With a "wet" system can you use steam heating rubber specification pipe as well as generic plastic pipe systems to circulate the water between heater matrix(s) and the heater?
Well I have such a system and the connection between the matrices and the hopeless plastic "quik push fit but guaranteed to leak in a few years as it gets moved about" grey plastic piping is rubber heat resistant pipe and it it the only bit that is entirely satisfactory. I use this stuff from ASAP and it works a treat http://www.asap-supplies.com/search/206416/hot-water-hose-and-steam-hose-206416
Either decent barrier pipe (JG Speedfit et al) copper or heater hose is regularly used, the temperatures in a normal wet system are actually quite low. If using hose be very careful not to introduce high spots just because it makes the run easier, a common cause of air locks. My personal preference is for barrier pipe except the calorifier when heater hose is my usual first choice. If you want a factory manual on how to install a wet heating system just PM me an email address.