Has any one any experience of Kroll oil condensing boilers - they're 3 times the price of the Grant Vortex at trade but claim they condense (and presumably run efficiently at whatever the return temp is. My system was never designed as such, but works ok, it's just that I'm unsure if the return temp currently will be under 50C. Are Kroll the Rolls Royce or restricted supply keep price up kit - I understand they have a german Nuway burner for instance