Hi Paul
I have the Alde system in my 98 Bessacarr 550gl. I can only say it is fantastic! The system will run off gas only, or 1Kw immersion heater, or 2 Kw or any combination. We tend to start our stay off with the 2Kw only on just to get things heated up, then leave it on 1 Kw, as it is thermostatically controlled. For summer only use you can turn down the water temperature to save energy.
The 1kw setting is enough on its own to run the central heating system quite adequately in spring/autumn.
The control system is similar to, but different from, a domestic controller. It is 'on' all the time but relies on different temperature settings at various times, to achieve an 'off' condition. This allows the system to come during the night if the temperature falls below your preference.
The radiators are situated at various places all around the perimeter of the van, and work by drawing air under bunk floors and passing the warmed air out both behind the seat cushions and and under them through the bed slats. This way, there are few, if any, cold areas and the warmed seats are very cosy.
The fluid in the system is ordinary 50/50 car antifreeze so no issues with topping up.
The system contains a miniature pump and header tank, which in our case are sited in the wardrobe, thus keeping clothes aired as well.
It realy is excellent-go for it...