Thanks- 44,616 would be the design heat load. They have installed a 60,000 BTU furnace, which at 94% efficiency will yield 56,400 output. With 1075 CFM across the coil, you'd need roughly 110 degree discharge air to meet the sensible load. This would require only about 5 GPM at a 20 degree delta from 175* EWT to 155* LWT. Add a domestic load to that however, and we have a problem with very low return temps. If I were me, and I'm pretty sure I am, I would still splurge for the 26-99 and keep the flow rate up at 8-9 GPM. That way there are never any surprises to your boiler, and never any trouble heating the house and DHW simultaneously. I don't mean to spend your money, but the extra couple hundred bucks for a proper system goes a long way when you're risking mistreating a $10k+ boiler. Why gamble?