How much efficiency is lost by putting 30% glycol through a boiler like a Triangle Tube Prestige? I understand glycol is inferior at heat transfer. After lots of googling all I found was that you may need more feet per second through the exchanger.
Would be 7% higher as well, so your 19 GPM becomes 20.3 GPM. Is this piped with a hydraulic separator or closely spaced tees in a primary-secondary configuration? If so (and assuming you don't have overly restrictive piping in that loop) you can usually use a 0010 or a Series 100 with the 399.