature rise is not proportional to the flux it received. It tells you which position is better and whether a change helped. It does not tell you by what factor.

Where this reasoning travels

The three multipliers are not an HVAC concept. They apply anywhere a hot or cold surface is in view of something you care about.

• A bare hot water line in a cool basement loses heat two ways at once, radiantly to every surface it can see and convectively to the air moving past it. Insulating it kills both, which is why insulation outperforms anything you do to the room air.
• A cold window in an occupied room runs the exchange the other way: the person is the warm surface and the glass is the cold one, so the person loses radiantly toward the glass regardless of what the room air is doing. The sibling article on air versus radiant temperature carries the occupant-side arithmetic for that case.
• An appliance cavity heats its contents by view. Something shadowed by a pan on the shelf above it is in the same situation as bench B behind the rack, and the fix is the same: change what sees what.
• A cold roof deck or a hot one is a large-area surface with a big view of everything below it, which is why a modest change in its surface temperature moves an occupied space more than its area alone suggests.

The portable rule: to change radiant transfer you change a surface temperature or you change a view. Changing the air changes convection, which is a different transfer path with its own arithmetic, and the two do not substitute for each other.

References

• Stefan-Boltzmann relationship for radiant exchange between surfaces, as presented in ASHRAE Handbook - Fundamentals, heat transfer chapter
• Manufacturer installation instructions and equipment labeling for listed clearance to combustibles on radiant equipment
• 29 CFR 1910.23 for ladder use in general industry, and 29 CFR 1926 Subpart X where the work falls under construction
• See related: How Air Temperature and Radiant Temperature Differ; What Emissivity Does to an Infrared Reading