Rick, I heated my house with ease last winter doing just what you are thinking. With a 3/4' line off a heat exchanger from an outside coal boiler. It works fine, just keep your coal boiler temp up high around 190 degrees. It appears that you have a relatively modern house by looking at the condition of the concrete wall behind the boiler. Or,,,,more better insulation. If this is true, then the 3/4" taps will work. If your house isn't insulated so well I would pipe it into the ends of your headers and increase the pipe size from the coal boiler.