The news that the 2008 model year twin axle Conquerors were to be fitted with wet central heating systems resulted in many Club members asking me to explain how the wet systems compare to the conventional blown air types that most of us are used to.

The first question usually is “Do they require mains electricity to work?” Fortunately the answer to that is no, although there is a small drain on the caravan battery. The next question usually is “Do they take long to heat the van up?” The answer to that is about 2 hours from cold in winter. More on these items later.

Talk to someone who’s got a wet central heating system in their van or motorhome and you’ll find they wouldn’t swap it for a blown air system for all the tea in China. Why? – Well, once warmed up, the heat is evenly spread throughout the van so there are no cold bedrooms or washrooms. Also the air is more comfortable to breathe than with blown air systems which tend to produce very dry air. What’s more wet systems are virtually silent so there’s no problem using them at night.

The system being fitted by Swift for the 2008 model year is based on the brand new Alde 3010 compact boiler. In essence this is a scaled down version of the central heating boilers that most of us have at home. It is also capable of producing 12 litres (2.6 gallons) of hot water every 30 minutes. It can be fuelled by LPG or mains electricity or both. It is a powerful and efficient unit producing around 6kw on gas and 2kw on electricity.

Alde 3010 boiler
Typical finned radiator

Heat is transferred from the boiler to the caravan by means of small finned radiators, mostly hidden away in bunks at floor level and adjacent to the outside walls. Circulating through these radiators is a mixture of water and anti-freeze (ethylene glycol) that is pumped round by a 12 volt pump. A small head tank keeps the system topped up and allows for expansion when hot. This is often mounted at the back of a wardrobe. The boiler can be used just for hot water or just for central heating (with the hot water side drained down) or both.

The radiators are controlled by a thermostat positioned to sense the air temperature inside the van. When the thermostat calls for heat the boiler and pump fire up and when the air temperature reaches the set point they shut down again.  It is possible to set the pump to run continuously in order to smooth out the heat output from the radiators. Naturally that puts a bigger drain on the battery, and so is only recommended where the van has a mains hook up. Beware though that this function limits the hot water supply, particularly when there is little need of heat in the van.

So what is the drain on the battery and will it last for a weekend rally without mains? When running on LPG the system requires a start current of 1 amp, which quickly drops to a running current of just 0.4amps. In practice this means that, with a good 85 amp hour (or more battery), weekend winter rallies (2 nights) should not be a problem. However, For rallies longer than this it is advisable to have a second good battery or some means of charging the main battery. Of course battery life is also dependent on what other equipment is switched on so, as ever, there is a need to be careful with power hungry items like televisions.

If the caravan is to be stored for long periods of time without a 240v supply then the 12v supply should be disconnected. This is because the boiler draws a small current (0.01amp) in standby mode.

control panelOne nice thing about the Alde system is that the electronic room thermostat can be programmed with a different heat setting for up to six separate time periods during the day. This means that you can set it to be cool at night but warm in the morning and evenings. The thermostat can be set a low as 5 degrees which is useful for frost protection if there is power where you store your van. The picture on the left shows the control panel. There are quite a few symbols to get used to!

One point here is that, to keep its programmed settings, the controller must have a permanent 12 volt supply. Many carvan manufaturers do not provide this meaning that the settings are lost as soon as the caravan is towed. This is beacuse the habitation relay switches off all 12 volt power within the caravan during towing to comply with European requirements. To get round this you can purchase a lead from Alde that connects into the back of the control panel at one end and allows you to connect a 12 volt feed to the other.

The only real drawback with wet heating systems is the time they take to heat the van from cold. The reason for this is that the most radiators are situated around the edges of the van (where it is coldest) and often out of sight under bunks. The result is that it takes time taken for the heat to get into the main part of the van. When it does though the whole van is evenly warm and it will stay so for quite some time after the heating is switched off. This means that putting the heating on for a couple of hours before setting out for a rally can significantly reduce the warm up time after arrival.  If this is not possible then one way to get some heat into the van quickly is to put the oven on high for a while (door closed for safety) and put the kettle on for a nice hot cup of tea!

Motorhomes and wet heating
Although this article concentrates on caravans with wet heating systems much of what is said is equally applicable to motorhomes. The only real difference is that motorhomes can be fitted with an engine heat exchanger that can power the wet heating system whilst on the move. This means the whole motorhome can be toasty warm on arrival at site.

Alde company profile
Alde AB was founded in 1949 by the Swede, Alde Rask, manufacturing systems in Sweden for cars and tractors. Over the next 10 years the company became more focused on caravans and boats. The company's first central heating system was developed in 1966. Over the next 30 yeas Alde expanded across Europe with independent companies acting as agents abroad. Alde International UK Ltd was founded in the UK in 1986. In 1997 Alde was purchased by the German company, Truma.

The antifreeze solution used in the boiler and radiators has a finite life and should be changed at regular intervals. Failure to do so can result in corrosion affecting these items. The change interval is usually two years depending on the type of solution used. Alde UK use Comma Super Coldmaster Concentrated Antifreeze with 2 year Longlife protection. Castrol Antifreeze NF (Grade Code 1729) with 2 year service life is also suitable. The mix is 60% water, 40% ethylene glycol antifreeze in the UK, or a 50/50 mix for the rest of Europe. The water should be clean and fresh. It is important to ensure that the antifreeze is suitable for use with aluminium components.

Alde say that their 3000 and 3010 Comfort boilers do not require servicing but older Alde Comfort and Slimline boilers should be serviced according to the schedule given in the manual.

More information can be found at