Case Study 19-2: The Chen-Williams Renovation — Choosing Between Central AC and Mini-Splits

Background

Priya Chen-Williams and Marcus Williams had gutted their 1963 ranch house down to the studs. It was a genuine blank slate: no existing ductwork, no existing HVAC equipment worth keeping, new insulation going in, new windows throughout. The mechanical systems were entirely up to them to design and specify.

This was an unusual and enviable position. Most homeowners are constrained by existing systems — they're choosing between systems A and B given what's already in the walls and ceilings. Priya and Marcus were choosing from first principles.

Their house was 1,850 square feet, single story. The renovation included: spray foam insulation in all exterior walls and the attic (R-21 walls, R-49 attic), triple-pane windows throughout, continuous exterior rigid foam on the wall assembly (additional R-7.5), and comprehensive air sealing. Their energy consultant estimated the final envelope would test at approximately 1.2 ACH50 — significantly tighter than typical new construction.

The Cooling Load Calculation

Before any system discussion, Priya insisted on a Manual J cooling load calculation from their HVAC designer, a woman named Diane who came recommended by the energy consultant. The results confirmed what Priya suspected from her research: a very tight, well-insulated 1,850 sq ft ranch house has a surprisingly small cooling load.

Design cooling load: 18,400 BTU/hour (1.53 tons) at design conditions (92°F outdoor, 75°F indoor, 55% RH setpoint).

"This surprised me until Diane explained it," Marcus said. "The better your insulation and air sealing, the smaller your HVAC system needs to be. We'd over-invested in the shell, which meant we could under-invest in the equipment."

This small load opened up system options that wouldn't be practical for a leakier house.

Option Analysis

Diane presented three options. Priya and Marcus documented the full comparison.

Option 1: Ducted Central Heat Pump System

A single ducted heat pump (air handler inside, heat pump compressor outside), sized at 1.5 tons, with approximately 280 linear feet of new sheet metal and flex ductwork. The ductwork would require soffits in two locations (the kitchen and the main hallway) and would run in the attic for a portion of the run.

  • Equipment cost: $6,200 (Carrier 19 SEER2 heat pump + air handler, 1.5-ton)
  • Ductwork materials and labor: $4,800
  • Electrical: $350
  • Total installed: $11,350

Pros: Single system to maintain. All rooms served from central unit. High MERV filtration possible. Familiar technology that future owners would understand.

Cons: Duct runs in the attic (even though the attic is now inside the thermal envelope due to spray foam — a significant advantage). Requires soffits that affect ceiling height in kitchen and hallway. Single zone unless zone dampers are added.

Option 2: Full Multi-Split System (No Ductwork)

Four wall-mounted mini-split indoor units (one per zone: main living area, primary bedroom, secondary bedroom, and a combined unit for two small back bedrooms/office) plus one outdoor unit.

  • Equipment: $8,400 (Mitsubishi MXZ-4 multi-zone system)
  • Installation: $3,200 (4 wall penetrations, 4 indoor units, outdoor unit mounting, electrical)
  • Total installed: $11,600

Pros: No ductwork at all. Individual zone control per room. Very high efficiency at partial load (Mitsubishi Hyper-Heat technology; effective heating to -13°F). No soffits needed. Better for a tight house because no duct leakage possible.

Cons: Four visible indoor wall units. Filtration is simple mesh only — no high-MERV capability. More complex system (four refrigerant circuits). Aesthetic impact in a renovation with careful interior design.

Option 3: Hybrid — Ducted System for Main Areas, Mini-Splits for Bedrooms

A 1-ton ducted mini-split for the main living/kitchen area (using a short duct run served through a single small soffit), plus two high-wall mini-split heads in the primary and secondary bedrooms.

  • Equipment: $5,900 (Mitsubishi ducted system for main area + 2 wall units)
  • Installation: $3,600 (shorter duct run, 2 wall penetrations, shared outdoor unit)
  • Total installed: $9,500

Pros: Shorter duct runs reduce duct leakage risk. Main living area gets conventional air distribution (no visible unit). Bedrooms get individual control. Less total ductwork than Option 1. Lower total cost than either pure option.

Cons: More complex than a single system. Two different equipment interfaces. Still some ductwork in the main area.

The Decision Process

Marcus was initially drawn to Option 1 — the familiar ducted system. He'd grown up with central air and understood it. Priya was drawn to Option 2 — no ductwork at all fit the clean-slate renovation philosophy, and she'd done enough reading to appreciate the efficiency of Mitsubishi's variable-speed technology.

They spent several evenings working through the considerations.

The aesthetic question turned out to matter more than they expected. Priya had designed the interior herself and had very specific ideas about wall surfaces and ceiling lines. The two soffits required for Option 1 would cut through ceiling runs she'd planned carefully. Option 2 would put four white rectangular units on walls — she could work with that, but she wasn't thrilled. Option 3 minimized both concerns.

The filtration question came up when they learned Marcus had moderate dust allergies. A ducted system with a 4-inch MERV-11 filter — which the HVAC designer confirmed was possible with the properly-sized central system — would clean whole-house air on every cycle. Mini-split mesh filters do not accomplish this. For Marcus's allergies, the ducted portion of Option 3 was meaningful.

The cost difference between Options 1 and 3 was $1,850. Option 2 was actually slightly more expensive than Option 1. The hybrid option was cheapest overall.

After three weeks of deliberation, Priya and Marcus chose Option 3. The reasoning:

"The ducted main area means our living space — where we spend most of our time — has filtered, even air distribution. The bedrooms have individual control and no ductwork so there's nothing to leak or seal. And we came in $1,850 under what the full duct option would have cost, which we put toward the HRV system." (The HRV is covered in Chapter 20.)

The Installation

The ducted mini-split for the main area required one soffit — a 12×10 inch box running 14 feet along the kitchen ceiling. Priya designed it with a floating shelf detail that turned it into a design feature rather than a compromise. The two bedroom wall units were mounted on interior walls where they wouldn't be the first thing seen upon entering the room.

All three indoor units connected to a single Mitsubishi MXZ outdoor unit. Total refrigerant piping: approximately 130 linear feet across all connections.

Diane conducted a final commissioning check and measured the static pressure on the ducted system: 0.44 inches water column — within design range. She confirmed refrigerant charge on all three circuits using superheat and subcooling measurements.

First Summer

The first cooling season validated every aspect of their decision. The main living area maintained both temperature and humidity within setpoint all summer. Marcus reported that his dust allergy symptoms were noticeably reduced compared to their previous house — the MERV-11 filtration on continuous fan mode made a difference he felt directly.

The bedroom mini-splits were convenient in a way they hadn't fully anticipated: Priya sleeps warmer than Marcus. Before, compromising on a bedroom temperature was a nightly negotiation. Now, each side of the bed had its own zone. "This is the feature I didn't know I needed," Marcus said.

Energy consumption for the cooling season: 1,840 kWh for the entire system. At their utility's $0.13/kWh rate, their cooling cost was $239 for the season — for an 1,850 sq ft house in suburban Virginia, a figure that surprised their neighbors who were paying $400–600 for the same period.

Key Questions for Discussion

  1. The Manual J calculation showed a design cooling load of 18,400 BTU/hour for a 1,850 sq ft house. A typical 1,850 sq ft older house might have a cooling load of 36,000–45,000 BTU/hour. What features of the renovation created this difference, and what does it imply for system sizing if the renovation hadn't been done?

  2. Option 1 and Option 2 cost almost the same amount. What does this tell us about the economics of mini-splits vs. central ducted systems for new installations?

  3. Why was the filtration question relevant to their system choice? Why can't a mini-split mesh filter replicate the filtration of a ducted system with a media filter?

  4. Priya and Marcus's cooling cost was $239 for the season in suburban Virginia. If they had kept the house unimproved (original insulation, original windows, original leakiness) and installed a conventional SEER 14 central system, estimate what their cooling cost might have been. What does this suggest about the relative value of building envelope investment vs. HVAC equipment efficiency?