Just about all of these boilers use sealed combustion, which draws combustion air directly from the outside and vents exhaust gases directly through the exterior wall to which they are mounted. Their compact heat exchangers are constructed of stainless-steel or cast-aluminum alloys that can handle the corrosive effects of condensing flue gases. Nearly all these boilers can also modulate their heat output from full power down to 20 percent to 30 percent of that rating.
Because of their relatively high flow resistance, these boilers should always have their own dedicated circulator. Figure 1a shows one approach. Install a pair of closely spaced tees (a.k.a. a primary/secondary interface) between the boiler circuit and the remainder of the distribution system. This allows for constant boiler flow, regardless of what's happening with the zone circuits.