Mod-con boilers & venting.

The plant, sized to load

A modulating-condensing boiler is a sealed-combustion appliance that condenses the water vapor out of its own flue gas to recover the latent heat, achieving annual fuel utilization efficiency (AFUE) ratings of 95% and higher. The condensing happens when the return water temperature is below the flue gas dew point (about 130°F for natural gas) — which is exactly the operating condition of a properly-designed hydronic radiant system. Mod-con plants are the right plant for radiant systems for that reason, full stop.

We size the plant to the Manual J peak heating load, not to a rule-of-thumb BTU-per-square-foot estimate. For southwest Montana residential work, the peak heating load in a well-built 3,500–4,500 sf custom home typically lands between 85,000 and 130,000 BTUH at the 99% design temperature; older 1920s–1940s housing stock at the same square footage typically runs 130,000–180,000 BTUH; very large or poorly-insulated buildings may exceed 200,000 BTUH. We then select a boiler whose minimum firing rate is at or near 25% of the peak load, so the plant can modulate continuously across the entire heating season without short-cycling.

The three boilers we install most often

Lochinvar KHB Knight — our default residential plant in the 80,000 to 199,000 BTUH range. Stainless-steel fire-tube heat exchanger, 10:1 turndown on the WHB series, onboard SMART SYSTEM control with built-in outdoor-reset and a snowmelt-friendly priority sequence. Approximately $4,200–$6,800 boiler-only at our distributor.

Triangle Tube Prestige Solo & Trimax — what we install when the homeowner has elected an indirect water heater on the same plant. Prestige's helical stainless heat exchanger handles the priority DHW call cleanly, and the Trimax control has a particularly clean outdoor-reset and snowmelt logic. $4,500–$8,200 boiler-only.

Viessmann Vitodens 200-W — what we install when the homeowner wants the longest-warranty German engineering and accepts a slightly more involved commissioning. Inox-Radial heat exchanger, 5:1–7:1 turndown, Vitotronic control with Lambda Pro Plus combustion management that auto-tunes to gas composition. $7,500–$14,500 boiler-only, but the plant is genuinely set-and-forget for 15+ years.

Venting

Mod-con boilers vent in CPVC, PVC, or polypropylene depending on the manufacturer's listing — never in metal vent intended for atmospheric appliances. The vent and combustion-air pipes are sized per the manufacturer's published equivalent-length tables, with elbow counts and pipe diameter set against the run. We run dedicated combustion air to outside in every install; we never use room-air combustion for a sealed-combustion boiler, even when the listing technically permits it. We pitch the vent back to the boiler at the prescribed slope so condensate from the vent runs back to the condensate trap, not pooled at a low point in the vent run.

Condensate handling matters. The condensate stream from a mod-con boiler is mildly acidic — pH around 3.5 — and is not legal to dump directly to a municipal sewer in many jurisdictions, including the City of Bozeman per its water reclamation facility guidance. We install a neutralizer kit (limestone media in a small inline canister) downstream of the condensate trap; the pH at the outlet runs 6.5–7.5, well within municipal limits. The neutralizer media is replaced annually at the spring service visit.

Commissioning

Commissioning happens in a single day with the system pressurized, purged, and the building enclosed. We start the boiler at minimum firing rate, check combustion with a Testo 320 analyzer (CO, CO₂, O₂, stack temp, draft), ramp through 25%, 50%, 75%, and 100% firing rate, and adjust the air-fuel ratio at the gas valve where the manufacturer's commissioning permits. We set the outdoor-reset curve at the boiler control: typical residential curve in this climate is 130°F supply at 0°F outside, 105°F supply at 50°F outside, with a minimum return-water protection floor at 90°F to prevent low-fire stalls in the very early shoulder. We document everything in the binder. The outside temperature on the commissioning day is stamped on the page.

What it costs

Boiler-only installed in our service area runs $8,500 to $22,000. The variable is the brand, the BTUH, the venting run length (every 25 equivalent feet of vent over the base run adds material and labor), the gas-line modification required to reach the nameplate input, and whether an indirect water heater is included on the plant. A complete plant including indirect, primary pump set, expansion tank, hydraulic separator, and condensate neutralizer typically lands at $14,000–$28,000 installed.

Maintenance — the part most homeowners don't budget

A mod-con boiler is a high-efficiency appliance with a stainless heat exchanger that benefits from annual professional service. We bring our existing clients onto an annual spring service calendar at $385–$525 per visit, including: combustion analysis, heat-exchanger inspection through the sight port, condensate neutralizer media replacement, vent inspection, igniter and flame-sensor cleaning, gas pressure check, expansion-tank pre-charge check, glycol concentration check on snowmelt systems, and a one-page service sheet added to the binder. We do not sell maintenance contracts as a profit center; we do them because they keep the systems running and the warranty intact.

Read across

See the mod-con boiler component sheet in the atlas for the part-by-part submittal. See the radiant floor and snowmelt scopes for what the boiler is feeding. See the Bridger Canyon residence for a fresh Lochinvar Knight commissioning at –14°F.