Question: I am adding a coal fired boiler to my home and am looking for recommendations as to the best way to pipe it into my existing oil fired system. I was planning to take the feed from the coal boiler into the return of the oil boiler and then go back to the coal boiler from the output side of the oil boiler. This would allow the mixing of the coal boilers output and the return water from my baseboard zones to occur in the oil boiler, also keeping my domestic water coil hot. Any suggestions? Answer: This is a good way to hook up a boiler. Even better is if you can find extra tappings on the boiler near the bottom and top where you can go in and out of. They must be large enough, usually 3/4" or 1" min. , to handle the load. If you do come of the feed and return, do it before any flow or check valves (close to the boiler) A circ must be added which does nothing more than circ water between the two boilers. This runs either full time, of off an aquastat hooked up near the output of the coal boiler (on at 180 or so, off at 160) Be sure to install the PR valve on the coal boiler and pipe it to near the floor, a drain or other approved config. I once had my coal boiler Pressure Releif valve go off, and the basement filled with steam in seconds! Couldn't even see.