If possible you should really try to have at least three feet behind the boiler. It makes it much easier to service the unit, and is required to easily pull the tankless coil out of the boiler. 36" clearance to combustibles is pretty standard, and
Please print out a copy of the EFM manual for reference: http://c326988.r88.cf1.rackcdn.com/DF52 ... lation.pdf
The minimum clearances are on page 3.