Case Study: Emergency Furnace Replacement at SunCrest, Draper — January

Location: SunCrest neighborhood, Draper, UT — 6,100 ft elevation
Season: January
Outdoor temperature at call time: 11°F valley floor; approximately –3°F at address elevation during overnight low
Presenting complaint: No heat; furnace starting and shutting down within 30 seconds; CO detector alarmed earlier in the evening
Equipment installed: Carrier Performance 96 two-stage gas furnace (59TP6C100S21-14), 100,000 BTU/hr nameplate, 96% AFUE
Technician: Diego Ramirez

The Call

The call came in at 8:47 p.m. on a January night. A homeowner in SunCrest reported that the furnace had been cycling on and off for several hours before stopping completely, and that their Kidde CO detector had alarmed briefly around 7 p.m. before clearing. Indoor temperature was 63°F and dropping; outdoor temperature on the valley floor was 11°F. At SunCrest’s 6,100-foot elevation, the overnight low was forecast at –3°F.

The household had two children under 10. A dropping home at SunCrest elevation with a CO detector alarm history and a forecast overnight low below zero is an immediate dispatch regardless of hour or day of week. Diego Ramirez was on-call; he was on-site at the SunCrest address 97 minutes after the call, which included the 35-minute drive up Traverse Ridge Road on a road that was dry but had patches of ice on the shaded curves.

The Diagnostic

The furnace was a Carrier 59TP6 installed in 2011 — 13 years old at the time of the call. Before pulling the access panel, Diego ran a combustion analysis at steady state on a trial lighting sequence. The result: CO air-free at 341 ppm, O₂ at 2.1%. That is not an altitude derate problem. At SunCrest’s 6,200-foot elevation, a correctly running furnace should show CO air-free under 100 ppm with O₂ between 5 and 9%. An altitude derate error at this elevation produces CO air-free in the 150–250 ppm range. A reading of 341 ppm with O₂ at 2.1% indicates a heat exchanger breach routing flue gas into the combustion air supply or the return air stream — combustion products recirculating back through the system.

Diego did not restart the furnace after the combustion analysis reading. He opened the primary heat exchanger access panel and conducted a borescope inspection. The borescope revealed a 3/4-inch crack at the secondary heat exchanger outlet, consistent with the overfire discoloration pattern visible at the primary access: 13 years of combustion without altitude derate at 6,200 feet, running at approximately 25% over-fuel from a sea-level manifold pressure setting, had compressed the thermal fatigue timeline substantially below the furnace’s rated 20-year heat exchanger service life. The crack was large enough to route combustion products into the return air — which explained the CO detector alarm earlier in the evening when the furnace was still running cycling attempts.

The assessment: the furnace could not be returned to service. Heat exchanger repair on a 13-year-old furnace with a breach of this size was not economically defensible. The unit needed to be replaced.

The Options Discussion

Diego called the homeowner — who had been waiting outside in a neighbor’s house with the children — at 11:23 p.m. to review the findings. The conversation covered three items:

Safety first: The CO detector alarm was not a false alarm. Combustion products had been entering the return air circuit. The home could not be reoccupied with the furnace operating. There was no path to a safe same-night repair of the existing system.

The replacement options: The homeowner’s priority was heat back on as quickly as possible given the overnight forecast. Diego confirmed that the Carrier Performance 96 two-stage condensing furnace in the same BTU class (100,000 BTU/hr nameplate, correctly sized at the altitude-corrected 74,200 BTU/hr delivered output for the SunCrest heat load) was available through our after-hours parts network for same-night delivery. The alternative — a modulating Carrier Infinity 59TN6 — would have been the long-term preference for a SunCrest home, but the modulating system was not available through the same-night parts channel. The homeowner authorized the Carrier 59TP6 replacement.

The upgrade opportunity: Since the furnace was being replaced and the existing installation had never been altitude-derated, the replacement would be commissioned correctly from the first ignition. Diego explained that the altitude correction for a 100,000 BTU/hr furnace at SunCrest reduces delivered input to approximately 74,200 BTU/hr — still adequate for the home’s calculated heating load at 6,200-foot design temperature, and correctly balanced for clean combustion rather than the 25% over-fueling that had been destroying the prior heat exchanger since 2011.

The Installation

Parts arrived on-site at 1:18 a.m. from our emergency parts network contact in the Salt Lake market. Diego began the installation at 1:27 a.m.

The existing furnace was an upflow configuration in the mechanical room of the home’s lower level. The installation scope:

• Existing furnace shut off and disconnected from gas, power, and condensate drain
• Combustion products in the duct system assessed: with a confirmed heat exchanger breach, the return air duct system had been exposed to flue gas. Diego ran the air handler in fan-only mode with all windows open for 20 minutes to flush the duct system before beginning installation
• Existing furnace removed to the exterior (placed outside for disposal, not through the living space)
• New Carrier 59TP6 positioned in place; gas and power connections made
• Existing two-pipe PVC venting inspected: the vent pipe slope was adequate, the exhaust termination on the exterior wall was correctly positioned above the roofline snow accumulation zone, and the intake was unobstructed. Venting reused without modification
• Condensate drain reconnected with descaling treatment and drain flush
• Gas valve manifold pressure set to altitude-corrected target for 6,200 feet: 24.8% reduction from sea-level specification per Carrier’s published altitude correction table for the 59TP6C100S21 model
• Combustion analysis at startup: CO air-free 38 ppm, O₂ 6.4%, stack temperature within specification. Documented.
• Static pressure measured: 0.28” WC total external static, within design range for the 59TP6 at this blower speed setting
• Temperature rise at steady state: 52°F, within the 35–65°F rated range

The homeowner returned to the home at 3:41 a.m. Indoor temperature had dropped to 55°F during the interruption. With the new furnace running correctly at its altitude-corrected output of approximately 74,200 BTU/hr at 96% AFUE, the home recovered to 68°F within 95 minutes against the –1°F outdoor temperature that prevailed at SunCrest elevation during the recovery period.

The Findings and What They Mean

The post-installation documentation confirmed what the combustion analysis had indicated: 13 years of operation at sea-level manifold pressure at 6,200 feet had produced the following documented consequences:

• CO air-free at 341 ppm from the cracked heat exchanger routing combustion products to return air — potentially 300+ ppm CO in the supply air during furnace operation in the weeks or months before the heat exchanger fully cracked
• Heat exchanger service life compressed from the rated 20 years to approximately 13 years by accelerated thermal fatigue from chronic rich-burn combustion
• Carbon deposits on the flame sensor requiring replacement as part of the installation scope, and overfire discoloration throughout the secondary heat exchanger cells consistent with years of localized hot spots from incomplete combustion
• An altitude derate error that had been present since the furnace’s 2011 installation by the original builder’s HVAC subcontractor, through at least two prior service visits documented in the homeowner’s records by two different contractors, neither of which had run a combustion analysis or checked manifold pressure

The last point bears emphasis. This furnace had been serviced at least twice in its 13 years — the homeowner had records showing a 2017 and a 2021 tune-up by two different local HVAC companies. Neither service report mentioned a combustion analysis, and neither mentioned manifold pressure verification. The altitude derate error that was present from 2011 ran uncorrected and undiscovered until it had done its work on the heat exchanger.

What This Cost

Total invoice for the emergency replacement, inclusive of after-hours emergency labor rate, parts, permit (applied for the next business day), and commissioning: $4,890. The homeowner’s home warranty covered $1,500 of the replacement cost, leaving a net out-of-pocket of $3,390 after the warranty reimbursement.

For context: a planned non-emergency replacement of this furnace with a modulating Carrier 59TN6 during the prior spring or fall would have cost approximately $5,800–$6,400 installed (modulating unit, slightly higher price than the two-stage 59TP6), without the after-hours labor premium, without the emergency parts premium, and with the opportunity to schedule the correct equipment rather than the equipment available through the same-night network. The actual emergency replacement on a January night at SunCrest cost approximately $1,500–$2,500 more than a planned replacement would have, in addition to a night spent in a neighbor’s house and a home that spent six hours at or below 55°F before heat was restored.

Takeaways

This case illustrates three issues that appear at different rates across our service area but concentrate at SunCrest and Traverse Ridge:

1. Altitude derate omission is common and consequential. The builder’s HVAC subcontractor did not apply the altitude correction in 2011. Two subsequent contractors did not identify or correct it in 2017 or 2021. The consequence was a heat exchanger that failed at 13 years instead of the rated 20 years, a CO detector alarm event in a home with young children, and an emergency replacement at –3°F.
2. A combustion analysis that includes manifold pressure verification detects this. The 2017 and 2021 service visits did not include combustion analysis. Had either of those technicians run a combustion analysis and found 180 ppm CO air-free (the approximate reading a 24.8% altitude derate error would produce at SunCrest), the manifold pressure correction would have been a 30-minute same-visit repair. The heat exchanger might have reached its rated service life. There would have been no CO alarm event.
3. Planned replacement is less expensive than emergency replacement. Not always by the same amount, and not every planned replacement prevents an emergency — but when a 10–13-year-old SunCrest furnace has no combustion analysis history and no altitude derate documentation, the probability of a heat exchanger event in the next 3–5 years is elevated. A planned replacement in the spring or fall is executed at lower cost, with better equipment selection, and without children sleeping in a neighbor’s house in January.

Contact

If you have a SunCrest or Traverse Ridge furnace that has never had a combustion analysis or altitude derate verification, contact us for a diagnostic visit before the next heating season. The visit takes 60–90 minutes, costs $89, and provides documented confirmation of safe operation or identifies a correctable problem before it becomes an emergency.

• Phone (24/7): (385) 336-1837
• Address: 12244 Business Park Dr #155, Draper, UT 84020
• Email: info@draperheatingairconditioning.xyz

Related Pages

• Furnace Repair in Draper
• Furnace Installation in Draper
• Emergency HVAC in Draper
• Draper HVAC Services Overview