BrockWilt wrote:maybe i could leave the gas boiler hooked up and use the coal boiler most of the time.
I don't have a coal boiler myself, but from reading in this forum, lots of people leave their gas or oil boilers in place. Then your coal boiler can just pump hot water to your gas boiler, and the existing circulation from gas boiler to radiators can continue unchanged. And if you can't feed the coal boiler for a week because Paris Hilton invites you to the Bahamas for a tryst, the gas will just kick on and keep the house and the wife warm while you are gone. In fact you will probably find the wife actually steaming upon your return. The other way to plumb it is as you describe, an independent feed to the radiators from each boiler. One is called a series connection, the other parallel, but I don't know the relative merits of each.