Grant eco 15/21 outdoor module. Grant developed the first oil-fired External Boiler in 1986 and although the concept was new to the oil market, it quite quickly became a popular addition to Grant's boiler range. It is therefore appropriate that Grant has taken the lead with three Grant Vortex Eco Condensing oil-fired external modules, with outputs between 15 and 35 kW (50-120,000Btu/h). This high efficiency Eco External Module range has some unique features including a multi-directional factory fitted low level flue, external powder coated robust casing, built-in frost protection, mains isolating switch and a test switch.