Use AI tools to explore and evaluate potential HVAC design strategies.
Example Prompt
Recommend several HVAC design strategies for a 50,000 SF 10-story residential tower in San Jose, California.
- Provide recommendations for heating, cooling, and ventilation strategies.
- Consider how natural ventilation, passive heating, and passive cooling can be used to conserve energy.
- Consider different layout options - central heating and cooling equipment versus separate heating and cooling located in individual units or rooms.
- Consider different distribution options - air-based systems versus fluid-based systems.
- Consider different stakeholder goals, including construction cost, construction speed, materials availability, carbon footprint, operational energy use, and aesthetics.
Create a table that contrasts the pros and cons of each strategy.
Results from Claude
For a tower this size, I’d lead with an envelope-first, mixed-mode approach, then pick between in-unit heat pumps (lower cost, faster to build) and a central hydronic heat pump plant (better long-term energy, carbon, and aesthetics), with balanced ERV ventilation either way.
Project context that drives the choices
- The floor plate is small. 50,000 SF over 10 floors is about 5,000 SF per floor, roughly 4–6 units per floor and 40–60 units total. Vertical shafts and central mechanical rooms take a large share of each floor, so compact, simple systems gain value.
- The climate is ideal for passive design. San Jose is mild and Mediterranean. Summers are warm and dry with large day-to-night temperature swings, winters are mild, and humidity is low. Cooling and heating loads are both modest. Night cooling and natural ventilation work most of the year.
- Plan for all-electric. San José’s reach code required new mid- and high-rise residential buildings to be all-electric starting August 2021, with limited exemptions. Council has since revised the reach code, so confirm the current requirements with the city. Separately, California’s 2025 Energy Code took effect January 1, 2026, and for multifamily buildings like this one, the compliance budget is based on heat-pump space heating. It also adds PV and battery storage requirements for high-rise multifamily.
- Local constraints on operable windows:
- Aircraft noise near SJC can make open windows impractical.
- Wildfire smoke means the building must be able to seal up and filter its air.
- Ten stories usually means a concrete or mass-timber structure. Exposed timber or concrete ceilings make ductwork visually costly.
Passive strategies (apply to every option)
Strategy | How it applies here |
Solar control | Exterior shading on south and west façades: horizontal fins on the south, vertical fins or deep balconies on the west. Use moderate window-to-wall ratios (about 30–40%) and spectrally selective low-e glazing. West-facing glass is the biggest cooling driver. |
Thermal mass and night flush | Concrete slabs, or a concrete topping on timber, absorb daytime heat. Opening windows or running ventilation fans at night purges that heat. This takes advantage of San Jose’s large day-to-night swing and can eliminate mechanical cooling for much of the year. |
Cross and stack ventilation | Corner and through-units allow cross ventilation, and the small plate makes more of these possible. Ceiling fans in every room extend comfort without cooling. Window contacts automatically turn off the heat pump when windows are open. |
Passive heating | South glazing with low-e coatings, combined with thermal mass, carries much of the winter heating load on sunny days. A tight envelope matters more than solar gain, so specify continuous insulation and thermally broken balcony connections. |
Smoke and noise mode | Every unit needs a mode with windows closed, MERV 13+ filtration, and recirculating cooling. Use acoustic glazing on façades facing the flight path. |
Active system options
A. In-unit ducted split heat pumps (decentralized, air-based). Each unit gets a small ducted air handler in a soffit or closet. Condensers sit on balconies or the roof, or each unit has a compact through-wall system.
B. Ductless mini-splits, single-room or multi-zone (decentralized, air and refrigerant). Wall or ceiling cassettes go in each room, with outdoor units on balconies or the roof.
C. Packaged vertical or through-wall heat pumps (decentralized, room-level). Self-contained units sit in a closet or under a window and connect to the outside through a wall louver. Examples include Ephoca, Friedrich VertiCal, and modern packaged terminal heat pumps (PTHPs). No refrigerant piping is run in the field.
D. Central VRF heat recovery (central, refrigerant-based). Roof condensers serve fan coils in each unit through branch selectors and refrigerant piping.
E. Central air-to-water heat pump plant with fan coils (central, fluid-based). Roof air-to-water heat pumps make hot and chilled water, which runs in a 2-pipe or 4-pipe loop to quiet fan coils in each unit. This plant can also share heat recovery with domestic hot water.
F. Hydronic radiant slab or ceiling plus a dedicated outdoor air system (central, fluid-based). The same central heat pump plant feeds radiant tubing in the slabs or ceiling panels, and ventilation air is delivered separately. San Jose’s dry climate makes condensation easy to control.
G. Ambient (water-source) loop with in-unit water-to-water or water-to-air heat pumps (hybrid). A building-wide water loop at moderate temperature links heat pumps in each unit with a central heat recovery plant or boiler/tower. It can also serve domestic hot water.
Ventilation options
V1. In-unit energy recovery ventilator (ERV) or heat recovery ventilator (HRV). A compact balanced unit in each apartment with short duct runs through the wall. It is easy to meter and needs no vertical shafts.
V2. Central rooftop dedicated outdoor air system (DOAS) with energy recovery. Air is ducted down shafts to each unit. Filtration is centralized and easy to maintain, but each floor loses shaft space and every floor crossing needs a fire/smoke damper.
V3. Mixed-mode ventilation. Operable windows serve as the primary ventilation in mild weather, with V1 or V2 as backup for smoke, noise, and extreme days.
Pros and cons
Strategy | Pros | Cons |
Passive / mixed-mode | Lowest operating energy and carbon. Shading and natural light add architectural character. Resilient during power outages. Reduces mechanical system size. | Depends on occupant behavior. Limited by noise and smoke. Shading devices add façade cost and design coordination. Building code and fire rules on window openings at height. |
A. In-unit ducted split | Familiar to contractors, competitively bid, and quick to install. Each unit is metered separately, so tenants pay their own bills. One failure affects only one unit. | Soffits lower ceilings. Outdoor units clutter balconies or the roof. Many separate systems to maintain. Field-installed refrigerant piping on every floor. |
B. Ductless mini-splits | High efficiency. Room-by-room zoning. Easy to buy and quick to install. Little ceiling space needed. | Wall cassettes are visible and often disliked. Ventilation has to be handled separately. Many refrigerant joints create leak risk. New lower-GWP A2L refrigerants have charge limits in small rooms. |
C. Packaged vertical / through-wall | Lowest first cost and fastest installation. Replacement is a simple swap. No field refrigerant piping and a small factory-sealed charge. Easy metering. | Louvers punctuate the façade. Noisier. Lower efficiency than splits. Shorter equipment life (about 10–15 years). Envelope leakage at the sleeves. |
D. Central VRF | High efficiency with heat recovery between units. Compact fan coils. Fewer outdoor units. | Large refrigerant charge, so high GWP leak risk and A2L compliance work. Billing each unit requires allocation software. Proprietary single-vendor controls. Long lead times on central equipment. Usually costs more than A. |
E. Central air-to-water + fan coils | Very low refrigerant inside units, since the refrigerant stays in the roof plant. Quiet. Long-lived piping. Simultaneous heating and cooling with heat recovery. Works well with central heat pump water heating (CO₂ units). | Higher first cost. Pipe risers and a mechanical room take space on a small plate. Coordinating piping trades slows the schedule. Billing units requires BTU meters or including utilities in rent. Some plants rely on a few critical units, so redundancy is needed. |
F. Radiant + DOAS | Best comfort and acoustics. No visible equipment, which suits exposed concrete or mass timber. Concrete slabs act as thermal storage, allowing load shifting to cheaper off-peak hours. Low fan energy. | Highest first cost. Slow response time. Tubing must be coordinated before concrete is poured. Smaller pool of skilled installers. Dehumidification and condensation controls required. Harder to zone individual units. |
G. Ambient loop | Shares heat between cooling units, heating units, and hot water. Very efficient. Can be expanded or connected to a district system later. Equipment in each unit is metered separately. | Complex to design and commission. Both central and in-unit equipment means two systems to maintain. Pumps run continuously. Rare in buildings this small. |
V1. In-unit ERV | No shafts. Easy metering and control per unit. Strong indoor air quality and energy recovery. | Many filters for residents or staff to change. A façade penetration for each unit. Small units can be noisy. |
V2. Central DOAS | Central filtration is easy to maintain and protects against smoke. Fewer façade openings. | Shafts take space on a small plate. Fire/smoke dampers at every floor. Balancing airflow across 10 floors is hard. |
Stakeholder comparison
● = strong, ◐ = moderate, ○ = weak
First cost | Build speed | Materials / lead time | Carbon (refrigerant + embodied) | Operating energy | Aesthetics | |
Passive / mixed-mode | ◐ | ● | ● | ● | ● | ● |
A. Ducted split | ● | ● | ● | ◐ | ◐ | ◐ |
B. Ductless | ● | ● | ● | ◐ | ● | ○ |
C. Packaged through-wall | ● | ● | ● | ◐ | ○ | ○ |
D. Central VRF | ◐ | ◐ | ○ | ○ | ● | ● |
E. Air-to-water + fan coils | ○ | ◐ | ◐ | ● | ● | ● |
F. Radiant + DOAS | ○ | ○ | ◐ | ● | ● | ● |
G. Ambient loop | ○ | ○ | ◐ | ● | ● | ◐ |
Recommended packages
- Value / speed (market-rate rental, tight budget): Passive measures, A or C, in-unit ERVs, and central CO₂ heat pump water heating. Units are metered individually and the work is easy to bid.
- Balanced (recommended starting point): Passive measures, ducted splits (A) in living areas, in-unit ERVs, and ceiling fans. Add night-flush controls and a smoke mode that runs from the building automation system.
- High performance / premium (condo, long-term owner, net-zero or carbon goals): Passive measures, a central air-to-water heat pump plant serving radiant slabs (F) or fan coils (E), a central DOAS with ERV, and heat recovery into domestic hot water. This system has low refrigerant risk, a clean interior, and is best for load shifting with the required PV and battery.
Early items to resolve
- Electrical service and transformer lead times. An all-electric building needs a larger utility service. PG&E interconnection and switchgear delays are often the real critical path, more than the HVAC equipment.
- Rental vs. condo. This decides whether central systems need BTU submetering.
- Refrigerant strategy. Systems using the new A2L refrigerants need room-volume and leak-detection checks. That favors E, F, or C over D in small bedrooms.
- Shading and window decisions. These set the HVAC capacity, so settle them in schematic design.