Atmospheric boilers combust their fuel the old-fashioned way.

Combustion air is gathered from the immediate vicinity of the boilers; so they are generally isolated in their own boiler room or facility. They require both primary air circulation (to mix the fuel) and secondary air circulation (to ignite the fuel), and air pressure is supplied by a simple draft system that uses flue gasses to provide negative pressure. 

Atmospheric boilers generally deliver efficiencies in the 80%-84% under high load and steady state conditions. Under variable loads, their efficiencies can drop below 60%. This is the most mature low-mass technology, but they do have trouble complying with low-emission rules that have begun to appear over the past decade. 

BioTherm offers several models of the Raypak RayTherm atmospheric boilers.


BioTherm RayTherm
Model Number Full Input in BTUH Flue Diameter Stages
H-926 926,000*
H-1083 1,083,000*
H-1178 1,178,000*
H-1287 1,287,000*
H-1414 1,414,000*
H-1571 1,571,000*
H-1758 1,758,000*

*Propane input is 94% of stated value