1 BTU is the amount of heat needed to change the temp of water 1 deg. How can you figure it out with air??????

You're going about it all wrong. first; you need to know how many btu's are in the fuel you are burning. Example; Oak contains 26 million btu's per cord. Then you need to divide that by the time it takes you to burn 1 cord in say hours. Then you would know how many btu's per hour it cranks minus of course any that escapes up the chimney. With oak there is a number that is used to calculate the heat (smoke and sparks) that are lost plus an average of 20% water. that number is 0.069 per 1,000,000 btu's. (sounds like Your fire place might be more effiecient then this). But this number is mostly used for figuring cost compared to other fuels. Example ; 0.069 X $150.00 = $10.35 for 1 million btu's. That's if you paid $150.00 for a cord. With this info you sould be able to make some comparisons

...Hope this helps...

NOTE; You might set aside 1/4 of a rick and time that. It would take a long time to burn a cord.

1/4 rick = 3,250,000 btu's

There is no good way to tell. Gas fireplaces have ratings on the order of 10,000 to 30,000 BTU/hr, and a wood burner probably puts out something like that. If you knew the temperature of the output air from your heating tubes, and assume that the 250 cfm figure is correct (which it isn't, exactly) you could figure it out knowing the specific heat of air, which is a figure I don't have handy.