Zilkr on the Park Condo Case Study: Fixing High Humidity

Quick answer: A 12-year-old, 1.5-ton single-stage system in a 1,039 sq ft Zilkr on the Park condo couldn’t control humidity: indoor relative humidity reached 65-72% even with the thermostat at 68°F. After an upgrade to a 2-ton variable-capacity inverter heat pump, the condo held 41-46% RH at a 74°F setpoint. This case study walks through the assessment, the equipment and the results.

This is a technical case study of a humidity-optimized HVAC system in a condominium at Zilkr on the Park.

What was wrong with the original system?

System Configuration (Pre-Upgrade):

• Goodman 1.5-ton single-stage condenser with scroll compressor (fixed capacity, on/off cycling)
• Goodman Wall-Unit 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:

In summer, the resident kept the thermostat at 68°F to try to manage humidity. Despite continuous cooling cycles, indoor relative humidity consistently reached 65-72% during Austin’s peak humidity months (June-August). At 68°F and 70% RH, the condo felt clammy and uncomfortable, with noticeable condensation on the windows and a persistent musty odor that pointed to high moisture levels.

The 1.5-ton compressor was undersized for the solar load on this corner unit, and the high ceiling made it harder. A larger cubic volume means more air to condition, and high ceilings create stratification: cool air settles while moisture and warmer air linger up high. The east and south exposures create peak sensible loads of 8,000-10,000 BTU/h in mid-afternoon, which left the fixed-capacity system cycling continuously without enough time to dehumidify. Compressor cycling was estimated at 8-12 cycles per hour.

Each cycle brought the coil to approximately 35-40°F, dropping the temperature quickly without enough coil dwell time for effective dehumidification. The high sensible heat ratio (SHR ~0.80) meant 80% of cooling capacity went to temperature control and only 20% to moisture removal – not enough for a moisture-prone corner condo.

How was the new system designed?

Around humidity, not just temperature. The upgrade increased capacity to 2 tons while treating humidity as a primary performance metric. The 33% capacity increase (1.5T – 2T) covers the solar load deficit, while variable-capacity control prevents overshooting and holds the coil at the right temperature for dehumidification. Rather than oversizing a single component, the design aimed for a balanced system where every element removes moisture efficiently.

What equipment was installed?

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 enough capacity headroom, while inverter modulation keeps the system from overshooting 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

How was the system commissioned?

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

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

Refrigerant Charge:
Base charge per nameplate: 3 lb 11.6 oz (for standard 25 ft line-set)
Lineset adjustment: 45 ft – 25 ft = 20 extra feet × 0.55 oz/ft = 11 oz additional charge
Total charge: 4 lb 6.6 oz (weighed on digital scale, ±0.1 oz tolerance)

Startup Verification:

How did the system perform after installation?

Humidity dropped from 65-72% RH to 41-46% RH, at a setpoint 6°F warmer than before. Here are the details.

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 held 44% RH (±2%) during 24-hour operation, including overnight when outdoor RH peaked. No condensation was observed on windows or interior surfaces, and the musty odor was gone within 48 hours of installation.

Energy Observation:
No formal energy audit was done, but the resident reported a slight drop in electricity use, along with the 6°F higher setpoint and much better humidity control. That fits with the part-load efficiency of an inverter compared with fixed-capacity on/off cycling, despite the 33% capacity increase.

Why does variable capacity control humidity better?

Because it can run slower and longer, keeping the coil cold and the air moving across it. A fixed-capacity system (like the original 1.5-ton) has two states: full compression or off. Under partial load (typical in condos with lower sensible loads), the compressor runs at 100%, shuts down and cycles rapidly. That 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 adjusts compressor speed to match the load. At 60-70% capacity during peak summer humidity, the compressor runs continuously at lower speed, keeping the coil at a steady 42-45°F and letting air spend 3-5 seconds across the coil – optimal for latent (moisture) removal. The resulting SHR of ~0.60 puts significantly more capacity toward moisture removal.

Capacity Implications for Corner Units:

During afternoon peak loads on this corner unit, the original 1.5-ton system was running at compressor maximum (100% displacement). That left zero capacity margin for dehumidification; all the cooling went to fighting solar gain. At 2 tons, the inverter can run at 60-70% displacement during peak load, freeing up 30-40% of capacity for moisture removal. The system now has both sensible and latent headroom.

Filter and Coil Surface Area: Why Bigger Helps Humidity Control

Filter: Filter surface increased from 252 sq in (14×18) to 500 sq in (20×25) – a 2× upgrade. That cuts face velocity at the filter from approximately 180 CFM/sq ft to 90 CFM/sq ft at the same airflow, so the blower works less to pull air through it and total airflow stays where the system was designed to run.

Evaporator coil: The new coil has about twice the surface area of the original. Moisture only comes out of the air where it touches fins that are colder than the air’s dew point, so more cold fin area means more places for water to condense. The bigger face also slows the air crossing the coil. Slower air spends more time against the cold fins, and condensed water has a chance to run down into the drain pan instead of being blown off the fins and back into the duct (called moisture carryover).

The inverter compressor is what makes the extra surface pay off. On a humid day it can slow down and keep running at low speed, holding the coil below the dew point for long stretches instead of cycling on and off. With the old single-speed system, the coil warmed up every time the compressor shut off and some of the water sitting on it evaporated right back into the apartment. Now the coil stays cold and wet, draining steadily, which is a big part of why the condo now stays so much drier than it did with the old system.

Lineset Charge Correction:

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

What did the resident notice?

The difference was immediate at startup. Instead of the heavy, sticky feel of the old installation, the air now feels crisp and dry – without feeling 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 setpoints.

It’s also much quieter. The variable-speed compressor and ECM blower create a lower baseline sound level throughout the living space, especially during afternoon peak loads, when the old system’s constant cycling was most audible.

Most notably, humidity has stayed stable and unobtrusive even through Austin’s most humid weeks. The resident said the condo now feels more comfortable at 74°F than it did at 68°F with the previous system. That’s a direct result of better humidity control: drier air lets a warmer setpoint feel comfortable.

What does this case study mean for other Austin condos?

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 the coil
• Oversized filter and coil to reduce face velocity
• Right-sized capacity to prevent overshooting and energy waste
• Precision commissioning so actual performance matches the design

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

Related guides: What “Relative” Humidity Really Means, Smart HVAC Design: Manual J, D & S Explained and What Do SEER and SEER2 Mean, and Why Do They Matter?.

About Gold Eagle Services: This guide comes from the team at Gold Eagle Services, a family-run Austin HVAC, indoor air quality and mold remediation company founded in 2017. We’re licensed (TACLA119235 and RCO1783), insured and bonded, and we serve homes across the greater Austin area.

Need help with this in your home? Learn about our air conditioning repair and installation, or call 512-489-4653 to schedule a visit.