Rob R. wrote:You could disable the oil burner via a switch and just let the coal boiler maintain temperature...that is the easy way. If you want to save some coal, disable the oil burner and install a DPST relay to split the zone control end switch signal to the two boilers....that way the coal boiler can sleep at 140 degrees (for example) and only run up to the high limit when there is a heat call.
Ours already come with the triple aquastat so that is how it will function. Normal low is 160 or somewhere close and the high is 180 or somewhere close. The aquastat will always maintain the low setpoint regardless of call for heat or not. Then when the TT terminals are shorted the Boiler will maintain the 180* or high set point.
So what you can do is either set your high set point of your oil boiler at 140* and the low to 120* and the coal to 180* and 160* and run the TT terminals in parallel so you have auto back up. Or you can just leave the TT wires off on the oil boiler and just have the low setpoint be the setpoint temp of the oil. And the coal unit will be primary.