Crestwood Modulating Furnace Upgrade: 2018 Single-Stage Replacement With 2-Year AC Retention Phased Approach

Case study summary: 1990 Crestwood ranch with end-of-life 30-year-old single-stage gas furnace (Bryant 313AAV from 1995); customer had recently replaced AC in 2023 (Carrier 24ABB6 2.5-ton, 2 years old) and wanted to retain new AC equipment while upgrading furnace; furnace-only replacement with new Carrier Performance 96 modulating furnace (60,000 BTU 96% AFUE); existing ductwork and AC retained; existing thermostat upgraded to Carrier Infinity touchscreen for variable-capacity furnace integration with existing two-stage AC; total project cost $7,800; completed in 1 day during November 2025. Outcome: 31-36% reduction in heating gas consumption; smoother furnace operation pairing well with existing two-stage AC; phased equipment replacement plan allowed customer to spread investment across multiple years.

The Home

• Built: 1990
• Style: Single-story ranch with attached two-car garage
• Location: Crestwood, MO 63126 (1980s-1990s subdivision)
• Size: 1,820 sf single level plus 1,400 sf finished basement (3,220 sf total conditioned with basement included)
• Construction: Wood frame with brick veneer; concrete block walls on basement; original windows replaced with vinyl in 2017; attic insulation upgraded to R-49 in 2018
• Existing heating: Bryant 313AAV (80,000 BTU/hr 80% AFUE single-stage gas furnace from 1995, 30 years old at time of replacement)
• Existing cooling: Carrier 24ABB6 2.5-ton two-stage AC from 2023 (2 years old, well within service life); previous AC was Bryant 463A 2.5-ton from 1995 (replaced 2023 due to R-22 compressor failure)
• Ductwork: Sheet metal supply trunks from 1990 with adequate sizing; some sealing improvement opportunity during installation
• Existing thermostat: Honeywell programmable thermostat installed during 2023 AC replacement; not communicating-equipment-compatible with new modulating furnace

The Situation

Customer contacted us in September 2025 about furnace-only replacement. Drivers:

• Furnace equipment status: 30 years old, well past typical 18-25 year service life; pressure switch replacement (2024) and inducer motor replacement (2025) signaled end-of-life pattern
• Recent AC investment: customer had spent $5,800 on AC replacement in 2023 due to R-22 compressor failure; this equipment was high-quality (Carrier 24ABB6 two-stage) with 8+ years remaining service life expected
• Phased equipment strategy: customer wanted to spread equipment replacement across multiple years rather than do simultaneous full-system replacement; furnace replacement now, AC replacement when needed in approximately 2031-2033
• Variable-capacity equipment opportunity: customer interested in modulating furnace benefits (longer cycles, better distribution, quieter operation) without doing full-system replacement

Initial assessment confirmed furnace-only replacement scope:

• Existing Carrier 24ABB6 AC compatible with new modulating furnace through Carrier Infinity touchscreen thermostat (communicating equipment protocol supports both)
• Existing ductwork and refrigerant connections retained without modifications
• Equipment location in utility room adequate for new equipment
• Thermostat upgrade required for variable-capacity equipment integration but otherwise minor scope

The Equipment Selection

Carrier Performance 96 Modulating Furnace (Selected)

• AFUE: 96%
• Capacity: 60,000 BTU/hr input (Manual J calculation showed 52,000 BTU/hr design heating load; previous 80,000 BTU was oversized for actual home load; 60,000 BTU provides appropriate margin)
• Technology: Modulating gas valve with variable-speed ECM blower; 40-100% modulation range
• Warranty: 10-year parts, 20-year heat exchanger

Carrier Infinity Touchscreen Thermostat

• Communicating equipment protocol compatible with both new Carrier Performance 96 furnace AND existing Carrier 24ABB6 AC
• Provides integrated variable-capacity control across heating and cooling modes
• Replaces previous Honeywell programmable thermostat (which couldn’t fully integrate with two-stage AC even before furnace upgrade)

Alternative Equipment Considered

• Carrier Performance 80 (80% AFUE non-modulating): lower cost ($1,800 less); selected against because efficiency gain wouldn’t justify investment and modulating operation provides comfort benefits
• Carrier Comfort 92 (92% AFUE two-stage non-modulating): middle option ($800 less than modulating); selected against because modulating equipment provides better long-term value
• Trane S9V2 modulating: equivalent equipment from different manufacturer; selected against because Carrier brand match with existing AC equipment provides integration benefits
• Heat pump alternative (Carrier Performance 38MURA): would have replaced gas heating; customer chose gas continuation based on operational familiarity and modest projected operational savings

The Project Scope

Pre-Installation

• Crestwood Public Works permit: Pulled within 4 business days (mechanical permit only for furnace replacement)
• Equipment ordering: 7 business days for Carrier Performance 96 furnace and Carrier Infinity touchscreen thermostat
• Installation scheduling: November 2025 before peak heating season

Installation Day 1 (November 14, 2025 – single-day installation)

• Morning (8:00 AM – 11:30 AM): Existing furnace isolation and shutdown; gas line capped; electrical disconnected; old furnace removal (300 lbs, removed via utility room to garage to outdoor disposal vehicle); chimney venting disconnected; existing AC connections protected during furnace work
• Mid-day (11:30 AM – 12:30 PM): Ductwork inspection and modest sealing improvements at supply and return trunk connections; old chimney venting verified disconnected from active heating equipment
• Afternoon (12:30 PM – 4:30 PM): Carrier Performance 96 furnace placement; gas line reconnection; electrical connection; PVC venting installation through utility room exterior wall (replacing previous chimney venting); condensate management routing to existing utility room floor drain; existing AC plenum and refrigerant connections verified intact
• Late afternoon (4:30 PM – 6:30 PM): Carrier Infinity touchscreen thermostat installation; furnace commissioning with Bacharach combustion analysis (CO 11 ppm air-free, CO2 9.6%, O2 5.5%); communicating equipment configuration; system performance verification in heating mode; AC integration verification (cooling mode test); customer walk-through
• End of Day 1: equipment fully installed and operational; chimney top sealed with stainless steel cap (chimney no longer used for venting; preserved as architectural feature)

Inspection (November 21, 2025)

• Crestwood Public Works inspector visit; installation passed inspection

The Pricing

• Carrier Performance 96 60,000 BTU modulating furnace: $3,400 equipment
• Carrier Infinity touchscreen thermostat: $580
• Installation labor (1 day, 2 technicians): $1,600
• Ductwork sealing scope: $280
• PVC venting installation through exterior wall: $480
• Crestwood Public Works permit: $200
• Old equipment removal and disposal (300 lb furnace): $240
• Bacharach combustion analysis and commissioning: $200
• Chimney cap installation (sealing unused chimney for moisture protection): $180
• Drywall and paint patches at thermostat location: $80
• Communicating equipment configuration and AC integration testing: $200
• Miscellaneous fittings, hardware, and consumables: $160
• Total project cost: $7,600 rounded to $7,800 with permit contingency and installation variances

This pricing fell within our standard range for furnace-only replacement with thermostat upgrade ($6,800–$9,400 for modulating furnace + communicating thermostat installations).

The Outcome

Heating Performance

• First-winter operation (November 2025 – March 2026): System maintained setpoint reliably with substantially quieter operation than previous single-stage equipment
• Modulation benefits: equipment running at 40-55% capacity during typical winter conditions provides longer cycles and more even distribution; customer notes basement temperatures more consistent with main level
• Quieter operation: customer reports dramatic noise reduction vs. previous 30-year-old equipment

Integration with Existing AC

• Two-stage cooling integration: existing Carrier 24ABB6 two-stage AC now integrated through Infinity touchscreen; previous Honeywell thermostat couldn’t fully utilize two-stage operation
• Improved cooling performance: even though AC equipment unchanged, control upgrade through Infinity thermostat enables previously-unused two-stage capabilities; verification during spring 2026 cooling-mode operation
• System longevity: 2-year-old AC equipment expected to provide 8-13 more years of service before replacement consideration; furnace-only replacement preserved this investment

Energy Performance

• First-winter gas consumption (Nov 2025 – Mar 2026): approximately 980 therms vs. previous winter (Nov 2024 – Mar 2025) consumption of 1,460 therms — 33% reduction
• Annual gas savings at $1.20/therm: approximately $575 reduction in annual heating costs
• Simple payback on installation: approximately 13-15 years on heating efficiency savings alone; faster considering equipment reliability and avoided emergency replacement risk

Customer Feedback

Customer reported high satisfaction with several aspects: furnace-only approach preserved 2-year-old AC investment; modulating operation noticeably quieter and more even than predecessor; smart thermostat provides remote monitoring and integration with existing AC capabilities; single-day installation minimal disruption; phased equipment plan allows future AC replacement at natural end-of-service-life without simultaneous full-system investment.

Ongoing Service

Customer enrolled in maintenance plan covering furnace and existing AC; both pieces of equipment serviced together annually under unified maintenance scope.

Frequently Asked Questions

Why replace only the furnace if both pieces of equipment are over 25 years old?

Customer’s situation involved recent AC replacement creating asymmetric equipment age that supported furnace-only replacement. Recent AC replacement history (this customer): 2023 AC replacement was forced by R-22 compressor failure on previous Bryant equipment; customer invested $5,800 in new Carrier 24ABB6 two-stage AC at that time; new AC equipment has expected 10-15 year service life remaining as of 2025. Decision analysis – replacing both vs. furnace only: replacing both equipment now would discard 2-year-old AC with 8-13 years remaining service life; that’s substantial wasted equipment value; furnace-only approach extracts full value from previous AC investment while addressing immediate need. When to replace both pieces of equipment simultaneously: equipment of similar age both at or approaching end of service life; matched-system benefits valuable for customer; budget supports full-system replacement; planning for long-term equipment longevity. When phased replacement makes sense: significant age difference between heating and cooling equipment (this customer’s situation); recent investment in one piece of equipment; budget constraints requiring spreading investment across years; customer comfortable managing equipment replacement timing actively. Compatibility consideration: new modulating furnace + existing two-stage AC integrates well through communicating thermostat; brand consistency (Carrier furnace + Carrier AC) supports integration; cross-brand integration possible but more complex; integration capability should be verified during quote development. For this customer’s situation: phased approach extracted full value from 2023 AC investment; matched-brand integration through Carrier Infinity thermostat provides system benefits without simultaneous full-system replacement; customer can plan AC replacement for 2031-2034 timeframe when natural end of service life approached. For other customers in similar situations: evaluate actual remaining equipment life; phased approach extracts more value from existing equipment when remaining life is substantial; full-system replacement appropriate when both pieces of equipment are at similar ages.

How does the communicating thermostat improve operation of existing AC?

Modern smart thermostat unlocks two-stage capabilities of existing AC that previous thermostat couldn’t access. Why previous Honeywell thermostat limited two-stage AC: Honeywell programmable thermostat from 2023 installation was basic equipment selected by previous contractor at time of AC replacement; thermostat provided basic two-stage AC control but without sophisticated coordination; equipment ran on simple capacity stage selection based on temperature difference; couldn’t optimize stage selection for variable-capacity equipment benefits. Carrier Infinity touchscreen thermostat improvements for existing AC: communicating equipment protocol allows direct AC interface (not just contact closure like basic thermostats); equipment runs at optimal capacity stage based on actual demand and operating conditions; smart staging logic provides longer cycles at low stage when appropriate; improved coordination with modulating furnace for transition between modes. Specific operational improvements: low-stage operation more frequent (~70% of cooling time at low stage vs. ~50% with previous thermostat); longer cycles improve humidity removal; more even distribution from longer cycle airflow; reduced cycling reduces equipment wear. Quantifying improvement: customer’s spring 2026 cooling-mode verification shows slightly lower energy consumption vs. previous summer despite no equipment change; estimated 6-10% improvement in cooling efficiency through better staging logic; minimal but measurable improvement. For other customers considering thermostat upgrades alone: communicating thermostats can improve two-stage and variable-capacity equipment performance even without other equipment changes; cost $400-$800 for thermostat upgrade vs. potential 5-15% efficiency improvement; worthwhile investment when paired with existing communicating-compatible equipment. Why thermostat upgrade matters specifically: many customers don’t realize that thermostat selection affects equipment performance beyond basic on/off control; sophisticated equipment requires sophisticated control to deliver full capabilities; thermostat is often overlooked component of equipment upgrade decisions.

What’s the value of single-day installation vs. multi-day projects?

Furnace-only replacement is dramatically simpler than full-system replacement, supporting same-day completion. Single-day installation feasibility for furnace replacement: gas line reconnection straightforward; electrical connection simple; venting installation moderate scope; thermostat upgrade quick; commissioning standard; total scope manageable for 2-technician team in single day. Compared to full-system replacement timing: typical 2-day project for matched furnace + AC replacement; refrigerant work adds time (recovery, vacuum, charge); two equipment installations require sequencing; additional commissioning scope. Customer benefit of single-day installation: minimal home disruption; no extended period without heating capability during winter season; customer can plan day for installation rather than multi-day inconvenience; less stress about equipment installation process. Single-day installation requirements: existing equipment in accessible location; no asbestos abatement coordination needed (this would require multi-day project); ductwork serviceable without major modifications; existing connections compatible with new equipment; clear weather for outdoor equipment work if applicable. When single-day installation isn’t feasible: complete heating system replacement (with new ductwork requirements); custom equipment configurations requiring fabrication; asbestos abatement coordination (multi-day project); unusual installation locations or access challenges. For this customer’s installation: standard utility room equipment location; existing ductwork retained without modifications; furnace-only scope; standard equipment configuration; single-day installation appropriate. Cost implications of installation duration: $1,600 labor for single-day installation vs. typical $2,400-$3,200 for 2-day installation; reduced labor cost reflects simpler scope rather than rushed work quality. For customers considering furnace-only replacement: single-day installation typical when scope is straightforward; reasonable customer expectation for furnace-only project; multi-day projects appropriate when scope warrants but shouldn’t be assumed required.

When does it make sense to replace AC during furnace replacement vs. waiting?

Customer-specific decision based on AC age, condition, and economic factors. Replace AC during furnace work when: AC is older than 12-15 years (within typical 15-20 year service life); AC has experienced significant repair history; AC uses R-22 refrigerant (becoming expensive and difficult to service); matched-system benefits substantial; customer prefers single project disruption; budget supports full-system replacement. Wait on AC when: AC is recent (under 5-7 years for typical equipment); AC has clean repair history; AC equipment is high-quality with substantial remaining life; budget supports phased approach better than full-system; customer comfortable managing equipment timing actively. For this customer’s situation: 2-year-old Carrier 24ABB6 AC with 8-13 years remaining service life; replacing now would waste substantial remaining equipment value; phased approach preserves AC investment; future AC replacement (2031-2034 estimated timeframe) can be matched with then-current technology. Cost-benefit analysis – replace now: $7,800 furnace only + future AC replacement in 6-9 years vs. $13,500 full-system replacement now; phased approach total cost approximately $14,000-$16,000 (current furnace cost + future AC); slightly higher total cost over period; benefits are preserved AC investment and ability to use latest technology for future AC. Customer decision approach: evaluate actual remaining equipment service life; consider operational benefits of full-system replacement (matched equipment, simultaneous upgrade); evaluate budget across phased vs. simultaneous replacement; consider customer’s planning preferences. For this customer’s situation: phased approach made sense given recent AC investment; we present both options during quote development and let customer make informed decision; many customers in similar situations choose phased approach; some prefer full-system for simplicity. For customers facing this decision: consider remaining equipment life as primary factor; brand integration possible across brands but Carrier-to-Carrier or Trane-to-Trane provides simplest integration; communicate timing preferences during quote development.

What’s the long-term cost comparison: gas furnace vs. heat pump for this scenario?

Heat pump alternative discussed but customer chose gas continuation for specific reasons. Heat pump alternative scope for this customer: would have replaced gas heating with Carrier Performance 38MURA heat pump (3-ton, 17 SEER2 cooling rating, electric backup heating); would have required replacing existing 2-year-old AC equipment (heat pump integrates with air handler, not gas furnace); cost approximately $13,200 for heat pump + air handler vs. $7,800 furnace-only approach. Heat pump pros for this customer: gas service eliminated (operating cost dependence on electricity only); environmental benefit of electrification; potentially lower long-term operating costs (if electricity prices remain stable relative to gas); preserves AC functionality through heat pump cooling capability (heat pump doubles as AC). Heat pump cons for this customer: would have required replacing 2-year-old AC equipment (substantial wasted investment); higher upfront cost ($13,200 vs. $7,800 = $5,400 premium); electricity cost during cold weather backup heating; uncertainty about long-term electricity cost trends; existing gas service infrastructure being abandoned. Customer decision factors: substantial AC investment already made (2-year-old equipment with significant remaining service life); existing gas service reliable and relatively economical at $1.20/therm; customer not actively pursuing electrification goals; phased equipment replacement strategy in place; cost-benefit ratio for heat pump conversion didn’t justify in current situation. Long-term cost projection: gas furnace approach over 20-year period – $7,800 furnace replacement + future AC replacement $9,000-$11,000 + 20 years of natural gas operating costs at $1,400-$1,800 annually = $44,800-$55,400 total cost over 20 years; heat pump approach over 20-year period – $13,200 heat pump installation + AC equipment service over 20 years + 20 years of electricity heating costs at $1,300-$1,700 annually = $39,200-$47,200 total cost over 20 years; heat pump shows modest long-term advantage but with significant uncertainty about energy price trends. For other customers considering this comparison: evaluate actual current equipment condition; consider long-term home plans (electrification preferences, anticipated home sale timing); consider energy price preferences (electric vs. gas); discuss with quote development to evaluate specific situation. For this customer’s situation: phased gas furnace approach was right decision given specific circumstances; heat pump remains future option when AC reaches natural end of service life in 2031-2034 timeframe. We present both options during quote development; customer makes informed decision based on their priorities.

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Contact Purisync Heating and Air

For your furnace-only replacement, phased equipment replacement strategy, or thermostat upgrade with existing equipment integration in Crestwood, contact our 325 N Kirkwood Road office at (314) 338-5111. Initial consultation includes Manual J load calculation, evaluation of existing equipment compatibility, equipment options across multiple manufacturers, phased vs. full-system comparison, and written quote with itemized pricing.

• Emergency Line (24/7): (314) 338-5111
• Address: 325 N Kirkwood Rd #245, Kirkwood, MO 63122
• Email: info@purisyncheatingairconditioning.xyz
• St. Louis County Mechanical Contractor License: #MC-2014-08439-STL
• Kirkwood Business Registration: #BL-2014-1187
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