Question: I have an old coal conversion gravity boiler. It has huge diameter piping coming from the boiler. I would like to hook up an hs tarm boiler in the loop. My current boiler is a two pipe system not a one pipe loop. Can I use this in a gravity system? And can I break into the loop at some point when the piping gets smaller . Would I need a circ. pump? Any info would be helpful Answer: Assuming your system is hot water (not steam), the system should work well. You have to look close, or have a pro look close, at your exact system and the current flow. For instance, how is the heat now controlled? I would guess that the thermostat calls on the oil burner which then fires and heat the water, which then rises through the house. A coal boiler is a bit different in that coal cannot react instantly like the oil, nor can it shut down like the oil. As a for instance, such a system as yours might be set up this way: 1. Coal boiler hooked up to other boiler with small circulation pump. Control set to keep both boilers at about 180 degrees. 2. Electric check valve (large zone valve) placed on output of main boiler which opens when house thermostat calls for heat. You also have to think about what to do with the oil controls. The oil burner could simply be switched off, or you could add a control which allows the oil burner to fire when the water temp gets lower than 150 degrees. This way, as long as the coal does the job it will not fire.