New Build vs. Retrofit: Different HVAC, Different Rules

The HVAC design decisions on a new-construction project are fundamentally different from those on a retrofit. Here's what changes.

New construction — what you can do

1. Duct design from scratch. Right-sized ducts, short runs, insulated in conditioned space (or in insulated attic assemblies), sealed at every joint. This is the single biggest performance lever most homes never get.
2. Equipment sized to actual load. Sizing based on the actual house — not a rule-of-thumb for what was there before.
3. Zoning integrated at design. Multiple thermostats, motorized dampers, and a communicating control system planned before any drywall goes up.
4. Balanced mechanical ventilation. ERV/HRV integrated into the design (increasingly required by California code).
5. Heat pump + solar-ready by default. Larger electrical service, dedicated circuits, panel capacity for future electrification.
6. Correct refrigerant lines. Sized and routed with future service access in mind. Not stapled to a joist behind a wall.

Retrofit — what you're working with

1. Existing ductwork. Usually undersized for high-efficiency equipment. Static-pressure test tells you whether it's usable, needs modification, or needs replacement.
2. Existing electrical service. A 200A panel is fine for most heat-pump conversions; older 100A services may need an upgrade.
3. Existing footprint. Outdoor unit pad, indoor equipment location, and refrigerant line routing constrained by what's already there.
4. Existing gas service. Which drives whether a gas furnace stays or the house electrifies.

Retrofit sequence that works

1. Static pressure and airflow assessment on existing ductwork
2. Proper load sizing based on the actual house
3. Equipment selection matched to load AND to what the ducts can carry
4. Duct modifications where needed (usually branch sealing, sometimes trunk upsizing)
5. Replace the equipment
6. Commissioning: refrigerant charge, superheat/subcool, static pressure, airflow, temperature split — all measured and documented

New-construction pitfalls to avoid

• HVAC contractor picked by builder based on lowest bid — the ducts get value-engineered down to the minimum, hot rooms follow forever
• No real load sizing — equipment picked by rule of thumb based on square footage. Same problem as any retrofit.
• Ducts in unconditioned attic — even with insulation, running ducts through a 140° attic is a permanent efficiency penalty. Design them into conditioned space when possible.
• Undersized returns — the most common builder shortcut. Symptoms show up 2 years later as "why is one room so hot?"

Retrofit success criteria

If a retrofit is done right, on final commissioning you should see:

• Every register within ±20% of design airflow
• Temperature split (return to supply on cooling) between 17°F and 22°F
• Static pressure across the air handler under 0.5" WC total
• Refrigerant subcool within ±2°F of manufacturer spec

Ask for these measurements documented on the commissioning report. If a contractor won't produce numbers, you have your answer.

Planning a new build or renovation? We do design consulting alongside install — happy to weigh in on the HVAC design at the framing stage.