I know its prbably a bit late now but there are wet underfloor heating mats now available as a complete kit, comes with valves, thermostats etc. call Heat and Cool or www.heatandccol.co.uk
Chances are it has got an open heating element and it's burning the dust thats being drawn into it. This causes staining on the wall and your heater! Only solution change them for rads with encased elements www.heatandcool.co.uk
