Hello Clive,
The typical blown air system (Carver/Truma) has been around for 30 odd years. They can be set up to cater for the majority of touring vans, and have been shown to improve the comfort levels significantly. Even though they were the standard kit for most caravan manufacturers, some caravan layouts exceed the practical limits for the Blown air systems.
The majority of the ducting used is approx 75mm diameter and is manufactured with a corrugated wall. This allows for easy bending and positioning within the confined spaces in a caravan. But corrugations also present quite a rough internal surface that produces significant drag that impedes the airflow through the duct.
The practical outcome of this is that the longest duct run should not exceed about 3M to 4M as the drag will reduce airflow to just a warm waft, rather than breeze or stronger.
There are other factors that also increase the ducts drag. Each 90-degree bend has the effect of adding the equivalent of about 0.5M length of duct, so the maximum length of duct that is effective is usually less than 3M.
Some manufactures prefer to use the under floor option. In some cases there is no alternative, but in some, there is an internal route, but it may be more complicated to fit, or use more parts.
Where the duct is taken under the floor, it should be sheathed in a second skin, which offers water protection, and some additional thermal insulation.
There is of course nothing wrong in looking to increase the insulation under the van, but it may be worth looking to see if there is a viable internal route.
Technically, although the surface of the duct may appear to be quite hot, the amount of heat actually lost through the walls is a small proportion of the total heat transfer by the air through the duct. I do concede that any heat loss should be avoided if possible.
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