North Bozeman 1908 Rehabilitation.

Project background

The house had been in the same family since 1947 and changed hands in spring 2022. The new owner — a return-to-Bozeman buyer who had grown up on the same block in the 1990s — knew the building well enough to understand that the radiator system was original to the house and worth keeping, and that the basement boiler had been short-cycling and weeping condensate from the flue baffle for years. The first call to Headwaters in May 2022 was framed as what would it cost to make this work right without losing the radiators.

The house is a contributing structure on the Bozeman Historic Survey, so any work visible from the street required review. Our scope was entirely interior and basement; we did not file with the Historic Preservation Office. We did, however, send the office a copy of the binder when we finished, as a contribution to their records of original-radiator survivability in the district.

Assessment phase · June 2022

We assessed the house over two visits totaling about ten hours of on-site work. Each radiator was inspected for casting integrity, internal scale, valve packing, bleed function, and visible weeping. Of the fourteen radiators:

• 11 radiators in good condition — to be pressure-washed, painted, valves re-packed, bleeds replaced, returned to service.
• 2 radiators with hairline casting cracks — to be replaced. (Kitchen north wall, mudroom.) Donor radiators sourced from Heritage Radiator Co. in Anaconda; their inventory included two matching American Radiator Co. units from a 1912 Anaconda house demolition.
• 1 radiator orphaned in the attic from a 1962 dormer addition — to be replaced with a Runtal R-30 panel because the attic floor framing would not support the original radiator's weight.

The distribution piping was a hybrid: 1¼″ black-iron supply main from the boiler heading north under the basement ceiling, branching to four legs, three of which were proper two-pipe direct-return and one of which had been converted to a monoflo single-pipe in 1981 to add the upstairs bathroom radiator. The monoflo branch was the source of the most consistent comfort complaint — the upstairs bathroom radiator never came up to temperature unless the boiler was firing at full output for at least forty minutes, which (given the system's oversizing) it rarely did.

Design

A Manual J load calculation against the actual envelope (R-19 walls after a 2014 cellulose blown-in upgrade, R-49 attic, U-0.31 glazing) gave 86,000 BTUH at –10°F. The original boiler was sized at 220,000 BTUH — about 2.5× the actual load, which explained the short-cycling behavior. We specified a Navien NCB-180 combi boiler, 180,000 BTUH input. The NCB-180 is a combi (heating + domestic hot water on the same unit) and was selected because the homeowner also had an end-of-life electric water heater they wanted to retire; the NCB-180's domestic-side performance comfortably matched the house's DHW peak demand.

Distribution redesign: delete the monoflo branch, replace with proper two-pipe direct-return ½″ copper to the upstairs bathroom radiator, run new ¾″ PEX-AL-PEX from the new boiler to each leg's connection at the basement main. Install a Δp-controlled Taco 00-VR ECM circulator on the primary loop, replace the old three-piece system pump. Install a Caleffi DISCAL air separator at the boiler outlet (the existing system had an antiquated scoop-type separator that had stopped removing air). Add a TRV at every radiator on the supply side, plus a lockshield on the return for as-needed balancing.

Installation · September 2022

The radiators were removed in three days using two technicians and a pallet jack on rolling boards (cast-iron radiators are heavy — a typical 38-tube column radiator weighs 380–520 pounds). Removed radiators went to the shop for pressure-washing, two coats of low-VOC paint matched to the homeowner's preference (a deep oxblood for the parlor and dining radiators; a cool gray for the bedrooms and kitchen), valve repacking, and bleed replacement. Refinishing took six business days at the shop in parallel with the distribution rebuild on site.

The donor radiators from Heritage Radiator Co. arrived on a flatbed truck on September 19; they were pressure-tested at the shop and required no additional refinishing. We installed them in the original positions on September 26. The two attic-replacement Runtal R-30 panels were installed on September 27, fed by a new ½″ PEX-AL-PEX home run from the basement up an interior wall chase.

The new Navien NCB-180 was set on October 8 in the position of the old Burnham (the chimney's existing sleeve was abandoned and capped; the new venting runs horizontally through the south basement wall in CPVC, with combustion air through a separate intake on the same wall). The mechanical room layout was simplified considerably — the old Burnham occupied roughly 24 cubic feet of floor space, the wall-hung Navien occupies roughly 6 cubic feet of wall space.

Commissioning record · 2022-10-28

Outside temperature: +31°F at 09:00 MT warming to +44°F by 16:30. House occupied. Pressure test of complete system at 60 psi cold for 24 hours preceding commissioning held within 1 psi (acceptable). Boiler started at minimum firing rate (~18,000 BTUH); combustion measured with Testo 320 at 25%, 50%, 75%, 100%; CO < 16 ppm air-free across the range; CO₂ 8.7–9.2%; O₂ 4.8–5.2%. Outdoor-reset curve set: 180°F supply at 0°F outside, 130°F supply at 50°F outside, return-water floor 95°F.

Each radiator was bled at the bleed port until water ran clear; supply Δt at the manifold against the radiator-by-radiator readings showed all eleven retained radiators and the two donor radiators emitting within 3°F of design at the test supply temperature. TRVs were set to a neutral setpoint of 68°F by default for occupant tuning. Domestic hot water was tested for first-hour delivery (96°F at 2.4 gpm sustained, well within the NCB-180's spec). House at setpoint by 14:20.

Operating record · three heating seasons

Through the 2022–23, 2023–24, and 2024–25 heating seasons, the system has shown a 33%, 38%, and 31% reduction in annual natural-gas consumption respectively, against the 2021–22 baseline measured before our work. The variance across years tracks heating-degree-day variance per the NOAA Climate at a Glance Gallatin County data. Cycle reduction (number of boiler ignition events per heating season) is approximately 68% relative to the as-replaced Burnham, measured via the Navien onboard counter.

Comfort: the upstairs bathroom (previously the worst-performing radiator in the house) is now the most-loved radiator, consistently 1–2°F above setpoint, with the TRV throttled back by the homeowner. The parlor radiator, which had been overheating the parlor in mild weather, now modulates appropriately with the outdoor-reset curve.

Credits

Engineer of record · Iris Cottonwood. Radiator assessment + removal + refinishing · Lars Treadway + Kel Begay + Marlowe Krenn. Donor radiator sourcing · Halla Bjørndal (coordinating with Heritage Radiator Co.). Distribution rebuild · Lars Treadway + Kel Begay. New boiler install · Lars Treadway. Commissioning · Wendell Pratt + Lars Treadway. Binder · Halla Bjørndal. Signed off by Lars W. Treadway, 2022-10-28.

Read across

Read the legacy rehabilitation scope for the framework, the mod-con boiler scope for the plant, and the about page chapter VII on the reading shelf for the books behind this work.