- Replaces Sterling TF100 and SF100 heaters - The Sterling XF series gas unit heaters replace the TF and SF series heaters and combine them into one unit that can be vented as standard open combustion (Exhaust pipe only) or separated combustion (Using both an exhaust and intake pipe). They are a great heating option for large greenhouses, warehouses, commercial structures, etc. Sterling XF heaters feature: tubular heat exchanger, power exhaust system, electronic ignition, advanced diagnostic...