Eberspächer Airtronic D2

Diesel air heater: it burns fuel from the vehicle's own tank and blows the heat into the cabin, so the 12 V supply runs the fan, the glow plug and the fuel metering pump rather than producing the heat. The datasheet gives four output stages from 850 W to 2200 W, drawing 8 W at the smallest and 34 W at full output. Starting is the electrically expensive part — no more than 100 W while the glow plug is on. The manual never puts a single figure on how long that lasts; it gives two intervals instead, fuel supply starting about 65 seconds after switch-on and the glow plug going off 60 seconds after the flame sensor detects the flame. An undervoltage cut-out shuts the heater down when the supply stays below roughly 10.5 V for 20 seconds.

Two leads go back to the battery, red positive and brown negative, and the manual sizes them as one pair rather than individually: 4 mm² where positive plus negative comes to 5 m, 6 mm² from 5 to 8 m. Those sizes exist to hold the round-trip loss under 0.5 V, not to carry the running current. If the positive lands on a distribution panel instead of the battery, the cable feeding that panel counts toward the same total length.

The supplied three-position fuse holder sits in that positive lead and holds the heater's own 20 A fuse alongside the 5 A fuse on the tap that feeds the control unit.

The control unit — EasyStart Select, Timer, Remote+ or Call — is a separate connector, not a switch in the power feed. It takes its own positive off the 5 A fuse, with a negative and signal wires beside it, and a second pair runs back to the heater. The fuel metering pump has no supply of its own; it runs from the heater's harness.

Nothing in the positive lead should be opened while the heater is running or running on. It needs power for about four minutes after it is switched off, and the manual allows that to be cut short only for an emergency stop.

Putting the supply behind a switch, and turning that switch off at bedtime. Switching the heater off starts a shutdown sequence — fuel off, glow plug on for about 40 seconds to burn residue out of the burner, fan running roughly four minutes to cool the heat exchanger. Eberspächer's instruction is that the after-run must not be interrupted prematurely, naming the battery disconnecting switch as the way it happens, and the marine installation advice says of an emergency cut-off that performing one might damage the heater. Boat owners land in the same place: a YBW thread on whether the feed should have a switch answers "I counsel against any switch that can cut off the power to heaters, including battery isolator", and settles on pulling the fuse when the heater has to be isolated.

The corollary gets misread. A permanently live feed is not the default: the marine installation guide prefers the boat's main battery isolator, wired so the heater cannot be used once the isolator is off. It moves the feed to the permanently live side only when a timer or remote does the switching, and the reason it gives is that the heater has to stay operable with the isolator off — not that the isolator would truncate a shutdown.

• The supplied three-position fuse holder in the positive lead: 20 A for the heater, 5 A for the control unit tap
• 4 mm² for a positive-and-negative pair up to 5 m combined, 6 mm² from 5 to 8 m, per the manual's 0.5 V budget
• Control unit (EasyStart Select, Timer, Remote+ or Call) on its own connector, fed from the 5 A fuse
• Where a timer or remote does the switching, a feed from the permanently live side of the isolator; otherwise the isolator itself is the preferred connection point