It depends on boiler pressure ...

It depends on boiler pressure and source water quality.  Below 600 psi, single pass RO with a softener on the feed or polishing the RO permeate is usually sufficient unless you have a source with very high TDS and/or silica.  For those waters, two pass RO is likely sufficient.  High-Pressure boilers, >800 psi, will require DI water.  600-800 is a transition zone and dependent on the feed quality.  BUT, even when DI is required, feeding the DI with an RO almost always provides an economic benefit.  I would say if you are over 150-200 ppm TDS in your feed water, an RO is going to offer a sufficient payback.