I haven't been able to calculate my home's heat loss yet, but in the meantime, I had emailed Slant/Fin about downfiring the TR-30 because the instructions that came with it show four nozzles for the Beckett with the same burner and same burner head: a .85, 1.00, 1.10, and 1.25. It came with the 1.10 installed and they also shipped a 1.25 in the accessory box.
Slant only said it could be indeed be run with the .85, but was hoping they would give more info as to real world fuel usage or efficiency. Also, I read that the boiler should be matched to the air handlers, correct?
Is downfiring the boiler good practice as long as it's setup for that nozzle size?
John
As long as the EWT at the boiler doesn't drop below 140F when both air handlers are running it's fine to dial it back. I'm estimating with a boiler output temp of 180F the air handlers would be pulling only 110,000 BTU/hr, whereas with the 0.85 gph nozzle the burner is delivering about 0.85gph x 85% efficiency x 138,000 BTU/gallon = 99,705 BTU/hr. That's about 10% shy of what the air handlers deliver at 180F EWT, but that only means the boiler would drop back to ~170F-175F out (roughly the EWT at which the pair of air handlers would be drawing the full 99,705 BTU/hjr) with a return water temp in the 150F range- there is still plenty of margin above 140F.
With the 0.85 gph nozzle you could conceivably run into hot water capacity issues when the air handlers are running if you're family is fond of taking extra-long showers. If that happens, setting up the air handler controls to inhibit the blower when the EWT drops below 155F - 160F would be safe for both the boiler and the person in the shower. Unless you have a real gusher of a shower you're not likely to be drawing the full 97.7K with just the tankless coil.