I would like to heat about 800 sq ft of living space at my country place in Northern California with a small (i.e., 45,00 BTU), minimum efficiency, late model, gas fired furnance that I have on hand. The house is insulated, has other heat sources, and isn't in a cold winter area, so I am willing to try out the furnace even though it's small. I appreciate that there is room for error with "rule of thumb" recommendations, but that's all I seek at the moment.

My first question relates to minimum size of the cold air return. If I pull the return air from an area in the living space, what is the smallest capacity duct (in square inches) with a filter installed that I could reasonably use? I have a limited amount of space to get the return air duct from the furnace to the point of intake.

My second question refers to sizing the supply air ducts. The furnace will be located at one end of the basement, underneath a very small kitchen, and a 250 sq. ft. eating area. I have room to route two ducts to two well spaced floor registers in the eating area. The ducts would be about 15' and 25' long, respectively. I was planning to use 6" round ducts to supply these two registers. Does that make sense, or given my second issue (see below) should I use a slightly smaller duct because these two registers will be much closer to the furnace than the rest of the registers in the living area?

To service the remainder of the living space above the basement, I need to run about 50' of duct (with one 90 degree turn) around the perimeter of basement to supply three, and perhaps four, floor registers. The first register will be positioned at about the 30' point, just before the 90 degree turn, and the remainder of the registers will be spaced evenly along the last 20' of run after the 90 degree turn. What size duct should I plan to run to the point of the first register, and then to the point of the second register? Finally, am I right to assume that the last register in the run can be supplied with a 6" duct, or should I plan to use a larger duct to finish off the run?

Thanks for your thoughts.