Hydronic and Central Heating Explained (Alde and Diesel Hydronic)

Hydronic heating is the quietest, most even way to warm a caravan, and it heats your shower water at the same time. Here is how it works, why Alde leads the market, where diesel hydronic fits in, and what it is actually like to live with one.

What Hydronic Heating Actually Is

Hydronic heating is central heating for your caravan, the wet kind, the same principle as the radiators in a cold-climate house. A central boiler heats a mix of water and glycol, and a small 12V pump circulates that warm fluid through a closed loop of pipework and slim convectors run around the van. The glycol matters: it stops the fluid freezing on a sub-zero night and inhibits corrosion inside the system.

The convectors usually sit low along the walls at skirting-board height, and the pipework often runs under the floor as well. Heat radiates and convects gently off all of it, warming the whole van from every side at once. Compare that to a ducted air heater, which blows hot air out of one or two floor vents. The air heater warms the van faster, but it does it unevenly, toasty right by the vent and cold at the far end.

The other half of the appeal is hot water. The same boiler almost always heats your domestic hot water at the same time, through a coil or heat exchanger built into the unit. One box heats the van and fills the shower. That dual job is a big part of why people option hydronic in the first place, and it is the single most useful thing to understand about the category. A plain diesel air heater does not touch your hot water. A hydronic system does both.

Alde, the Market Leader, and Diesel Hydronic

When people say hydronic heating in a caravan, they usually mean Alde. The Swedish brand (now owned by Truma) is the reference point for true whole-van wet heating, and its current boiler, the Alde Compact 3020 HE, is what you will find factory-fitted in higher-end European imports and a handful of premium local builds. It heats an 8.4 litre glycol reservoir and pushes it out through convectors laid to the van’s floor plan, warming the cabin by natural convection with nothing more than a quiet pump doing the work. In the living space it is effectively silent.

The alternative path is diesel hydronic. Instead of a gas boiler, these systems use a small diesel combustion furnace, typically an Eberspacher or Webasto unit from the same families used in truck and marine coolant heating, to warm a tank of coolant. That heat then runs through a heat exchanger to make hot water, and optionally through fan-fed heat exchangers or convectors for cabin air. So they are hydronic at heart, they heat a fluid, but they are usually delivered as strong diesel hot water plus optional blown air rather than Alde’s whole-van radiant convector layout. In Australia the notable specialist is Dieselheat in Tasmania, who build their systems around Eberspacher furnaces.

There is a middle option worth knowing about too. The Truma Combi is not full wet hydronic, it is a hybrid air-plus-water heater that blows warm air through ducting and heats a 10 litre tank, but it is what most lappers actually compare hydronic against. The diesel version, the Combi D, does the same job burning diesel instead of gas. It is lighter and cheaper than a full Alde system, which is exactly why it is the mainstream default.

Energy Modes: Gas, Electric, Diesel

This is where hydronic earns its keep for Big Lappers, because the boiler can usually run on more than one energy source. Which modes you get depends on the system, but the choices break down like this:

• Gas (LPG): the workhorse mode when you are off grid. Efficient heat with no mains needed, though it draws on your gas bottles. This is how an Alde runs at a free camp.
• 240V electric: plug in at a powered site and heat on the site’s power instead of your own fuel. On a long-stay powered site this effectively saves your gas or diesel entirely.
• Diesel: draws from a dedicated tank, or the vehicle tank in a motorhome. This is the single-fuel appeal, one fuel for the tow vehicle and the heating, and no LPG to manage.
• Combinations: the better systems let you run gas and electric together for a faster warm-up on a cold morning, or pick whichever fuel is cheapest for the situation. Alde and the Truma Combi both do gas plus electric.

The practical upshot: if you spend a lot of nights on powered sites, the electric element is close to free heat and you barely touch your gas. If you free camp for weeks at a time, you lean on gas or diesel and the flexibility to switch means you are never stuck. A diesel hydronic system is the pick for the committed off-gridder who wants to run one fuel and forget about bottle swaps in remote towns.

Why the Warmth Feels Different

Anyone who has spent a winter in a van with a ducted air heater knows the pattern: a jet of hot air near the vent, a cold patch by the bed, and a fan that hums all night. Hydronic solves all three at once, and the difference is the main reason people pay for it.

The heat is even. Because the warmth comes off pipework and convectors spread around the whole perimeter, there are no hot or cold spots. The van sits at one comfortable temperature end to end. Run the pipework under the floor and you get warm floors underfoot, which is a genuine luxury on an alpine or Tasmanian winter morning.

The heat is gentle and silent. There is no fan roaring air across the cabin, just quiet convection, so the living space stays properly quiet. For light sleepers and couples that alone can justify the system. The only combustion noise is at the boiler, and that is usually well away from where you sleep.

It also helps with condensation. Convectors placed under the windows throw up a curtain of warm air against the cold glass, which cuts the radiant chill and reduces the condensation that plagues vans in cold weather. For full-timers living aboard through winter, drier windows and warmer surfaces are a real comfort and health benefit, not just a nicety.

The Trade-Offs: Weight, Cost, Install

Hydronic is the best heat in a caravan, and it is also the most expensive and the heaviest way to get warm. Those trade-offs are real and worth going in with your eyes open.

Weight is the big one. A boiler, metres of pipework, convectors around the van and several litres of glycol all add up to real payload. On a rig already close to its GVM, that is weight you may not have to spare, and a light diesel air heater starts to look very sensible.

Cost is the next. Hydronic is the dearest heating you can put in a van, both the hardware and the labour. In Australia it is realistically a factory-fit option that adds several thousand dollars to the price of the van, because Alde in particular is almost never sold as an off-the-shelf retrofit here.

That leads to the install. Routing pipework and convectors around and under a van is a serious job, which is why it is nearly always done at the factory or by a specialist. It is not a weekend DIY retrofit the way a diesel air heater can be. If you are upgrading an existing van rather than buying new, a diesel air heater or a Truma Combi is far easier and cheaper to fit.

Living With One

Day to day, a hydronic system is easy to live with, and most of the interaction comes down to choosing an energy mode. Pull onto a powered site and switch to electric to save your fuel. Head bush and run on gas or diesel. On a bitter morning, run gas and electric together to bring the van up to temperature faster, then drop back to a single source to hold it there. Set a timer so the van is warm before you climb out of bed and you have most of what makes hydronic pleasant to own.

There is a bit of setup and upkeep the category demands. A new or freshly serviced system needs to be primed and bled, filling the loop with glycol and working the air out of the pipework, so the fluid circulates properly and you do not get cold sections. It is not hard, but it is a step a straight air heater does not have. Get it wrong and part of the van stays cold.

The main ongoing task is the glycol itself. It should be changed roughly every two years to keep its corrosion and bacterial protection working. That is a genuine ownership cost and a job to budget for, on top of the usual furnace servicing any combustion heater needs. None of it is onerous, but hydronic has more parts in the loop than a simple air heater, and more parts means more to look after.