How it came in
The forecast on the 18th from the NWS Billings forecast office called for an Arctic air mass to settle into the Gallatin Valley by Wednesday evening (1/21) and to hold through Friday morning (1/23). The forecast lows were –18°F to –24°F at the valley floor, with somewhat colder readings expected at the canyon-bottom stations up Bridger and Hyalite. Lars rotated the on-call schedule on Monday morning per the standing winter protocol: full crew on phone, two pre-staged trucks at the shop with extra actuators and the spare Caleffi NeutraSafe canisters, and the test-bench boiler taken offline so the commissioning kit could be reassigned if needed.
The event came in clean — wind shifted to the northwest at 18:00 MT on Wednesday, temperature dropped through 0°F at 19:00, and the air mass settled with surface winds dropping to 6–8 knots. The valley turned into a cold pocket. By midnight Wednesday it was reading –12°F at the shop and –15°F at the AWOS at the airport.
Thursday, 03:15 MT · the Bridger Canyon call
The first call of the event came in at 03:08 MT on Thursday. Bridger Canyon Residence homeowner, reporting the primary bath reading 64°F (setpoint 70°F). Outside reading on the homeowner's deck thermometer: –22°F. The homeowner had checked the boiler — firing, not in error — and noticed the great-room slab felt warm under bare feet but the primary bath floor felt cool. Confirmed by the homeowner's wall thermostat at the bedroom: holding setpoint. Confirmed by the bath's slab-surface read: 64.2°F (target 76–80°F).
Lars took the call. The system was familiar — Lars had been on the commissioning crew in November 2024 and knew the manifold M-2 served the primary bath. Diagnosis from the call, before any truck moved: a seized actuator on loop C-6, the primary bath slab. Kel and Marlowe rolled Truck 2 at 03:24 with a stock Caleffi Z-One thermoelectric actuator and the loop schedule for the M-2 manifold on a clipboard. Drive time from the shop to Bridger Canyon at –22°F with a clean dry road: 27 minutes.
On site at 03:51. Mechanical room temperature 58°F, boiler firing at 84% (within range for the design load at –22°F). M-2 manifold open at 03:54: actuator on loop C-6 showing the call-LED green but the actuator stem at its rest position — a stuck wax element, the failure mode we have seen six times in fourteen years. Actuator removed and replaced with the stock unit at 03:56. New actuator drew against the manifold and opened the valve at 03:58. Supply/return Δt on C-6 measured 4°F at 04:05, building to 18°F at 04:32 as the loop came up. Slab surface at 78°F by 07:00. Truck back at the shop by 06:14, coffee at Cateye Café on the way back.
Thursday, 11:00 MT · the second call
A homeowner from the 2021 Bridger Canyon caretaker-cabin install (JB-2021-12) called at 10:54 reporting the cabin's pellet stove had not lit overnight and the hydronic system had been the sole heat source — concerned that the cabin was reading 58°F at the thermostat (setpoint 64°F). Outside –18°F. Lars took the call. Asked the homeowner to read the boiler display: firing at 100% modulation, no error codes, supply temperature 138°F (within design at the cold reset point). Asked the homeowner to feel the slab in the kitchen: warm. Bedroom slab: warm. Bathroom slab: warm. Diagnosis: the cabin's heating load at –18°F was being driven by both the pellet stove and the hydronic system together; with the pellet stove down, the system was alone and the cabin's heat-loss rate was outrunning the slab's emission rate.
This is not a system failure — it is a sizing question that the owners had been told about at commissioning four years before. The slab is sized to maintain setpoint with the pellet stove assumed off; the cabin's deep cold setpoint hold is 60°F in the absence of the pellet stove, recorded in the binder. The homeowner found the binder, found the page (commissioning report, January 8, 2022, "deep-cold no-stove hold: 60°F at –20°F outside"), called back at 11:34 to confirm the system was operating as designed, and arranged for a pellet-stove technician separately. No truck rolled.
Thursday, 16:20 MT · the Cottonwood snowmelt
Halla called the Cottonwood Drive homeowner at 16:00 to confirm the snowmelt slab was idling appropriately through the cold event. The homeowner walked outside, confirmed the slab was clear of frost despite the surrounding ground being ice-glazed, confirmed the slab sensor was showing 33.1°F surface (target idle 32–34°F), and reported the boiler was running at about 32% firing rate. No action. The snowmelt at the Bridger Canyon residence was running similarly per the Tekmar 654's connected display. All four operating snowmelt systems carried the event idle-on without intervention.
Friday, 06:00 MT · the warm-up
Air mass began advecting east at 06:00 Friday. Temperature climbed through –10°F at 09:00 and through 0°F at 11:30. The boiler at the shop dropped from 100% modulation to 64% at 13:00. The week's commissioning paused on Wednesday resumed Friday afternoon. Iris worked the commissioning page for JB-2026-02 (Hyalite Foothills Phase 2) which had been scheduled for Thursday and rolled to Saturday.
What we wrote down
Three things from the cold-snap event went into the master notebook for the post-season debrief:
- Actuator stock — running with 6 spare Caleffi Z-One actuators in the truck and 12 at the shop. The Bridger Canyon call used one from the truck; that needs replacement. Restock to baseline by 02-15 (Halla).
- Outdoor sensor calibration — the Cottonwood snowmelt's outdoor sensor showed a 1.5°F drift from the slab sensor at peak draw in the storm on 02-04, and a 0.8°F drift during the cold-snap event. Two data points are not a trend, but worth a sensor swap at the spring service to confirm.
- Phone protocol — the Bridger Canyon caretaker cabin call resolved without a truck roll because the homeowner had the binder and could read the page. This is the design intent and it worked. Lars wrote a one-paragraph note for the apprentice handbook on phone-call diagnosis.
Read across
Read the Bridger Canyon residence for the full as-built; the manifold sheet for the actuator failure mode discussion; the commissioning scope for what we look at in audit visits.