"On a properly sized two-stage system, it may never need second stage — high stage is basically there for peak-load days (design conditions), fast recovery, or unusual loads, so at 88°F it can often hold setpoint on low stage all day and that's normal."

If you want to back it up on-site, I'd ask two quick things: what thermostat/control is it (staging logic differs a lot), and do you have any runtime history showing a call for Y2/W2? If you've got access to data, you can also confirm the equipment can go to high by forcing a test call (per OEM procedure) and verifying 24V at Y2 plus a noticeable step in compressor amps and airflow — typically you'll see suction pressure and evap saturation temp rise, along with a drop in sensible split if airflow ramps correctly.

If it never hits high even when forced, then you pivot to wiring/setup (Y2 not landed, thermostat not configured for 2-stage cool, outdoor unit board fault, or staging locked out), but most of the time the honest answer is simply: it's meeting load on first stage, which is exactly the comfort/efficiency goal.

Two targeted questions for you so I can tailor the "most likely" to what you're seeing: is the thermostat a communicating control or conventional Y1/Y2, and have you verified whether Y2 ever energizes (even briefly) during a long call?

If the plan shows a 14x8 trunk in a 10-inch bay, the practical reality is you don't have enough "real" height once you add insulation, sealing, and hangers — so you're in field-routing mode, not "make it squeeze."

Your best options are to (1) change that segment to a wider/shorter duct profile (or approved oval) that preserves roughly the same capacity, then transition back where you've got depth, (2) drop it below the joists in a soffit/chase and coordinate with framing/drywall now, or (3) split into two smaller parallel trunks in adjacent bays and reconnect downstream to keep velocity/static from getting ugly.

The key rule we use with framers is the chase/bay has to be sized for the duct dimensions plus insulation, sealing, hangers, and access — not just the metal size, and this is exactly the kind of thing that needs a same-day coordination huddle before rough mechanical inspection so you don't get stuck reworking after a fail.

Two quick questions so I don't steer you wrong: are these open-web floor trusses, I-joists, or 2x10s, and is the duct internally lined or externally wrapped (and how thick)?

I can give you solid planning targets, but the exact room size and clearances should follow the Manual J load calculation and selected equipment (air handler type, ducted vs. ductless, filtration, dehumidification, electric heat kit, and whether there's a separate ventilation unit).

For a 4,200 sq ft heat pump with a ducted air handler, I usually ask the architect to reserve a dedicated mechanical closet/room that can comfortably fit a full-height air handler with a service side plus a return plenum/filter cabinet, a drain pan and trap with good access, and space for a distribution plenum — think "walk-up service" rather than a tight closet. Ceiling height in the 8'6" to 10' range makes duct transitions and filter service far easier and avoids ugly soffit surprises.

Also plan an exterior condensing unit location with a clean intake/discharge path, sound-sensitive setbacks (bedrooms, patios, neighbor lines), and an architectural screening approach that doesn't choke airflow.

For next week with the MEP engineer, I'd come with these questions:
• Are we doing a fully ducted heat pump with one or multiple air handlers, or a Variable Refrigerant Flow system?
• What are the target equipment locations and required service clearances on the access panels and above the unit for duct connections?
• What are the largest duct sizes leaving the mechanical room and the main return path strategy?

Quick reality check: 75% CDs is exactly when you want the engineer to mark up required access/clearances on the architectural plans so you don't end up with an air handler that can't be replaced without demolition.