Short answer: route the exhaust so it slopes down all the way from the heater to the outlet, point the end of the pipe down, and if your muffler has small drain holes, turn them to face down. A little fuel mist condenses inside the exhaust; it has to be able to drain out. If it pools, unburned fuel builds up and the heater can start to smoke later.

Why the slope matters

When the heater starts and shuts down, and on cold days, some fuel mist in the exhaust gas condenses on the cold pipe wall. With a continuous downward slope, that liquid runs out of the outlet. If the pipe rises toward the outlet, even only slightly, the liquid runs back and collects at the lowest point. Over time this unburned fuel accumulates and can lead to white smoke at start-up later on.

The small drain holes on the muffler

Many standard exhaust mufflers have one or two small holes in the body, often right next to the seam. They are drains: install the muffler so the holes face down and condensation can drip out. Turned up, the holes do nothing and the liquid stays inside.

A real muffler: the two small drain holes sit on either side of the seam. Mount it so they are underneath.

Integrated exhaust silencers: slope is even more important

The HLN integrated exhaust silencer does not have these drain holes. That makes the routing the only way for condensation to escape, so give the whole run a clear downward slope with no low points, and point the end down.

Exhaust installation checklist

• The exhaust outlet comes out from under the vehicle into open air, so the exhaust gas is carried away and does not collect underneath.
• The pipe slopes down along its whole length: no low points, and not even a slight rise toward the outlet.
• The end of the pipe points down.
• Muffler drain holes (if there are any) face down.
• Total exhaust length no more than 2 m. If you need to go longer, use a tube of at least 32 mm diameter.
• No bend tighter than 90°.
• The outlet does not face the direction of travel.
• Keep 300 mm below the outlet clear, and make sure snow or mud cannot block the pipe end.
• Fix the pipe securely with clips so it cannot vibrate, and keep wiring, rubber hoses and other non-heat-resistant parts away from it.

Seeing white smoke at start-up? An incorrectly installed exhaust is one of the most common causes, so go through the checklist above first. Source: HLN installation manuals (Aerolyn 2kW/4kW air heaters, Aquano coolant heaters) and HLN technical support.