Legacy system rehabilitation.

Why these systems are worth keeping

Cast-iron radiators are among the most durable heat emitters ever installed in American housing. The 1908 castings in a typical Bozeman north-side craftsman house are still flat, still pressure-tight, still emitting BTU at the same rate they did when President Roosevelt was in office. The piping is generally still serviceable; the radiator valves typically need new packing and the bleeds need replacement, but the bones are intact. What is at end of life in most of these houses is not the radiators — it's the atmospheric boiler in the basement, sized to about 220,000 BTUH against an actual heating load that, after the homeowner's last insulation upgrade, runs closer to 80,000.

The single biggest win in a legacy rehabilitation is that mismatch. A correctly-sized modulating-condensing boiler, capable of dropping to about 20,000 BTUH at minimum firing rate and modulating continuously across the heating season, will reduce annual cycling by 50–70% and reduce fuel consumption by 25–40% on a typical 1920s house. The cast-iron radiators behave better at lower supply temperatures too, because their surface area is generous and they are happy emitting at 140°F supply rather than the 180°F the original system was set for.

What we replace, what we retain

We replace: the atmospheric boiler (typically a 1970s–1990s cast-iron sectional from Weil-McLain, Utica, or Burnham); the expansion tank and air-elimination components; the system pump (typically a Taco 007 cast-iron three-piece running on full-speed for 40 years); the monoflo tees on series-loop one-pipe systems that resist proper balancing; the zone valves where present; the thermostat and any wired controls older than 2005.

We retain: the cast-iron radiators themselves, after a pressure-wash, paint or powder-coat, and bleed-replacement; the supply and return mains in copper or steel, assuming they pass a 50 psi pressure test for 24 hours; the radiator valves where the packing is replaceable; the trim. We sometimes retain the original chimney as a draft path for a fresh-air intake on the new sealed-combustion boiler; more often we abandon it and run the new boiler's intake and exhaust horizontally through a basement wall.

The redesign

Every rehab starts with a Manual J load calculation against the actual current envelope of the building — including any insulation upgrades the homeowner has done since the original installation, current window U-values, and measured infiltration if a blower-door test is feasible. We then re-do the distribution layout: we delete monoflo loops in favor of a proper two-pipe direct-return, we set a Δp ECM circulator (Taco 00-VR or Grundfos Alpha2) per zone, and we install a TRV at every radiator so the homeowner has room-by-room control without a thermostat-per-zone wiring nightmare.

The new mod-con plant — typically a Lochinvar Knight in the 110,000–155,000 BTUH range for a 2,400–3,400 sf 1920s house — is set on an outdoor-reset curve calibrated to the building's actual envelope, not the boiler's factory default. The curve typically lands at 180°F supply at 0°F outside, falling to 130°F at 50°F outside, with a minimum return-water protection floor at 95°F. A boiler running this curve will condense for more than 90% of the heating season, which is the entire point.

Steam systems · a separate conversation

Bozeman has a handful of remaining single-pipe steam systems in commercial and pre-1925 residential buildings. We do not convert steam to hydronic. We do rebuild steam systems in place: replace the boiler with a properly-sized atmospheric steam boiler (Peerless WBV or Smith Cast Iron Series 8), repack radiator valves, install new vents (Heat-Timer or Vari-Vent), pitch the mains properly, and skim the boiler on commissioning. Dan Holohan's The Lost Art of Steam Heating Revisited is the canonical reference for this work and the book we hand the homeowner before we quote it.

What it costs

A typical mod-con conversion on a 2,400–3,400 sf 1920s craftsman runs $22,000 to $38,000 installed, including the new boiler, new distribution, new circulators, new controls, condensate neutralizer, and the binder. Steam-system rebuilds run $14,000–$28,000. Both prices assume that the radiators themselves pass a pressure test on assessment day — if a radiator is leaking through a casting, we replace the whole radiator with a salvaged match from our partner Heritage Radiator Co. in Anaconda.

The carbon argument, briefly

A mod-con conversion reduces annual natural-gas consumption on a typical 1920s house by approximately 25–40%, depending on the original AFUE and the new outdoor-reset curve. The embodied carbon of the retained cast-iron radiators is zero — they are already in the house. The embodied carbon of the new mod-con plant is small relative to the operating savings over the plant's 15-year service life. This is the cleanest path to reducing residential heating emissions in pre-1950 housing stock that we know of, and it is one we recommend even when the homeowner is comparing it to a heat-pump retrofit.

Read across

See the North Bozeman 1908 rehabilitation for a worked example. See the mod-con boiler scope for the plant. See the commissioning scope for what we do at the end.