[Griff]

I'll go look at his design again. The design from Mother Earth News that I looked at uses recirculated air. My basic goals are to warm the floor upstairs and make a nice sit down space/solarium. I may just use darker heat absorbing materials in the sunspace but the thermosiphon has a simplicity that is just attractive. Heat losses through windows are higher but I could add insulated window shades for nights. The idea is probably better for heating water and workshops. Thanks for the input.