Oil to NG Conversion

BostonGTR

I’m converting from oil to NG.  13 yr old house with good insulation, windows and doors with 3 hydro-air units.  Given that the hydro-air units have aquastats set at 140 degrees; what is the best boiler solution?  Am I correct that a modcon unit will not be

in the “sweet spot” i.e. not condensing, due to the need to generate approximately 170 degree water for the air handlers; not to mention the need to generate hot water for an indirect DHW tank?  Even if the modcon is not condensing and therefore not operating at 96% efficiency; will it still be more efficient, or at least as efficient, as a new cast iron boiler?  I understand that the modcon would require some annual maintenance.  Is the cast iron boiler maintenance free?  The 96% modcon unit offers a rebate of $1,500; the rebate drops to $1,000 for a rating of 90%+; and then to $0 below 90%.  Might you be able to recommend a “medium efficient” cast iron boiler with a 90% rating? 

SWEI

rest of system

Are other emitters (radiators, floor tubing, etc) involved, or just the hydro-air units?

BostonGTR

Only Hydro-Air

Just the 3 hydro-air units.  

SWEI

outdoor reset

Works on air coils too.  The curve will be higher and steeper, but it will noticeably improve comfort and efficiency.  Depending on the coil sizing and the climate, a mod/con could be condensing more of the time than you'd expect.



Edit: looks like the aquastat controls the fan, so making ODR work properly would require some additional controls.

BostonGTR

Feedback

Does anyone have any experience using a modcon boiler to feed a hydro-air system? It seems that it shouldn't perform any worse than a cast iron unit; correct?  I just want to make the correct boiler decision (modcon vs. cast iron) before proceeding with this heart transplant.