Truma Blown Air Heating – Lagging the external hose

We have a Bailey Pegasus II Verona Caravan which has about 2 metres of air ducting located beneath the caravan floor which enables the Truma Blown Air system to be connected to outlet vents in the front seating area.  We have found that when the outside air temperature is low then the air being discharged from these front vents is no more than a cold draught and as a result we close them off and let the heat from the rear outlets permeate through to the front.  Utilising this method takes quite a while to get a pleasant overall temperature and is less than satisfactory.

There have been a number of threads on the Club Together forums about this issue and the remedy appears to be the application of some pipe lagging to the external ducting.  Why this isn’t being done by the manufacturer seemed to be at best an oversight on their part or more realistically probably a cost saving measure.  Then again maybe they don’t know that the construction method used causes a problem to the end users.

In order to get some idea about this I spoke with the nice people from Bailey on their Stand at the recent NEC show and asked about why the external ducting was causing the air to cool down so much to the point where it was actually quicker to heat up the caravan interior by closing off the front seating area vents and why the external ducting wasn't insulated as standard with the pipe lagging. Their response was that the external hose if double walled and although black coloured on the outside surface of the outer hose, the inner surface of this cover is shiny. This method of construction is supposed to limit the cooling effect on the warm air flowing inside the inner duct by the cold air outside the caravan.  We then went on to discuss the “Climatic Chamber” test; which it turns out is only looking to see if the interior temperature reaches an overall acceptable level within the required timescale and does not factor in the comfort effects of having cold air being discharged if the front seating area vents are open.  It would appear from the response received that it was the first time that anyone had bothered to ask that particular question.  So that made me wonder who dreams up the testing regime and also if there are other tests being carried out which do not consider the realistic operation and use of the caravan.

So after getting hold of some of the appropriate sized lagging (it comes in 2 metre lengths) I decided that it was time to remedy the situation and hopefully get a comfortable caravan that would allow use of all the air vents irrespective of the outside air temperature.

So after disconnecting the complete external duct assembly with the plastic end fittings which go through the caravan floor by removing the 2 screws at each elbow, there were a few cable ties which needed to be cut too.  I then found that the hose had been squashed between the caravan axle and the caravan floor (the gap is too small for the hose to fit through without distorting it). It would appear that the duct must have been in place in the floor assembly when it was attached to the Chassis.

I would also guess that having this deformed section of the ducting is allowing the heat to transfer to the outer hose directly which then acts as a radiator due to the nice matt black surface. This also prevented me from removing the assembly without incurring extra damage to the ducting.  Unfortunately it had the knock on effect of having to work entirely underneath the caravan and also it prevented the lagging from being installed in one continuous length without any cutting.

In order to install the lagging I had to cut the insulation across its length in the correct position in order to push it on from each end up to the point where the axle sits. Profiling the cut end by careful shaping at the point where the Axle was squashing the outer duct and preventing the lagging going on any further I was able to get the vast majority of the outer hose covered.  When this was done I used cable ties to minimise the chance of the edges lifting; the end fittings were reattached to the underside of the caravan floor and the ducting held firmly in place with cable ties at the locations where it was secured originally.

The testing time came recently when we used the van for a 3 nights at Grafham Water, the difference was very noticeable. We now get warm air out of the front seating area outlets rather than a cold draught.