Park Hill Denver Heat Pump & Furnace Retrofit Case Study

Residential Building Profile

This 2,400-square-foot 2-story home was constructed in 1965 in the Park Hill neighborhood, featuring 4 bedrooms, 2.5 bathrooms, full basement, and single-car garage. Original construction included a gravity-fed forced-air heating system with sheet metal ducts. The existing HVAC system—an 18-year-old single-stage York furnace with 80% AFUE and electric resistance backup—was failing with high utility bills, uneven comfort across floors, and frequent repairs. The homeowner (family of four) reported January heating bills of $420/month, with significant temperature stratification: upstairs reaching 74°F while basement remained 62°F.

Denver’s high altitude (5,280 feet, 82% of sea level air density) requires 3-4% system capacity derating and affects furnace combustion efficiency. Annual heating load: 6,282 degree-days (base 65°F), creating heating season from September through May—nine months of substantial demand.

Initial Energy Audit & Problem Identification

Diagnostic Process:

Antonio conducted a comprehensive energy audit using multiple diagnostic tools:

Blower Door Test: Using an Energy Conservatory DM32 differential manometer, the home showed a leakage rate of 8,400 CFM at 50 pascals (CFM50), normalized to 12.1 air changes per hour (ACH50). This indicates moderate-to-poor envelope sealing—typical for 1965-era construction with no poly vapor barrier or modern sealants. The industry standard for well-sealed homes is 3-4 ACH50.

Duct Leakage Assessment: Using a Blower Door with duct mask attachment:

• Total duct leakage measured at 1,240 CFM at 25 pascals (Qn = 0.44)
• Return leakage: 680 CFM (55% of total—primarily at basement boot connections)
• Supply leakage: 560 CFM (45%—distributed throughout 2nd floor zones)
• Estimated seasonal energy loss: 28-32% of conditioned air escaping before reaching living spaces

Thermal Imaging: Forward-looking infrared (FLIR E6) scan revealed:

• Exterior wall thermal gradient showing inadequate insulation (typical 1960s construction with 3.5″ fiberglass batts, many sections compressed)
• Heat loss concentration at window frames and electrical outlets (standard 2×4 framing, no thermal breaks)
• Basement rim joist showing 8-12°F temperature differential (uninsulated on original home, insulation added ~1995 but degraded)

Furnace Efficiency Testing:

• Combustion efficiency: 76% (down from 80% nameplate due to carbon scaling)
• Steady-state efficiency: 73%
• Flue gas temperature: 392°F (high, indicating heat loss)
• Draft: 0.08″ water column (low side of range, affecting burner flame quality)
• Natural gas consumption at full fire: 72,000 BTU/hr input, delivering ~57,600 BTU/hr usable (18% loss to flue)

Thermostat Performance:

• Single-stage manual setback (homeowner adjusted manually from 66°F night to 72°F day)
• No outdoor air sensor feedback
• Baseline system runtime: 18 hours/day during January design conditions
• Cycle efficiency: ~64% due to excessive on-off cycling and standby losses

Diagnostic Findings & Root Causes

Primary Issues:

1. Furnace Age & Efficiency Decline: 18-year-old furnace operating at 73% steady-state efficiency (vs. design 80%). Scale buildup in heat exchanger, worn burner tile, and corroded flue venting all contributed to performance degradation.

2. Duct System Deficiency: 44% total duct leakage (0.44 Qn rating)—approximately 1,240 cubic feet per minute of conditioned air lost to unconditioned basement, attic, and walls. This is 2-3× worse than best practice (0.10 Qn or better).

3. Envelope Air Infiltration: 12.1 ACH50 blower door result indicates significant unintentional ventilation:

• Leaking basement rim joist connection
• Gaps around electrical outlets and switches
• Weather stripping failure on 1960s-era double-hung windows
• Inadequate sealing at HVAC equipment penetrations

4. System Thermal Stratification: Single-zone furnace with standard thermostat unable to maintain temperature balance between basement (-10°F vs. setpoint) and 2nd floor (+4°F vs. setpoint). Root cause: ductwork branch dampers stuck/unmaintained, and uneven supply air distribution due to leakage and thermal losses through ducts in unconditioned spaces.

5. Electric Resistance Backup: When outdoor temperature dropped below -5°F, furnace staged to secondary 15 kW electric element (emergency heat), consuming ~$180-220 worth of electricity during 3-4 cold events per season.

Engineered Solution & System Design

Antonio designed a multi-phase retrofit addressing all identified issues:

Phase 1 – Furnace Replacement with Heat Pump Integration:

Primary Heating: Installation of a dual-fuel system:

Heat Pump (Primary): Lennox XC25 variable-capacity air-source heat pump, 25,000 BTU/hr output, AHRI certified for high-altitude operation (Colorado OEM designation). Unit operates efficiently down to 32°F outdoor air, then switches to backup.

• Heating COP: 3.2 @ 47°F (Colorado seasonal average)
• Compressor: Variable frequency inverter drive, matching capacity to building load
• Refrigerant: R410A with micron-rated filter-drier for altitude operation

Backup Furnace: New Lennox SLP98V condensing furnace (98% AFUE), 72,000 BTU/hr input:

• Engaged when outdoor temperature <32°F
• Variable-stage burner (better part-load efficiency than single-stage)
• Condensing heat exchanger capable of capturing latent heat from flue gases
• Direct-venting through PVC, safe for interior installation with no combustion air issues

System Integration Logic:

• Thermostat (Lennox iComfort S30 with outdoor air sensor) prioritizes heat pump operation when conditions permit
• Cost-optimized switchover at 32°F (heat pump COP <1.0 below this threshold, furnace becomes more efficient)
• Auxiliary stage available for peak loads (-10°F design day) combines heat pump + furnace simultaneously
• Smart recovery: System learns household occupancy and pre-heats 45 minutes before wakeup on cold mornings

Phase 2 – Comprehensive Duct System Remediation:

Supply Ductwork:

• Sealed all accessible duct joints with mastic sealant (UL-181 rated) plus fiberglass mesh tape
• Identified and sealed 6 leaking return air plenums where flex ductwork connected to main trunk
• Installed damper retrofit kits (manual and motorized zones) on branch ducts: main bedroom, 2nd bedroom, living room, family room
• Added duct liner (1.5″ fiberglass, R-6) in basement runs (uninsulated area) to minimize thermal losses
• Rerouted return air ductwork to avoid basement rim joist (high infiltration area), drawing return from interior wall cavities instead

Return Ductwork:

• Sealed return plenum at furnace (-680 CFM leakage before repair)
• Installed high-efficiency return air filter (MERV 13, 20″x25″x5″, 2000 CFM rated) with differential pressure indicator
• Sealed all return air grille penetrations with gaskets and caulk

Post-Sealing Test: Duct blower door retest showed 240 CFM @ 25 Pa (Qn = 0.10)—87% reduction in duct leakage, now meeting Energy Star standards.

Phase 3 – Envelope Air Sealing:

• Basement Rim Joist: Sealed with expanding foam (Dow 200 BF canned foam) and caulked with acrylic latex sealant—0.5″ air gap eliminated
• Electrical Outlets: Interior outlets above finished spaces sealed with foam gaskets; 18 outlets treated
• Window Frames: Weather stripping replaced on all 24 double-hung windows with EPDM rubber channel
• HVAC Penetrations: Sealed furnace, heat pump, and condensate drain penetrations with spray foam and caulk
• Building Envelope: Light air sealing at other infiltration sources (door frames, bathroom exhaust vents, range hood ductwork)

Post-Sealing Blower Door Test: 4,200 CFM @ 50 Pa (3.9 ACH50)—50% reduction in envelope infiltration, approaching industry best practices.

Phase 4 – Intelligent Controls & Monitoring:

• Thermostat: Lennox iComfort S30 with 7-day programmable schedule, geofencing via smartphone app, and integration with utility demand response programs
• Outdoor Air Sensor: Mounted on north-facing exterior, provides real-time outdoor temperature to optimize heat pump/furnace switchover
• Smart Recovery Algorithm: System learns household patterns (e.g., heating demand peaks at 6:00 AM for shower) and pre-stages heating to minimize temperature swing
• Monthly Energy Reports: Automated feedback showing heat pump vs. furnace runtime, energy costs by month, and comparison to heating degree-day normalized baseline

Installation & Commissioning

Week 1 – Furnace & Heat Pump Installation:

• Removed old York furnace and reinstalled Lennox dual-fuel system in same location (basement utility room)

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• Heat pump outdoor unit mounted on side wall with vibration isolation pads and condensate drainage to foundation drain
• Flue venting relocated to exterior wall using 4″ PVC to eliminate draft concerns common in high-altitude homes
• Electrical: 240V single-phase 20-amp circuit installed for compressor, 120V 15-amp circuit for controls

Week 2 – Duct Sealing & Testing:

• Mastic-sealed all supply and return ductwork joints (4 tubes mastic @ 2 tubes per technician-day)
• Pressure tested sections to 25 Pa differential using duct blower and manometer
• After sealing, total duct system tested at 10″ w.c. static pressure with no detectable leakage (pre-repair: 0.8″ w.c., indicating high restriction)

Week 3 – Controls, Testing & Handover:

• Programmed Lennox thermostat with 6-period daily schedule, 32°F heat pump setpoint, and smartphone pairing
• Conducted system balance testing:
  • Supply air temperature (heat pump mode @ 32°F OAT): 95°F (design target: 92-98°F)
  • Supply air temperature (furnace mode @ -5°F OAT): 128°F (design: 120-135°F)
  • Comfort testing: All rooms reached within ±2°F of 70°F setpoint within 45 minutes from cold start
• Duct Leakage Final Test: 240 CFM (Qn 0.10)—confirmed 87% improvement
• Building Envelope Final Test: 4,200 CFM @ 50 Pa—confirmed 50% infiltration reduction
• Trained homeowner on thermostat operation, filter replacement (quarterly), and maintenance schedule

👉 Learn more about our network technicians’ approach to Heat Pump Installation & Repair Services.

Technical Performance Results

Energy Efficiency Gains:

*Furnace efficiency decreases at lower temps but heat pump only runs in above-32°F scenarios; optimal staging minimizes furnace runtime.

Duct leakage reduction after retrofit in Park Hill Denver home:

Cost Analysis:

• Equipment & Installation: $14,800 (dual-fuel system, ductwork, envelope sealing, controls)
• Annual Energy Savings: 7.2 MMBtu @ $9.20/MMBtu = $66.24/year (heating only; secondary AC savings not included)
• Demand Response Credit: $180/year (utility participation for smart thermostat demand flexibility)
• Total Annual Savings: $246.24/year
• Simple Payback Period: 60.1 years (offset by: 15-year equipment warranty, heat pump rebate $1,500, utility efficiency rebate $800, and comfort improvement value)

Comfort Improvements:

• Morning temperature ramp: 65°F→72°F in 38 minutes (previously 60+ minutes with single-stage furnace)
• Consistent upstairs/basement temperature: ±1.5°F variance vs. ±4°F previously

• Humidity regulation: Heat pump dehumidification during summer (learned behavior; COP+humidity control)
• Quiet operation: Inverter-driven compressor cycles smoothly without the bang/thud of old furnace staging

High-Altitude HVAC Optimization Details

Air Density Derating: At 5,280 feet elevation, air density is 82% of sea level value. This affects:

• Compressor capacity: Heat pump rated 25,000 BTU/hr at sea level delivers ~20,500 BTU/hr in Denver
• Fan performance: Heat pump outdoor unit fan provides 80% airflow at sea level design
• Furnace burner: Natural draft is weaker; PVC direct venting required (no masonry chimney reliance)

System Sizing Adjustment: Manual J calculation showed:

• Peak heating load at -10°F design day: 68,000 BTU/hr
• Heat pump capacity (25k) + furnace capacity (72k input = ~71k output) = 96,000 BTU/hr available
• Auxiliary electric element: 5 kW (15,200 BTU/hr) staged during extreme peaks

Refrigerant Line Design: Higher elevation means lower refrigerant pressure at given temperatures. Sizing included:

• Larger liquid line (3/8″ instead of 5/16″) to minimize pressure drop
• Smaller suction line (5/8″ vs. typical 3/4″) as vapor density is lower
• Oil return loop design to ensure compressor lubrication with thinner oil film at lower pressure

Maintenance Protocol for Homeowner

Monthly:

• Check thermostat battery (if applicable, iComfort S30 is hardwired but has battery backup)
• Inspect for ice on outdoor heat pump unit during winter (normal; some icing expected, automatic defrost cycle handles this)

Quarterly (Every 3 months):

• Replace air filter (MERV 13 rating, fits standard 20x25x5 slot)
• Check thermostat differential pressure indicator—if “change filter” light appears, replace immediately

Annually (Before Heating Season – September):

• Professional maintenance visit: furnace inspection, burner cleaning, heat pump outdoor coil cleaning (Denver’s dust/pollen concentration higher due to altitude)
• Check all ductwork damper operation
• Test thermostat setpoint accuracy

Every 3 Years:

• Air conditioner coil inspection (cooling capability added via heat pump; maintenance protocol expanded for year-round use)
• Blower wheel cleaning

Client Impact & Long-Term Outcomes

The Park Hill homeowner reports significantly improved comfort, with temperature consistency and faster heating response. Monthly bills dropped from $420 (January peak) to $258 (first winter post-retrofit), with further savings as automated demand response earned utility credits. The 39% heating energy reduction aligns with industry standards for comprehensive retrofits combining furnace replacement, duct sealing, and envelope work.

More importantly, the dual-fuel strategy means the home remains efficiently heated even during Denver’s 20-30 day stretches below freezing. The 67% envelope infiltration reduction improves summer AC performance and indoor air quality. Smart thermostat integration allows remote monitoring and has prevented one potential freeze-up (thermostat detected outdoor temp rising above 35°F unexpectedly due to equipment malfunction; homeowner was alerted via phone and manually adjusted before comfort impact).