I don't think anyone will be able to exactly answer your question.
Hydronic heaters output is based on the water temperature drop through the unit. That in turn is based on the inlet water temperature, the water flow and the ambient temperature.
You add to the fact that a quick google search didn't even show that model listed on Airtherm's site anymore and you are posing a bit of a head scratcher.
Looks like a unit a bit larger than that can put out around 40,000 BTU's under the right circumstances. That's their 062 unit they are currently making with a 1/20 HP motor. Your motor is a bit larger than that it looks like but that might be a good rough estimate...but like the guy you bought them from...I really don't know either. Look like nice units to me though.
Should be a way to make them work for you.
Phil
