coalkirk wrote: Besides, doesn't a df520 have an oil burner also?
Only if you get the oil package...which I would not recommend if there is an existing oil boiler in the house.
Steve, I have an EFM 520 and oil boiler in my house as well. My first winter I did exactly what you suggested and turned down the aquastat on the oil boiler...it didn't have to run once that winter. The next winter I just shut the switch off to the oil burner and connected the thermostat wire from the zone control box to the L8124 on the EFM...that way when a zone calls for heat, the EFM stoker and circulator between the boilers starts. This is my third winter with the EFM, and I have still not had to bring the oil boiler online for heat. No outfires, no problems.