Calculating btu's?

How does one accomplish this? We have a 2700 sq/ft home insulated of course, and a detached garage 2500' /13' ceilings/ 6" insulation in the walls 10" in the ceiling.
I was wondering if a 520 sized unit could heat these areas? The home needs to be 70 degrees come winter and the garage 55 degrees. The garage has two 66,000 btu Modines maintaining this now, feed by a OWB were going to sell, to much work.

To properly size your heating appliance, stove, boiler, furnace you need to do a heat loss calculation. In the HVAC industry it's called a Manual J, after the section of the Air Conditioning Contractors of America design manuals. See: http://www.acca.org/

Do a search on this site and you will find many references to Manual J. Some companies that supply products for the heating industry have design procedures for heat loss calculations. One is Taco. See: http://commercial.taco-hvac.com/

You will need to register using your e-mail address. It's free and there is video training available.

What you are looking for is the "Taco HVAC Load Calculator". It's a software download analysis tool. To use it you enter your local weather conditions and the construction of your building. There is a learning curve. It can be used to determine entire building heat loss (called block load) or individual rooms. Good luck.

Here's the program we use. I can give you a calculation based upon your answers.