Calculating radiator sizes


Not as easy as you may first think....
i'm fitting wet central heating into my motorhome, i'm going to be using a 5 KW eberspacher water heater as the boiler, this will feed a calorifier for hot water, and then via a 12 volt 3 way motorised valve... if i can find one, it'll divert the hot water from the boiler to the radiators in the van, or to the hot water cylinder, or both.. basicaly how a home heating system works... only in minature.
I'm trying to work out what size rads i need to fit, tried one on-line radiator calculator, and to heat the living area it said i needed 3500 btu's to keep it tropicaly hot, ok i had to fudge some of the variables, north facing wall... said no, but i could well have, depends where i'm parked, it had a watt to btu calculator, and it came out at 1 watt needed to produce the 3500 btu's needed, if that's so then a candle will heat the van.
Can someone who knows how to do the calcs help me out.
The van has 2 areas, the main living area is about 4 meters long, 2.15 meters wide by 2 meters high, and the bathroom area is about 1.8 meters long, 2.15 wide and 2 high.
The floor is 15mm ply bottom, 1 inch of polystyrene, 15mm ply top, with a standard felt backed carpet on top in the living area, and those stick down vynl tiles in the bathroom.
all windows are double glazed, but not like house units, but they are better at heat retention than the single glazed glass windows, these are the plastic windows, about half an inch of gap between the 2 panels.
There's a double glazed roof vent in both area's the one in the living area i put an insulated cover on the underside at night in winter,
The walls are made of about 1mm thick alli sheet, 1 inch of polystyrene, and 3mm ply wall boards, with 1 x 1 timber battons for the frame,
I can be parked anywhere from england in summer, to berlin in winter, we had a gas fired convection and blown air heating system before, output of 3400 watts, and tht kept us warm in berlin when there was 2 foot of snow on the ground and a bitter siberian wind blowing, but the air heating was a pain to regulate, either too hot in the living area and too cold inthe bathroom, or the other way round, hence i'm going for the wet central heating, and will be fitting thermostatic valves on the rads top allow the 2 rooms to have their own temps due to the size differance.
i plan to have a towel radiator in the bathroom, not one of these expensive designer ones, i was thinking of a normal small rad with one of the add on towel rails fitted to the top fittings if you can still get them,
and i'd like 2 rads in the living area, a tall slim one near the door (the coldest part of the vanas the entry door is thinner than the walls) this one will 'point' towards the front of the van, and another rad in the kitchen area which is still in the same room, this one will point towards the rear of the van towards the bathroom door.
We don't use the luton area for anything but storage, so that's not an area that needs calculating on.. it's closed off with a door.
i know i could just put in the largest rads i can fit, but i'd like to size the system right so the eberspacher isn't cycling on and off too much, as they pull a lot of current when starting up, and they say the system must be sized so they're not cycling all the time, but dont offer any help sizing the system.
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http://www1.dimplex.com/heatcalculator.asp
http://www.thermalinc.com/math/aitkenform.htm
Don.
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