Just as a comparison; the heater core in your car is very similar in size to the standard Kitfox heater core, and the coolant temp flowing thru it is also roughly the same temp. Your car stays warm in very cold temps but the car cabin volume is much bigger than a Kitfox cockpit. I agree the heater is fine, the main problems are the leaks and drafts. The slots and openings in the tail for the stab trim are major scoops of cold air that then blows forward on the back of your neck. Seal these up with some flexible rubber like Floog did and its like doubling your heater size.