ZILKR ON THE PARK CONDOMINIUMS

Technical Case Study: Humidity-Optimized HVAC System

Existing Conditions

System Configuration (Pre-Upgrade):

• Goodman 1.5-ton single-stage condenser with scroll compressor (fixed capacity, on/off cycling)
• Goodman Wall-Unit with with PSC blower motor and 14×18×1 filter media
• Original installation: 2014 (12-years-old at time of assessment)

Space Characteristics:

• 1,039 sq ft corner unit, 2nd floor
• East and south-facing windows (high solar gain, particularly 10 AM-4 PM)
• Elevated ceiling height (higher than typical Zilkr units on this floor)
• Shared walls with 1 adjacent unit and staircase (reduced infiltration but higher humidity migration from neighboring spaces)

Performance Baseline:

During summer operation, the resident maintained a thermostat set-point of 68°F in an attempt to manage humidity. Despite continuous cooling cycles, indoor relative humidity consistently reached 65-72% during Austin peak humidity periods (June-August). At 68°F and 70% RH, the residence felt clammy and uncomfortable, with noticeable condensation on windows and a persistent musty odor indicating elevated moisture levels.

The 1.5-ton compressor was undersized for the solar load on this corner unit, and the elevated ceiling height compounded the challenge. Larger cubic volume increases the air mass that must be conditioned, while high ceilings create stratification – cool air settles while moisture and warmer air linger at upper levels. East and south exposures create peak sensible loads of 8,000-10,000 BTU/h during mid-afternoon, leaving the fixed-capacity system cycling continuously without adequate time to dehumidify. Compressor cycling estimated at 8-12 cycles per hour.

Each cycle brought the coil to approximately 35-40°F, causing rapid temperature drop without sufficient coil dwell time for effective dehumidification. The high sensible heat ratio (SHR ~0.80) meant 80% of cooling capacity went toward temperature control and only 20% toward moisture removal-inadequate for a moisture-prone corner condo.

Design Strategy

The upgrade increased capacity to 2 tons while prioritizing humidity as a primary performance metric. The 33% capacity increase (1.5T – 2T) addresses the solar load deficit, while variable-capacity control prevents overshooting and maintains optimal coil temperature for dehumidification. Rather than oversizing a single component, the design focused on creating a balanced system where every element removes moisture efficiently.

Equipment Selection & Specifications

Capacity Upgrade Rationale:
The original 1.5-ton system was undersized for a 1,039 sq ft corner unit with significant solar exposure. Peak summer sensible load (east/south windows, afternoon sun) approaches 10,000 BTU/h, leaving minimal margin for dehumidification. Upgrading to 2 tons provides adequate capacity headroom while inverter modulation ensures the system doesn’t overshoot on cooler days or during shoulder seasons.

Indoor Air Handler:

• Model: ADP SM7A2505R (25,000 BTU, 5 kW secondary heat)
• Blower: 5-speed ECM motor (variable frequency drive, 30-100% modulation)
• Evaporator Coil: Oversized aluminum tube-and-fin (2× surface area vs. original)
• Filter: 20×25×1 media (500 sq in filtration surface vs. 252 sq in original)
• Maximum Static: .5″ W.C. / 722 CFM @ 0.5″
• Configuration: Wall-mount compact unit, sealed ductwork connections

Outdoor Unit:

• Model: GE Connect NS15H24 (variable capacity inverter heat pump)
• Efficiency Rating (when paired with GE NAM24V): Rated SEER2: 16.0 / EER2: 9.0 / HSPF2: 8.5/6.8
• Refrigerant: R-454B (A2L low-GWP blend)
• Compressor: Inverter-driven variable displacement scroll
• Cooling Capacity Range: 10,200–24,000 BTU/h (modulates from ~42% to 100% of rated)
• Heating Capacity Range: 7,200–24,000 BTU/h (modulates from ~30% to 100% of rated)
• Operating Range: Heating 0°F to 80°F outdoor; Cooling 40°F to 125°F outdoor

Lineset Configuration:

• Length: 45 feet (field-measured)
• Suction Line: 3/4″ OD, foam-insulated
• Liquid Line: 3/8″ OD, foam-insulated
• Routing: Sealed roof penetration

Commissioning Protocol

Nitrogen Pressure Test:
Refrigerant circuit pressure-tested to 450 psig (low-side design pressure) using dry nitrogen. System held pressure for 60 minutes with zero decay, confirming integrity of all brazed and mechanical connections.

Evacuation:
Deep vacuum evacuation using certified pump to 450 microns. Evacuation time: 45 minutes. Post-evacuation pressure rise: <40 microns over 10-minute isolation period, confirming moisture and non-condensable removal.

Refrigerant Charge:
Base charge per nameplate: 3 lb 11.6 oz (for standard 15 ft line-set)
Lineset adjustment: 45 ft – 25 ft = 20 extra feet × 0.55 oz/ft = 11 oz additional charge
Total charge: 4 lb 10.6 oz (weighed on digital scale, ±0.1 oz tolerance)

Startup Verification:

System Performance Post-Installation

Operating Conditions (6-week summer baseline):

• Outdoor ambient: 88-98°F
• Indoor setpoint: 74°F (resident comfort preference)
• Runtime: 60-70% compressor capacity during peak load hours

Results:

Humidity Stability:
The system maintained 44% RH (±2%) during 24-hour operation, including overnight when outdoor RH peaked. No condensation observed on windows or interior surfaces. Musty odor eliminated within 48 hours of installation.

Energy Observation:
While formal energy audit was not conducted, the resident reported a slight decrease in electric consumption, correlated with the 6°F higher temperature set-point and significantly improved humidity control. This is consistent with inverter part-load efficiency gains over fixed-capacity on/off cycling, despite the 33% capacity increase.

Technical Analysis

Why Variable-Capacity Outperforms Fixed-Capacity for Humidity:

A fixed-capacity system (like the original 1.5-ton) operates in two states: full compression or off. During partial-load conditions (typical in condos with lower sensible loads), the compressor runs at 100% then shuts down, cycling rapidly. This creates two problems:

1. Insufficient Coil Dwell Time: Rapid cycling means short contact time between air and cold coil, reducing moisture removal per cycle
2. High SHR: The system overshoots temperature control while undershooting dehumidification

An inverter system modulates compressor speed to match load. At 60-70% capacity during peak summer humidity, the compressor runs continuously at lower speed, maintaining steady coil temperature (42-45°F) and allowing air to spend 3-5 seconds across the coil-optimal for latent removal. The resulting SHR of ~0.60 allocates significantly more capacity to moisture removal.

Capacity Implications for Corner Units:

The original 1.5-ton system was operating at compressor maximum (100% displacement) during afternoon peak loads on this corner unit. This left zero capacity margin for dehumidification – all cooling went toward fighting the solar gain. Upgrading to 2 tons allows the inverter to operate at 60-70% displacement during peak load, freeing up 30-40% of capacity specifically for moisture removal. The system now has both sensible and latent headroom.

Coil Surface Area Impact:

Filter surface increased from 252 sq in (14×18) to 500 sq in (20×25) – a 2× upgrade. This reduces face velocity from approximately 180 CFM/sq ft to 90 CFM/sq ft at the same airflow. Lower velocity = longer contact time = more moisture removal. The oversized evaporator coil reinforces this effect, providing additional surface area for refrigerant-air heat transfer at lower velocities.

Lineset Charge Correction:

At 45 feet, the system required 11 additional ounces beyond base charge. Many technicians skip this or estimate by pressure, risking undercharge. Precise weight-based charging is critical for R-454B, which has a narrow charge tolerance (±0.5 oz can affect performance). Undercharge would have prevented the system from reaching optimal coil temperatures; overcharge would have reduced capacity and efficiency.

Resident Feedback

The resident noticed an immediate difference in air quality upon system startup. Rather than the heavy, sticky feeling of the previous installation, the air now feels crisp and dry-without the sensation of being over-cooled. Sleep quality improved noticeably; the resident no longer wakes up clammy or damp, a chronic issue with the old system even at lower temperature set-points.

Operational noise is significantly reduced. The variable-speed compressor and ECM blower create a much quieter baseline throughout the living space, particularly noticeable during afternoon peak loads when the old system’s constant cycling was most audible.

Most notably, humidity has remained stable and unobtrusive even during Austin’s most humid weeks. The resident commented that at a 74°F set-point, the condo now feels more comfortable than it did at 68°F with the previous system. That is a direct result of the improved humidity control allowing higher temperature perception without discomfort.

Lessons & Applicability

This case study demonstrates that humidity control in multi-unit residential requires system design intent, not just equipment capacity. The original 1.5-ton system at Zilkr had insufficient tonnage and lacked the control strategy to use it efficiently for moisture removal.

Variable-capacity inverter systems achieve optimal dehumidification through:

• Modulating compressor speed to maintain steady coil temperature
• ECM blower modulation to optimize air velocity across coil
• Oversized filter and coil to reduce face velocity
• Right-sized capacity to prevent overshooting and energy waste
• Precision commissioning to ensure actual performance matches design

For Austin condominiums with similar age of construction, this equipment stack represents current best practice for residential humidity control without mechanical dehumidifier addition.