How an Alde Hydronic System Works

The Alde system is being installed these days on more and more RVs, especially Roadtreks,  so it might be useful to explain what an Alde system is and how it works. Follow along and be astounded by the ingenuity of this Swedish marvel, and how much easier it makes life for the Roadtrek owner.

This is a cutaway view of the Alde unit.

This is a cutaway view of the Alde unit. The propane combustion area and electric heating elements are in the center, surrounded by the glycol plumbing, surrounded by the water jacket.

The easiest way to understand the Alde is to think of it like the steam heat systems in old buildings – only much fancier. You have a boiler where you burn fuel (propane in the case of the Alde), which heats water that circulates through pipes to radiators, which warm the rooms. The Alde boiler heats a fluid which is a mixture of water and glycol, much like the antifreeze in your engine. This warm glycol is piped to radiators and other things to warm them up.

The Alde boiler also heats water for your shower and sink in another jacket around the outside of the boiler. It will hold a couple of gallons, which when mixed with cold water at your shower faucet is enough for a (quick) shower.  This water is usually very hot – 85 degrees Celsius, or 185 degrees Fahrenheit.  If you want hot water in a hurry, you can turn off the pump circulating the glycol, and all the heat goes to warming the shower water.


The Alde heat exchanger, which transfers heat from the engine to the Alde system.

However, there are alternative sources of heat besides burning propane. The Alde also contains an electric heating element, sometimes two, of about 1000 watts each, which will allow you to use shore power when available instead of burning propane. But the slickest part of the Alde system is the way it has an option to scavenge excess heat from the chassis engine through a liquid-to-liquid heat exchanger, transferring heat from the engine coolant to the Alde glycol fluid.  When you’re running your Roadtrek engine it produces twice as much heat as it does power to the wheels – internal combustion engines are about 30% efficient. This heat is normally dumped into the air flowing through the chassis radiator, so why not use it? You’ve already paid for the fuel to produce it, right?  Save your propane – when you’re driving or idling the engine, your Alde system has all the already-paid-for heat it needs.

Typical Alde radiator, which they call convectors. These are out of sight throughout your Roadtrek.

Typical Alde radiator, which they call convectors. These are out of sight throughout your Roadtrek.

To transfer the heat from the glycol to the Roadtrek interior, Alde uses a customizable combination of under-floor heating and little radiators they call convectors. Alde also sells  small forced-air units with fans blowing air through radiators, which was used on 40th Anniversary 190s and early Adventurous CS models, but Roadtrek does not use these on new CS models – who wants to listen to a noisy fan when you can have silent heat? The floors of Roadtreks with Alde systems are metal with glycol tubes embedded in them, so heat spreads rapidly across the floor surface and the warm air rises, circulating the air without a fan. There are radiators under the bed, in the side step area, in the bathroom, and behind the driver’s seat, depending on your model.

Don't worry - it displays in Fahrenheit, too.

Don’t worry – it displays in Fahrenheit, too.

The best part about the Alde, though, is the control unit. It’s a very smart unit – no more banging on the pipes to request more heat. You can configure it to come on at a certain time, raise the temperature at 2 AM to kill any bacteria in the fresh water system, selectively use electricity, propane, or both to heat the water and glycol – all kinds of crazy things. It’s a thermostat and a butler all in one. The controls are all on an intuitive menu inside a small touch screen display, which is the only visible part of the system inside the Roadtrek once the Alde is installed.

Alde vent.  This will typically be on the driver's side maybe halfway down the vehicle.

Alde vent. This will typically be on the driver’s side maybe halfway down the vehicle. Only the white part shows, and it’s painted to match your Roadtrek.

Outside, the Alde system is also very unobtrusive compared to the old Suburban forced air propane units on older model Roadtreks. No fan noise inside or outside, and instead of a big clunky exhaust vent, there’s this neat, circular, color-coordinated vent that’s an intake and an exhaust all in one, as well as a countercurrent heat exchanger for all you design freaks. No propane exhaust fumes ever enter the interior airspace.

Alde frost control valve.

Alde frost control valve.

Europeans are way ahead  of us in camping comfort because they’re more accustomed to cold weather camping. This is a fully developed system that has been refined for decades by Alde, and Roadtrek knew a good thing when they saw one in their efforts to bring the latest technology to their customers. As a final feature, the Alde has a frost control valve, which dumps all the water from the hot water tank onto the ground below the unit when the unit is off and the outside temperature gets near freezing. You can’t break it by forgetting to winterize it – they think of everything.  Now, the rest of your Roadtrek plumbing still DOES need winterizing, but the Alde takes care of  itself.


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  • Dave

    Nice article Campskunk. Can you tell me if it is possible to eliminate the propane from an E Trek (or CS Adventurous with E Trek electrical package) and run the Alde system on just electricity from the batteries/solar panel/engine generator/inverter? I know it depends on many factors like what are you running simultaneously etc., but for spring/summer/fall camping in northern states, when the heater can be minimized and you just need the Alde system for heating water, would onboard electric power (no shore power) generation be enough? Thanks.

    • Dave

      I meant when coach heating can be minimized and you just need the Alde system for heating water.

      • Campskunk

        i’m always afraid of using electricity for heating – 2000 watts is a LOT of electricity. i suppose you could turn the heating part off, put it on electricity to warm up the hot water, take a shower, and turn it back off, but you’d probably need to either be plugged in or running your engine generator while it was on. it’s primarily a propane and shore power system. the solar’s not going to keep up with the electricity demand of that heating element.

  • bill walters

    Great article. Thanks. Question though. How fast does the propane “domestic” hot water recover during a shower?

  • bill walters

    I know that the diesel fueled Webasto heaters make complete water heaters for heat and domestic hot water. Roadtrek uses the warm air and domestic water Webasto for the RS E-Trek, but if they considered the full hot water Webasto and combined with the Alde heating/plumbing they could eliminate the propane tank and Alde boiler.

  • Vicki

    “Alde has a frost control valve, which dumps all the water from the hot water tank onto the ground below the unit when the unit is off and the outside temperature gets near freezing.”

    We’re looking at buying a CS Adventurous Roadtrek in the next year or so. We live in Wisconsin and are interested in winter camping. We also have family three states away that we visit during Thanksgiving/Christmas with our children. Would the Alde system work in sub-zero temperatures or are we better off with the furnace in the E-trek that Mike Wendland uses during winter camping?

    • Campskunk

      Vicki – both systems will work well below freezing. The frost control valve dumps the water in the water heater ONLY when the unit is off and the valve itself nears that temperature. When you’re using it, it’s always heated. Either unit will keep you warm.