Boilers

Residential and light commercial boilers: modulating condensing, cast iron atmospheric, and combination units that produce both heating and domestic hot water. From 85,000 through over 200,000 Btu/h.

Condensing boilers run at 90%+ by returning water cool enough to condense the flue gas, and that is the catch worth understanding — a condensing boiler connected to a high-temperature cast iron radiator loop returning water at 160°F never condenses and never reaches its rated efficiency. It needs low return temperature, which means radiant floor, panel radiators sized for lower water, or an outdoor reset control.

Cast iron boilers tolerate high return temperature and rough water, last decades, and are the right answer on an old gravity-converted system. They also require a bypass or mixing arrangement to protect against thermal shock.

Check whether a pump is included: 27 of the units here ship with no circulator and 14 include a Grundfos. Combi units size the domestic side by flow, so a combi that heats the house adequately can still be short for two showers.

All 90%+ units need PVC, polypropylene or stainless venting plus a neutralized condensate drain.

Brands include US Boiler, Navien and IBC.