We are in the process of replacing a failing 1.2 million BTU gas fired boiler in a gravity fed hot water system. To get a sense of the amount of water in the system, the pipes exiting the boiler up to the attic are 3 inches. Pumps were added at some point in the past, so I guess it isn't technically a gravity fed system anymore. There are two main zones and one small zone. Several contractors have proposed replacing the boiler with two Lochinvar Knight boilers or one Knight XL. Has anyone had any experience in replacing a large cast iron boiler in a gravity fed system with a high efficiency boiler? I am concerned that the boiler will have to run constantly to heat the large volume of water, thereby stressing the boiler and losing the efficiency gains. Any feedback would be greatly appreciated. Thanks.