Horse Trailer Heat: The Stopped-in-Traffic Bound — Cabin Warming, Ventilation Requirements, and Conditions Under Which No Passive Vent Design Suffices

Horses are routinely transported in enclosed trailers that are adequately ventilated only while the vehicle is moving. When traffic stops on a hot day, that ventilation largely disappears at the moment the heat load is greatest. This paper models the air temperature and humidity inside a stated two-horse trailer, stationary in sun and moving, using a lumped energy and moisture balance with stated assumptions throughout. In full sun the combined sensible load is 3282 W, of which solar gain through the roof and sun-side wall contributes 2292 W and the two horses contribute 990 W of sensible heat together with 810 W of latent heat, equivalent to 1.20 kg/h of water vapour. With the fitted passive vents the stationary cabin warms at an initial rate of 1.78 K/min, rises 5 K above ambient in 3.1 min, crosses 40 °C in 3.1 min when the ambient temperature is 35 °C, in 7.4 min at 30 °C, and in 15.3 min at 25 °C, and tends toward an equilibrium 18.5 K above ambient. Holding the cabin within 3 K of ambient under the same load requires a continuous airflow of 0.87 m³/s, about 305 air changes per hour. Buoyancy through vents alone would need a total free vent area of 8.55 m² to supply that flow, which cannot be fitted to this trailer, and the fitted ram-air inlets supply the required flow only at a road speed of about 60 km/h. A powered fan delivering the required flow draws 87 W at the stated pressure rise and is the only stationary option found that holds the stated limit indefinitely. The honest bound is therefore plain: on a hot sunny day a loaded trailer with passive vents only must not stop for more than a few minutes, and beyond those minutes the horses must be unloaded into shade or the vehicle must keep moving. Nothing in this paper is a measurement. SIMULATED, NOT MEASURED: nothing described has been built or tested. AI disclosure: modelling, computation and drafting assistance were provided by Dave, an artificial intelligence system, at the author's direction.