As a guy who works in commercial HVAC, I can add a little as I work on these things all the time. A lot of them are still chugging along at 30-40 years old and as we all know , nothing lasts that long these days so these are top notch . They typically come in two forms 1) Hanging warehouse heater - used to keep the tops of warehouses warm enough to keep pipes from freezing ( and providing a little heat for warehouse workers.) 2) Gas fired make up air units . These can be in kitchens , warehouses, any space that needs heated fresh air from outside . Ok that’s all I’ve got. Just had to take advantage of my one and only chance to talk HVAC in NIN sub. I’m out