Case Study 21-2: Designing HVAC Zoning from Scratch — The Chen-Williams Renovation

Background

Priya and Marcus Chen-Williams are three months into the gut renovation of their 1963 suburban ranch house. With exterior walls open to the studs and new ductwork going in from scratch, they have an opportunity that most homeowners never get: the chance to design their HVAC control system properly, from the beginning, rather than retrofitting zones into an existing system.

Their house is 2,100 square feet on one level, with a basement that serves as Marcus's home office and a workshop. The main floor has an open-plan living/dining/kitchen area at the front, two children's bedrooms and a shared bath in the middle, and a primary bedroom suite at the rear.

The previous HVAC system — a single 4-ton system with one thermostat in the central hallway — was a textbook example of wrong-sized equipment with a poor control strategy. The 4-ton system was almost certainly oversized for the 1963 ranch (as the renovation's Manual J calculation would later confirm). The single hallway thermostat produced predictable results: the living area and the bedrooms were never simultaneously comfortable. When the bedrooms were at 70°F, the living area was 74°F. When the living area was comfortable, Marcus's basement office was freezing.

Identifying the Real Zones

Priya, as an architect, starts the zoning design by mapping occupancy patterns:

Zone 1: Living/Dining/Kitchen — Used from about 7 AM to 10 PM on weekdays, all day on weekends. This is the "always occupied when home" zone. Set point: 70°F when occupied, 65°F setback overnight.

Zone 2: Children's Bedrooms — Used primarily for homework (4–8 PM) and sleeping (8 PM–7 AM). During the school day (8 AM–4 PM on weekdays), completely unoccupied. Set point: 68°F evening/night, 62°F setback during school hours.

Zone 3: Primary Bedroom Suite — Occupied primarily for sleeping (10 PM–7 AM). During the day, the door is kept closed and the room doesn't need conditioning. Set point: 67°F sleeping, 65°F setback during day.

Zone 4: Basement Office — Marcus works from home Tuesdays, Thursdays, and Fridays, 8 AM–6 PM. Mondays and Wednesdays he's at his firm's office. The basement runs naturally cooler than the main floor by 4–6°F due to earth contact. Set point: 68°F during Marcus's work hours, 60°F setback at other times.

Four zones. At any given time, typically only two zones are actively calling for conditioning — and almost never all four simultaneously.

Equipment Selection

The Manual J calculation Priya commissioned (from the same contractor who won the bid — see Chapter 23) produced a heating design load of 46,000 BTU/hr and a cooling design load of 38,000 BTU/hr for the entire house.

The contractor recommended a 4-ton system (48,000 BTU/hr cooling nominal). Priya pushed back, noting that 4 tons of cooling for a 38,000 BTU/hr load is 26% oversized. After discussion, they agreed on a 3.5-ton (42,000 BTU/hr nominal) variable-speed air handler with a 3.5-ton variable-speed cold-climate heat pump outdoor unit.

The variable-speed selection was critical for zoning. A single-speed system moves a fixed volume of air — closing zone dampers without a bypass means excess static pressure. A variable-speed system can modulate its airflow to match however many zones are calling, eliminating the bypass damper problem entirely. The system ramps down when only one zone calls; ramps up when all four call simultaneously. Static pressure stays in range throughout.

The Zoning Hardware

The zone control system consists of:

• Zone control board: Handles logic for four zones, receives calls from four thermostats, coordinates damper positions and communicates with the variable-speed air handler
• Four motorized zone dampers: Round dampers sized to each zone's supply duct (8-inch for living area, 6-inch for each bedroom zone, 8-inch for basement)
• Four smart thermostats: Ecobee SmartThermostat Premium for Zone 1 (living area, with display and remote sensor); Ecobee SmartThermostat Essential for Zones 2, 3, and 4

Because the renovation involved opening all walls, the thermostat wire runs were done with 18/8 cable — eight conductors — providing plenty of capacity for any current or future wiring need.

Installation and Commissioning

The contractor's HVAC team installed the system over two and a half days. The commissioning process was extensive — more so than a standard single-zone system:

• Each damper was tested independently: open signal verified, close signal verified, blade position visually confirmed
• Zone 1 was activated alone: airflow at Zone 1 registers measured with an anemometer; air handler modulation confirmed (blower ramped to approximately 60% of full output)
• All four zones activated simultaneously: blower confirmed at full output, all dampers confirmed open, static pressure measured at the supply plenum
• Temperature rise test (heating): with Zone 1 only calling, temperature differential between supply and return measured (confirmed within specification)

The contractor noted that the commissioning step is often skipped on residential zoning projects — and that skipping it is why so many residential zoning systems underperform. Without verification, it's possible to have a damper that's installed backward, a zone control board with incorrect wiring, or a blower that doesn't modulate properly.

First-Season Performance

After the first winter, Marcus runs the numbers:

• Total HVAC energy cost (heating + cooling, 12 months): $1,240
• Estimated pre-renovation HVAC cost (using similar homes in the neighborhood as comparison): $2,100–$2,400
• Estimated savings from envelope improvements alone (insulation, windows, air sealing): $400–$600
• Estimated savings from zoning and variable-speed equipment: $300–$500

The zoning contribution is difficult to isolate from the envelope improvements — they were done simultaneously. But the pattern of energy use tells the story: the system runs shorter cycles with lower intensity, and the basement zone is almost entirely idle on the days Marcus is at the firm's office.

Comfort, however, is unambiguously improved. The living area is consistently 70°F when the family is there. The primary bedroom stays cooler than the rest of the house during the day (the zone is in setback mode) and drops to 67°F well before bedtime. Marcus's basement office maintains 68°F on his work-from-home days without overcooling the rest of the house. The children's bedrooms are comfortable for homework and sleeping without wasting energy during the school day.

What Made This Project Work

The renovation context made proper zoning possible. With walls open, running zone control wiring and properly sizing duct branches for each zone cost far less than retrofitting an occupied house. The incremental cost of the zoning system over a single-zone installation was approximately $2,500.

Variable-speed equipment is essential for single-system zoning. The Priya and Marcus case illustrates why: with a four-zone system, some zones will always be calling and some won't. A variable-speed system handles this gracefully. A fixed-speed system needs a bypass damper and still doesn't handle it as well.

Manual J prevented oversizing. The original single-speed 4-ton recommendation would have produced a system that short-cycled constantly, particularly when only one or two zones were calling. The correctly sized 3.5-ton variable-speed system runs longer cycles, conditions the air more thoroughly, and dehumidifies effectively.

Commissioning is not optional. The two and a half days of installation included a full half-day of commissioning. Priya insisted on being present. Every damper was tested. Every zone was verified. This is what separates a zoning system that performs from one that causes more problems than it solves.

The Counterpoint: When This Would Not Have Made Sense

Priya is clear that this investment made sense for their renovation because: 1. Walls were already open, radically reducing installation cost 2. The family plans a 20-year horizon in the house 3. The comfort improvement addressed a genuine occupancy pattern mismatch 4. Variable-speed equipment was the right choice anyway for correct sizing

Had the Chen-Williamses been retrofit-adding zoning to a finished house with an existing duct system not designed for zone control, the economics would have been far less favorable. The ductwork would have needed significant modification to support proper zone sizing, the wall penetrations for control wiring would have been expensive, and the existing single-speed air handler would have required a bypass damper that reduces the efficiency benefit. The $2,500 incremental cost could easily have become $6,000–$8,000 with uncertain comfort improvement.

The lesson: zoning during new construction or major renovation is often cost-effective. Retrofitting zones into an existing finished house requires careful evaluation.