The 60mm ducting is compatible with the Autoterm Planar heaters up to 2KW and also the Propex LPG Air heaters, as well as Webasto, Eberspacher and other makes of diesel air heaters.
The 90mm ducting is required for 4kw heaters and above.
It is recommended that you use the thermal lagging to reduce any heat less, particularly on long pipe runs.
See here for the Autoterm heaters.
See here for the HS2000 LPG heater or the HS200e which has the additions of a 240v electric heating option.