When we set out to improve the heating and air conditioning in the Land Rover Defender, we quickly realized that understanding some basic thermodynamics was key—especially something called Delta-T (ΔT).

Delta-T measures the temperature change between the air entering the heater and the air coming out. For example, if the outside air is 54°F and the air from the vents is 69°F, then your Delta-T is +15°F. Sounds good—but here’s the problem.

The Defender heater pulls air from outside, not from inside the cabin. This is known as an “open circuit”. That means on a cold day, say 23°F, even with an upgraded Delta-T of +15°F, the cabin only gets to 38°F. No matter how powerful the heater is, it’s still trying to warm freezing air from outside.

Modern vehicles use a “closed circuit” system, which recycles air already inside the cabin. This allows the air to be heated repeatedly, making the interior much warmer and more comfortable, while using less energy.

Another important factor is the fan. Bigger isn’t always better. If air moves too quickly through the heater, it doesn’t have time to warm up properly. A well-matched fan with adjustable speeds provides better control and comfort.

These same principles also apply to air conditioning—and even more so. That’s why running AC with the windows down is so inefficient, it’s now illegal in places like New York City.

Better temperature control starts with smart design, and that’s exactly what we’ve built into our HVAC systems.

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