HVAC Design for Commercial Tenant Improvements: Working Within the Base Building System

The base building HVAC system was sized for generic office use in 1998. Your new tenant wants a 20-person conference center, a server room, and a coffee bar with commercial espresso equipment. Three things that the original design never anticipated, all in 6,000 sq ft of space that already has VAV boxes installed. This is the standard commercial tenant improvement (TI) HVAC situation, and it doesn’t resolve by just running Manual J on the new tenant plan.

TI HVAC design has a different workflow than new construction: you’re working within constraints the base building already established, and the decisions compound quickly depending on whether you can reuse existing infrastructure or need to start fresh.

Start with the Base Building HVAC Inventory

Before doing any calculations, pull three things from the landlord or property manager:

1. The as-built mechanical drawings showing the existing VAV box schedule (airflow range, reheat coil size if any), duct layout, and AHU schedule for the floor or zone you’re working in.
2. The AHU capacity remaining for the zone. What’s the total supply CFM the AHU provides, and how much of it is already committed to other tenants or zones?
3. The building’s DDC controls platform (BACnet, LonWorks, proprietary) and whether the property has a controls contractor who owns the system integrations.

If the landlord can’t produce as-builts, you’ll need to field-verify VAV box sizes and AHU capacity before committing to any design approach. Designing to a spreadsheet assumption of available airflow and then discovering the AHU is running at 95% capacity on a hot day is a change-order situation.

Can the Existing VAV System Handle the New Loads?

This is the central question. Work through it systematically:

Step 1: Calculate the new space loads. Run Manual J (or ASHRAE 62.1 for ventilation requirements) on the proposed tenant plan. Conference rooms have high occupancy-driven ventilation requirements — ASHRAE 62.1 requires 7.5 CFM/person plus 0.06 CFM/sq ft for general office, but conference rooms may use the assembly room category (5 CFM/person + 0.06 CFM/sq ft at 50 occupants/1000 sq ft default density). A 20-person conference room needs a different ventilation calculation than the same square footage as private offices.

Step 2: Compare to existing VAV box capacity. Each VAV box in the space has a maximum airflow rate (typically stamped on the box or in the as-built schedule). Sum the maximum CFM across all existing boxes serving the new tenant footprint. If the required supply CFM from your load calc exceeds that maximum, you’re either adding new VAV boxes or you have a problem with the base building system capacity.

Step 3: Check the AHU has headroom. Even if the VAV boxes can handle the load, the AHU serving the floor must have unallocated capacity for the additional airflow. This often requires a coordination call with the property’s mechanical engineer or the HVAC contractor who maintains the building.

The Server Room: Always a Separate System

Server rooms are never served by the base building VAV system for one fundamental reason: server cooling is a 24/7 load, and building VAV systems typically shut down or go to minimum airflow after hours. A 10-kW rack load in an unventilated 300 sq ft room will exceed 90°F within hours of the building VAV system going to night setback.

For a small server room or IT closet in a TI context:

• Below 5 tons: A mini-split or dedicated packaged DX unit with condensate drain and remote condenser is the standard approach. Size the unit at 1.2–1.5x the IT equipment nameplate load to account for the room envelope load and future growth.
• Above 5 tons: Consider in-row cooling or a Computer Room Air Handler (CRAH), particularly if the client expects rack density to grow.

The condenser location for a mini-split or packaged unit in a mid-rise office building requires landlord approval — rooftop space is often allocated, and refrigerant line routing through mechanical chases needs to be confirmed with the building team.

Coffee Bar and Commercial Kitchen Equipment: Makeup Air is the Issue

Commercial espresso equipment and any cooking hood over an open flame triggers makeup air requirements under both fire codes and ASHRAE 62.1. The kitchen exhaust hood must be balanced by makeup air that’s typically 80–90% of the exhaust volume (10–20% negative pressure differential is acceptable to prevent odor migration into adjacent spaces).

For a small coffee bar with a commercial espresso machine (not cooking), you’re typically looking at:

• Type II (non-grease) exhaust hood: 50–150 CFM exhaust per linear foot of hood
• Makeup air unit: sized to balance exhaust, typically conditioned (tempered) to prevent drafts
• Coordination with the building’s exhaust system: can you tie into existing exhaust infrastructure, or do you need a dedicated riser?

Penetrating the floor or ceiling for exhaust risers in a leased TI is a landlord-approval issue. Get those approvals in writing before finalizing the mechanical design — discovering the landlord won’t allow a new penetration after you’ve committed to a design creates expensive redesigns.

When to Design Around the Existing System vs. Propose Supplemental

Use this decision framework:

• Reuse existing VAV + AHU: Tenant load is within the VAV box maximums, AHU has available capacity, conference room ventilation requirements are met by supply CFM from the box serving the room, and the tenant schedule aligns with the base building operating hours.
• Propose supplemental VAV boxes or duct extensions: Tenant layout significantly changes the thermal zones (open plan to cellular or vice versa) and the existing box placement no longer serves the new zone geometry. You’re adding boxes to the existing AHU, which is straightforward if AHU capacity exists.
• Recommend a standalone system: The tenant operates 24/7 (call center, medical use), the base building system won’t support after-hours operation without significant cost, or the landlord’s AHU doesn’t have available capacity. A rooftop unit or split system dedicated to the tenant space is often cleaner than fighting a system that wasn’t designed for the use.

ASHRAE 62.1 Ventilation Requirements for Mixed Occupancy

See also: air changes per hour ventilation calculations covers the ACH-based ventilation method for different space types.

When a TI space has multiple occupancy categories — office, conference, break room — ASHRAE 62.1 Section 6.2 requires a zone-by-zone ventilation calculation. Each zone’s outdoor air requirement is:

where R_p is the people outdoor air rate (CFM/person), P_z is zone population, R_a is the area outdoor air rate (CFM/sq ft), and A_z is the zone area. For a mixed-occupancy TI, this typically exceeds what a generic "office" calculation would yield, and the AHU must be able to deliver the required outdoor air fraction.

The load calculation tool handles the Manual J component; for ASHRAE 62.1 ventilation rate compliance, work through the zone-level calculation by occupancy category before confirming the AHU can supply adequate outdoor air under all operating conditions.

Coordination Checklist Before Permit Submission

• AHU available capacity confirmed in writing with the property’s mechanical engineer
• VAV box reuse vs. replacement decision documented with as-built verification
• Server room dedicated system approved by landlord (condenser location, refrigerant routing)
• Kitchen/coffee exhaust and makeup air routing confirmed (riser penetration approvals)
• DDC controls integration scope defined (who programs the new VAV boxes into the BAS)
• ASHRAE 62.1 outdoor air calculation completed per occupancy category
• Energy code compliance path identified (IECC or ASHRAE 90.1 for commercial, depending on jurisdiction)