Station HVAC Load Calculator

Formula Used

• Conduction: Q_s = U × A × ΔT (W), converted to kW.
• Solar through glazing: Q = A × I × SHGC × shading factor (W), converted to kW.
• Internal gains: people, lighting, and equipment are added as sensible and latent kW.
• Outdoor air flow: V̇ = infiltration (ACH×V/3600) + ventilation (L/s terms), converted to m³/s.
• Outdoor air total cooling: Q = ṁ × (h_out − h_in), using moist-air enthalpy.
• Outdoor air sensible: Q_s ≈ 1.2 × V̇ × ΔT (kW). Latent is the remainder.
• Totals: (sensible + latent) ÷ diversity, then multiplied by safety factor.
• Capacity conversions: tons = kW ÷ 3.517, BTU/h = kW × 3412.142.

How to Use This Calculator

1. Enter station room area, height, and indoor setpoints.
2. Provide outdoor design conditions for your location.
3. Fill envelope areas and U-values for walls, roof, and glazing.
4. Adjust glazing SHGC, solar level, and shading to match exposure.
5. Set occupants and internal power densities for lighting and equipment.
6. Define ventilation rates and infiltration ACH for station tightness.
7. Use diversity to reflect non-simultaneous peak usage.
8. Add a safety factor for uncertainties and future growth.
9. Press Calculate, then export CSV or PDF for records.

Technical Article

Why Stations Need Load Clarity

Public stations combine high footfall, equipment rooms, ticketing areas, and variable schedules. Oversizing increases capital cost and short-cycling, while undersizing creates comfort complaints. This calculator provides an early-stage cooling and heating estimate in kW, tons, and BTU/h, suitable for concept design and budgeting.

Key Sensible Load Drivers

Sensible cooling is usually dominated by envelope conduction, glazing solar gains, lighting, and plug loads. Typical lighting power densities for station concourses can range around 8–15 W/m², while equipment allowances often start near 5–15 W/m² depending on kiosks, IT racks, and controls. Envelope conduction is modeled with Q = U×A×ΔT, making U-values and exposed area critical.

Latent Load and Humidity Control

Latent load comes from occupants and outdoor-air moisture. The calculator estimates moisture effects using humidity ratio and moist-air enthalpy, then separates outdoor-air sensible and latent portions. For busy waiting areas, typical per-person latent gains can be on the order of 40–70 W per person, which can materially affect coil sizing and dehumidification capability.

Outdoor Air and Infiltration Impacts

Stations often experience door cycling and platform pressure effects, so infiltration matters. Infiltration is converted from ACH using V̇ = ACH×Volume/3600. Ventilation is added using per-person and per-area rates, commonly starting around 10 L/s per person plus an area component. Outdoor-air cooling is computed by ṁ×(h_out−h_in), capturing both temperature and humidity differences.

Using Results for Equipment Selection

The total cooling output applies diversity and a safety factor to reflect non-simultaneous peaks and uncertainty. Convert capacity using 1 ton ≈ 3.517 kW and 1 kW ≈ 3412 BTU/h. Use the breakdown table to test scenarios, such as improved glazing shading, tighter envelopes, or reduced lighting density, before finalizing detailed HVAC modeling and code checks.

FAQs

1) Is this suitable for final equipment selection?

It is intended for early sizing and option comparisons. Confirm with detailed room-by-room calculations, ventilation code requirements, and equipment performance at design conditions before procurement.

2) What does diversity factor do?

Diversity reduces the assumption that all loads peak at the same time. A value like 0.90–0.98 is common for mixed-use station spaces, depending on scheduling and occupancy patterns.

3) How should I choose outdoor design conditions?

Use local design-day temperature and humidity from reliable climate data. For coastal or humid regions, humidity can drive latent load significantly, even if temperature difference is modest.

4) Why are there separate sensible and latent results?

Sensible affects air temperature, while latent affects moisture removal. Cooling coils and reheat strategies depend on both, especially in crowded stations or humid climates.

5) How do glazing and shading change results?

Solar load scales with window area, irradiance, SHGC, and shading factor. Reducing SHGC or adding shading can lower peak cooling kW, often more effectively than small U-value changes.

6) What infiltration rate is reasonable?

Tight conditioned rooms may be below 0.5 ACH, while frequently opened entrances can exceed 1.0 ACH. Use site observations, door traffic, and pressure effects to refine the assumption.

7) What is the difference between kW, tons, and BTU/h?

They are capacity units. The calculator converts automatically: tons ≈ kW/3.517, and BTU/h ≈ kW×3412. Use the unit your contractor or specification prefers.