I have an original 1914 coal fired (converted to gas) boiler that runs my gravity hot water heating via cast iron radiators. I have a pantry, and and a small breakfast room off of the kitchen that are currently unheated by the main boiler. The pantry is roughly 4’x9’, and the breakfast room is roughly 9’x12’. After doing the calculations the pantry requires about 3,500 BTU, and the pantry requires about 7,500 BTU for heating requirements. I will be replacing my old boiler at some point, and converting the entire system to pump fed, with zones. But I don’t have that kind of change just yet. I am wondering how hard it would be to go ahead and buy the proper sized rads for these two unheated rooms, plumb them in as a “zone”. But since I can’t tie them into my old system, I thought I could run it though a heat exchanger fed and run it off of my existing 50gal, 40,000BTU DHW heater on a closed system, separate from my DHW. This way, I can get functional, period correct heat in these room, with “minimal” investment, and when I do replace the old boiler I can easily tie it into the new system, no need to “redo” anything. Am I crazy?