Operating temperature / resulting dry temperature

The operative temperature (also called dry resultant temperature) is the indicator that combines the effect of air and wall radiation to describe what an occupant actually feels. It combines the air temperature (Ta) and the average radiant temperature (Tr), with a weighting that depends on air velocity: when the air is almost still, radiation counts as much as air; when the air moves more, it contributes more to the perceived temperature. In short, it's the useful unit for managing comfort in buildings, especially with radiant floors, large windows, or active ceilings.

Operative temperature: definition, formulas and measurement

General formula : To = A·Ta + (1 − A)·Tr, with A a function of air velocity.

At low speed (≈ ≤ 0.2 m/s), we often take To ≈ (Ta + Tr)/2.

At moderate speed (≈ 0.2–0.6 m/s), we can aim for A ~ 0.6 (air counts a little more).

Average radiant temperature (Tr) : weighted average of visible surface temperatures (walls, windows, ceiling, floor) according to their solid angles. Cold or hot glazing can cause Tr to drop or rise… even if Ta has hardly changed.

Practical measurement : black globe thermometer (approximate To), air probe + radiant probe pair, or combined operational sensors; position the probes well (occupied area, away from direct inputs).

Control : in radiant zones, aim for a To setpoint rather than a pure Ta; integrate air speed, sunshine and occupancy into the regulation.

Advantages, limitations and points of attention for operative temperature

Benefits (for design and operation)

• Comfort relevance: correlates better with the actual sensation than a single temperature, especially with cold/hot surfaces.
• Targeting actions: reveals radiative problems (unprotected glazing, cold walls), therefore a better ROI than "pushing the regulation".
• Intelligent control: allows lower settings in winter and higher settings in summer with equal comfort (kWh savings).

Boundaries

• Instrumentation to be provided (globe/radiating probe) and careful installation.
• Local heterogeneities: radiant asymmetry near a bay, local air current → The "average" does not tell the whole story.
• Dependent on usage: clothing and metabolism remain determining factors (To does not include all PMV).

Points to consider

• Building envelope: address Ubat, thermal bridges and external solar protection (g/BSO) to stabilize Tr.
• Glazing: aim for TL/g adapted by orientation; in winter, limit cold walls (high-performance glazing, diffuse warm air curtains).
• Diffusion: avoid drafts (speed < 0.15–0.2 m/s in offices), otherwise A rises and comfort drops.
• Local radiation: low temperature ceilings/floors, uniformity of active surfaces to limit asymmetry.
• Regulation: integrate sunlight (facade sensor) and occupancy (presence/CO₂ sensors) to correct the operational setpoint.

Anecdote — “The glass that heated Nancy”

In Nancy, southwest-facing offices reported an air temperature of 24°C, but persistent discomfort at 3 p.m. The ambient temperature (OT) measured by the globe climbed to 26.5°C due to a high thermal resistance (unprotected windows + interior blinds installed too late). Solutions: external, sun-controlled, adjustable sunshades, operational rather than air-based temperature control, and a slight increase in localized air velocity during the afternoon. Result: the perceived temperature was maintained at 24°C, complaints plummeted, without forcing the production of cooling.