"Fredericton man builds $300 solar furnace, decreases heating bill"

[Kaleva]

7. One generally tries to move 100 cfm per 6" heat run

With that amount of air, the 6" round duct is desirable because anything smaller would lead to increased noise.

Here is a link to a btu calculator:

http://www.calculator.net/btu-calculator.html

One will see that even a small, normally insulated 10' X 10' X 8' room in a northern climate will need a fair amount of btu in order to maintain the room at a comfortable temp when it's cold outside.

You could also try and calculate how many btu's your solar heater is producing by first determining the the temperature rise (compare the temp of the input to the output), then measuring the cfm (or it may state the cfm on the fan itself) and using this formula:

CFM= ____BTU/Hr/(1.08 x Temperature Difference)