One thing to mention in this whole heater discussion. I have a thermobob thermostat on my Kitfox to keep coolant temps up in cold weather. There is a bypass line on the thermo bob, that is run through the heater core in my installation. When things warm up enough and the thermostat starts to open, I do loose some heat from the heater core. For my next Kitfox build, I have a small radiator with 1" fittings on it that I will use for the heater core. It will be plumbed in before the thermostat, so it will always get a good flow of hot coolant. JImChuk