12,000 m² Factory HVAC Design: Zone Loads, OA, and DOAS Decision
Introduction #
When this guide fits: You are scoping new or retrofit HVAC for a high-bay factory (production + warehouse + QC) and need a documented zone load table, ventilation strategy, and system archetype before equipment hits the bid room.
When it is not suitable: Explosion-rated classifications, ISO cleanroom validation, or stamped refrigerant inventories require licensed mechanical engineers and code-specific submittals beyond this field record.
Verified: 2026-05-25 — design basis for the 12,000 m², 12 m clear-height packaging campus referenced in HVAC Load vs Capacity (500 kW coincident peak, later rightsized from 650 kW installed).
Design-day inputs we froze (revision C, 2025-11) #
| Input | Value | Source / note |
|---|---|---|
| Floor area (conditioned) | 12,000 m² | BIM shell v3.2 |
| Clear height | 12 m | High-bay packaging |
| Outdoor design | 32 °C DB / 24 °C MC WB | ASHRAE 0.4% dry + coincident wet |
| Indoor target (production) | 22 °C DB, 50–55% RH | QA moisture spec |
| Shifts | 2 × 10 h + 1 h overlap | Not 24 h full load |
| Dock doors | 6 × 4 m, scheduled open | Infiltration schedule in model |
Zone load schedule — basis for coincident 500 kW #
Do not sum the “Peak kW” column for equipment sizing—that was the original 650 kW bid error. Use coincident row at bottom.
| Zone | Floor m² | Sensible peak kW | Latent peak kW | Hours occupied | Non-coincident? |
|---|---|---|---|---|---|
| Lines 1–4 (packaging) | 6,800 | 248 | 42 | 06:00–22:00 | No |
| Warehouse mezz | 2,400 | 52 | 8 | 08:00–18:00 | Partial |
| QC / metrology | 900 | 38 | 22 | 07:00–19:00 | No |
| Offices + MCC | 1,100 | 28 | 6 | 07:00–18:00 | Yes (off-peak) |
| Dock vestibule | 800 | 18 | 4 | Intermittent | Partial |
| Arithmetic sum of peaks | 384 | 82 | 466 kW (wrong basis) | ||
| Coincident design (Aug 15 h) | 412 | 88 | 500 kW ✓ |
Sanity-check zones in the HVAC Capacity Calculator; electrical feeders via Factory Load Calculator.
Post-bid outcome and RH fixes: HVAC Load vs Capacity case.
Ventilation and exhaust — OA that drove latent load #
| Source | Design flow (m³/s) | Tempered? | Latent impact |
|---|---|---|---|
| Occupant OA (production) | 12.4 | Yes | Moderate |
| Dock infiltration (scheduled) | 8.2 | Yes | High humid mornings |
| Seal-bar exhaust | 6.5 | Makeup required | Dominant latent |
| General roof exhaust | 4.0 | Partial | Dust control |
Decision: Dedicated DOAS unit for OA dehumid (latent wheel), sensible cooling on three staged DX zones—avoid mixing all OA through oversized packaged DX.
| Space type | Planning ACH (conceptual) | Governing driver |
|---|---|---|
| Production hall | 6–8 | Heat + dust dilution |
| QC lab | 12+ changes | Spec + pressurization |
| Warehouse mezz | 4 | Inventory heat only |
Always verify against local code and occupational exposure limits—not this table alone.
Air distribution choices #
| Zone | Issue | Selected approach |
|---|---|---|
| Production | Stratification at 12 m | High-throw diffusers + destrat fans at 8 m |
| Warehouse | Low internal gain | Smaller sensible coil, night setback |
| QC | Tight RH | Independent zone, DOAS dry OA |
| Offices | Misleading if tied to bay | Separate thermostat, not bay SP |
Filter train: MERV 8 pre + MERV 13 main on production AHUs; monitor ΔP—fouling mimicked “undersized HVAC” in year-one ops.
System archetype comparison (why we rejected single RTUs) #
| Option | First cost index | Humidity control | Part-load | Verdict |
|---|---|---|---|---|
| 2 × large packaged DX | 1.0 | Poor latent | Short cycle | Rejected (became 650 kW problem) |
| Central chiller + AHUs | 1.35 | Good | Excellent | Deferred (capex gate) |
| DOAS + staged DX | 1.18 | Good | Good | Selected in retrofit |
Electrical and energy coordination #
| Load item | Connected kW (HVAC stack) | Notes |
|---|---|---|
| Compressors (staged) | 380 | Nameplate sum |
| Condenser fans + VFD | 48 | Include on one-line |
| DOAS wheel + fans | 62 | Do not omit on factory sheet |
| Pumps (future CHW) | 0 | Spare breaker only |
Coarse cost swing: Energy Estimator after demand kW moved in the site energy audit.
Browse HVAC calculator hub.
Instrumentation specified at turnover #
Minimum BMS points so rightsizing debates end with data:
- Filter ΔP each stage
- OA damper position feedback
- Zone RH at coil leaving air path
- Fan kW (not % speed alone)
- CHW ΔT if hydronics added later
Commissioning: 15-minute trends for two production weeks, not TAB design-day only.
Related articles #
- HVAC Load vs Capacity — 650 kW → 520 kW outcome
- HVAC Sizing Guide
- How to Calculate HVAC Capacity
- HVAC Sizing Common Mistakes
- Energy Cost Optimization
Next steps you should take #
- Publish a one-page zone schedule with coincident peak highlighted—not column sums.
- Model humid-hour latent separately from afternoon sensible peak.
- Match system type to OA + exhaust chemistry before requesting bids.
- Specify trend points in the controls spec—not a post-occupancy argument.
- Cross-check tonnage in the HVAC Capacity Calculator (page 2026-05-25).
Do industrial bays always need more tonnage than offices?
Not automatically—high internal gain can be offset by stratification, exhaust, and shift schedules. Model each zone; avoid copying office W/m².
When is DOAS worth the complexity?
When tempered OA + process exhaust drives latent load that packaged DX cannot handle without short cycling—this site is the reference case.
What commissioning mistake cost us RH control?
Treating **design OA as installed minimum without proving damper position and filter DP under full exhaust.
Should electrical design use compressor kW only?
No—include condenser fans, DOAS, pumps, and auxiliaries on the factory load sheet.
How do zone peaks relate to equipment nominal?
Size on the coincident envelope—see the load vs capacity case for what happens when you sum zone maxima.