Diesel Hot Water & Heater Combos: How Long Does 10 Litres Really Last in the Shower?

It is one of the most common questions caravanners ask before laying out three or four thousand dollars on a diesel hot water and heater combo — does ten litres really get you a decent shower, or are you going to be standing under a freezing dribble after ninety seconds?

The short answer: yes, you get a decent shower from a 10-litre combo unit — but only if you understand the maths, pick the right showerhead, and use it sensibly. Here is the honest breakdown.

What a Combo Unit Actually Is

A combo unit is a single appliance that does two jobs from one diesel burner: it heats the cabin air and heats a small hot water tank, usually 9 or 10 litres. The most common units fitted to Australian caravans are:

• Truma Combi D 4 and Truma Combi D 6 — the dominant fit on Jayco, Avan, Coromal, Trakmaster and many other Australian builds. 10-litre tank. The "4" and "6" refer to heat output in kilowatts.
• Webasto Dual Top Evo — a 9-litre tank, less common in original equipment but a solid aftermarket choice.
• Autoterm Air & Water — a newer entrant, popular for off-grid builds with a separate calorifier.

These are different from a standalone diesel heater (like the budget 5kW Chinese units) which only heats cabin air, not water. If hot water is the goal, you specifically need a combo unit or a separate hot water service.

The big win of a combo is fuel economy and space — one burner, one fuel line, one flue, drawing diesel from your main vehicle tank. No second gas bottle, no second appliance, no separate flue.

The 10-Litre Shower Maths

Here is where most people get confused. A 10-litre tank does not mean a 10-litre shower. The tank heats water to around 60 to 70 degrees Celsius. Your shower needs to be at roughly 38 to 40 degrees Celsius to be comfortable. The mixer tap blends the hot tank water with cold from your fresh water tank.

The rough mix ratio from a 70-degree tank to typical cold water is about one part hot to one and a half parts cold for a comfortable temperature. So your 10 litres of tank water becomes roughly 25 litres of usable warm shower water at the showerhead.

Now the showerhead matters. A standard household shower runs at 9 to 12 litres per minute. A caravan low-flow showerhead — the kind fitted to most modern caravans — runs at around 4 to 5 litres per minute. Some ultra-low-flow camp showerheads run at 2 to 3 litres per minute.

Working the maths:

That is continuous running. With the navy-shower technique — water on to wet, water off to soap, water on to rinse — most people get two adults through one 10-litre tank with a low-flow rose. It takes practice, but it is the difference between a usable shower and a frustrating one.

How Long to Reheat

A Truma Combi D 6 reheats a 10-litre tank from 15 degrees to 60 degrees in roughly 25 minutes on diesel. The Combi D 4 takes a bit longer — closer to 35 minutes. The Webasto Dual Top sits in a similar range.

That means back-to-back showers are not realistic. Two adults can stretch a single tank between them with discipline. Three adults will need to wait. Four adults need a different plan altogether — either a second tank fill cycle between showers, or supplementary heating from a powered site.

For a couple, this rhythm works well. Shower in the evening, shower in the morning, with a heat cycle in between. The unit picks up the load while you sleep or while you are out walking.

Diesel Consumption — Less Than You Think

The combo runs off your main diesel tank. Consumption is modest:

• Truma Combi D 6 at full burn (water heat-up cycle): around 0.6 litres per hour
• Maintaining a hot tank or trickle heating: 0.2 to 0.3 litres per hour
• Typical 24-hour winter use (cabin heat overnight + two shower cycles): around 1.5 to 2 litres of diesel

That is well under a dollar a day at current prices. Cheaper to run than gas in most scenarios. The fuel line draws from your vehicle's main tank via a dedicated pickup tube, with a safety reserve so the heater shuts off before you can run the vehicle out of fuel.

If you are towing on diesel, the heater integration is seamless. If your tow vehicle is petrol, you will need a separate diesel jerry can plumbed in — Truma and Webasto both support this.

Combo vs Gas Hot Water vs Instant Gas

A combo is not the only way to get hot water in a caravan. The alternatives:

Gas hot water service (Suburban, Atwood, Truma B-14 storage units): typically a 22-litre tank, gas only or gas-and-240V. Twice the shower capacity per cycle. Twice the recovery time. Needs a gas bottle. Cheaper to buy. Bigger and heavier in the van.

Instant gas hot water (Truma B-14 Easy, Joolca Hottap, portable units): unlimited hot water as long as the gas bottle is fuelled and the cold supply keeps up. No recovery wait. Excellent for free camping with longer showers. Gas only — no diesel option. Mounts externally on some setups.

240V at park: the slowest recovery of all on storage units, but free if you are paying for site power anyway.

A combo wins when:

• You are touring in cold country and want air heat anyway
• You free-camp often and want one fuel source for everything
• Weight matters more than capacity (combos are lighter than 22L gas units)
• You are already on diesel for the tow vehicle

A separate gas storage HWS wins when:

• You only need hot water, not heat
• Two-plus adults need back-to-back showers
• You are mostly on powered sites where the 240V element handles recovery

Common Gotchas

A few things worth knowing before you commit:

Altitude derates output. Above about 1,500 metres, diesel combo units start losing heat output and can struggle with stable combustion. If you tour the Snowy or the Victorian high country in winter, factor this in.

Fuel quality matters. Cheap or stale diesel will cause carbon build-up in the burner and shorten service intervals. Both Truma and Webasto specify annual service if you use the unit regularly.

Cold-soaked startup in deep winter. A combo sitting at minus 5 degrees Celsius takes a few minutes to fire reliably. Some units have an electric pre-heater module to help. Not usually a problem in mainland Australia, but worth knowing for Tasmania and alpine touring.

The cabin heat and the water heat compete. When the burner is reheating the tank, less heat goes to the cabin air. Most users notice this for the 25 to 35 minutes of a heat cycle — not a problem, just expected behaviour.

Compliance plate weight matters. A Truma Combi adds around 14kg dry, plus 10kg of water when filled. If you are at the edge of your tare or payload, factor this in. KamperHub's compliance plate OCR captures the as-built tare from the plate — if your van shipped with a combo fitted, the weight is already in your figures.

The Bottom Line

A 10-litre diesel combo gives you a real, usable shower of around 5 minutes continuous, or 2 adults using the navy-shower technique, for under a dollar a day in diesel. It is the right choice for cold-country touring couples who free-camp and want one fuel source. It is not the right choice for families of four who shower back-to-back every night.

If you are weighing it up against a 22-litre gas storage unit, ask yourself how often you camp somewhere cold enough to want cabin heat. If the answer is "more than a couple of weeks a year," the combo earns its keep. If you only ever tour the coast in summer, a gas storage unit will probably serve you better.

If you are running tight on payload or want to map out where every accessory adds weight in your van, KamperHub's garage and weight distribution tools let you stack the combo unit (or any aftermarket gear) against your ATM and Tow Ball figures before you commit. The compliance plate OCR captures your as-built figures from a single photo — so you can see exactly how much room you have for additions.

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KamperHub Team

Helping travellers tow safely and confidently. KamperHub provides tools for trip planning, weight compliance, and adventure management.