To answer that question correctly we need to know somthing about your heating load, typically quantified with a heat loss calculation, http://www.tombling.com/heaters/heatloss.htm
, basically you determine the heat loss for each component of your structures and add them together. The more effort the more accuracy.
But since we all like to shoot form the hip.
How big is this Modine hydronic heater, how much heat loss can we expect form the piping, that information and the BTU rating of your coal can help you determine how much coal will be needed to run the Modine for one hour. Add some coal for inefficiency.
Good luck, it sounds like it is already built, so why worry about it, turn it on and see what happens.
You can probalby find a better heat loss calcuator, by searching this site in the upper right.