I recently reviewed a project in which a municipal building was being connected to a district heating system supplied by pellet-fueled boilers. The building’s existing system had two propane-fired mod/con boilers piped into the system as shown in Figure 1.
Each boiler has its own circulator that runs only when that boiler is called to operate. Each boiler piping assembly contains a check valve to prevent reverse flow when one boiler circulator is on and the other is off. There’s also valving to isolate either boiler if necessary. What is less typical, but still fine, is that the designer chose to include an air separator on the outlet pipe from each boiler.