Rob R. wrote:What are you looking to heat?
I am heating a 1400 sqft ranch home. single story with full basement. it was built in 1987 so it is fairly well insulated. I use 54 ft. of fin tube baseboard, and there is a cast iron rad. in the basement that measures about 42" x 30" x 8". the oil boiler is rated at 134,000 btuh and my hand fired coal boiler is rated at 85,000'. they both heat the house fine. Also i look to add 2 rooms to the system that are currently not heated, and eventually my 1 car garage.
With only 54 feet of baseboard you seem to be more in the range of a Keystoker Kaa-2. At 600 BTU's per foot of fin tube baseboard radiator, you may find that you only need 32,500 BTU's of reliable boiler output
to max them out. Even the Kaa-2 is more than twice what you need, though it sounds about right if you intend for it to also supply DHW.
An EFM DF520 would potentially heat 6 homes that each require only 54 feet of baseboard radiators.
I have 120 feet of fin tube radiators in my home, plus another roughly 30 feet in my garage, and I think my S130 is oversized for my application. A DF520 would be seriously oversized for my application.