Building Energy Audit Tool
Estimate annual energy savings, payback period, NPV and CO₂ reduction from a building energy improvement.
Building Energy Audit Tool
Estimate annual energy savings, payback period, NPV and CO₂ reduction from a building energy improvement.
Results
Enter values and click Calculate
Results will appear here
Fill in the inputs and press Calculate
🧮 Building Energy Audit Tool — Formula
📐 Based on ASHRAE standards and Manual J/N methodology. Full load study required for equipment selection.
📌 Code Reference & Standard
Applied Standard
ASHRAE 90.1, ISO 50001, IPMVP
Disclaimer
For preliminary & reference use only. Final designs must be reviewed by a licensed Professional Engineer per applicable local codes.
📊 Quick Reference
| Input / Parameter | Description | Example Value |
|---|---|---|
| Floor Area | Building or zone conditioned area (m² or ft²) | 450 m² |
| Cooling Load Density | Typical: 80–150 W/m² office, 100–200 W/m² retail | 120 W/m² |
| Supply Air Temp (Ts) | AHU supply air temperature (°C or °F) | 12°C |
| Return Air Temp (Tr) | Space return temperature (°C or °F) | 24°C |
| Duct Velocity | Main duct: 5–10 m/s, branch: 3–6 m/s | 7 m/s (main) |
| COP / EER | Efficiency: chiller COP 3–6, split AC EER 3–4 | COP = 4.2 |
| Output | Cooling load (kW), duct size (mm), airflow (L/s) | 54 kW / 900 mm duct |
ℹ️ About This Calculator
The Building Energy Audit Tool helps mechanical engineers, HVAC designers, and building services consultants calculate cooling and heating loads, size ductwork and piping, evaluate equipment efficiency, and assess building energy performance. Inputs include building dimensions, insulation U-values, occupancy and equipment gains, climate design conditions, and system operating parameters; outputs include thermal loads in kW or BTU/hr, duct sizes, flow rates, COP/EER values, and annual energy consumption estimates.
These tools implement ASHRAE engineering fundamentals: the sensible heat equation (Qs = ṁ × Cp × ΔT), psychrometric relationships for mixed-air and dehumidification calculations, the LMTD method for heat exchanger sizing, and the Manual J/N load calculation methodology for residential and commercial systems respectively. Duct sizing follows the ASHRAE equal-friction method targeting 0.8–1.2 Pa/m pressure drop. Ventilation rates reference ASHRAE 62.1 Table 6.2.2 values. U-value calculations follow ISO 6946 and ASHRAE 90.1 procedures for building envelope thermal resistance.
Important limitations of these preliminary tools: accurate cooling and heating load calculation requires detailed building envelope modelling with current local TMY3 (Typical Meteorological Year) weather data, precise solar orientation analysis, thermal mass effects, infiltration path modelling, and occupancy diversity factors. These tools provide simplified estimates that may be ±20–40% compared to a full ASHRAE detailed calculation. Equipment performance degrades at part-load conditions not captured in design-point calculations — a chiller rated at COP 6.0 at design may achieve only COP 3.0 at 25% load.
These tools serve HVAC engineers performing preliminary equipment sizing for budget development, energy auditors estimating building loads for audit reports, contractors providing indicative quotes before detailed engineering is completed, facility managers assessing equipment replacement options, and building services students learning thermal engineering principles and ASHRAE calculation methods.
Full HVAC system design requires a detailed Manual J (residential) or ASHRAE/CIBSE load calculation performed by a licensed Mechanical Engineer using current local climate data, complete building envelope specifications, and accurate internal load data. Equipment must be commissioned, balanced, and tested after installation, and regulatory compliance with energy codes (ASHRAE 90.1, Title 24, Part L, NZBC H1) requires documented energy analysis, not preliminary estimates.
All calculations run in your browser only. No building geometry data, energy consumption figures, equipment specifications, or project details are transmitted to any server or stored in any database.
📋 How to Use This Calculator
- 1
Collect building envelope data
Enter floor area, ceiling height, wall and roof U-values (or R-values), window area and glazing properties, and building orientation. For existing buildings, this information comes from as-built drawings or on-site measurement.
- 2
Set climate and occupancy inputs
Enter outdoor design temperature (from ASHRAE Fundamentals climate data for your location), number of occupants, equipment heat gains, and lighting power density. Internal gains can contribute 20–50% of the total cooling load.
- 3
Specify HVAC system parameters
Enter supply air temperature, chilled water supply/return temperatures, or duct dimensions as required. For duct sizing, enter the design airflow rate in L/s or CFM and select the equal-friction or constant-velocity design method.
- 4
Calculate and size equipment
Click Calculate to get cooling or heating load (kW or BTU/hr), duct velocity, or equipment COP/EER. Equipment capacity should be sized at 90–100% of peak calculated load — oversizing degrades part-load efficiency and humidity control.
- 5
Validate with a full load study
These preliminary calculations should be validated with a full Manual J (residential) or ASHRAE 90.1-compliant energy model (commercial) before equipment procurement. Engage a licensed HVAC engineer for final equipment specification.
🎯 When to Use This Calculator
Building HVAC load estimation
Estimate cooling and heating loads for office, retail, or industrial buildings at the concept stage to select approximate equipment capacity.
Duct sizing and air distribution
Size main and branch ducts for equal-friction design to ensure balanced airflow distribution and acceptable duct velocity across all zones.
Energy efficiency evaluation
Evaluate COP, EER, and SEER values when comparing equipment alternatives, and calculate annual energy savings from upgrading to higher-efficiency units.
Chilled water system design
Size chilled water pipes, calculate flow rates, and determine pump head requirements for central plant distribution systems in large buildings.
Building energy audit
Estimate building thermal performance and identify envelope improvements (insulation, glazing, air sealing) that offer the greatest energy savings per dollar invested.
💡 Engineering Pro Tips
Rule-of-thumb cooling load estimates (W/m² or BTU/ft²) can be off by ±50% depending on glazing area, building orientation, and occupancy density. A south-facing open-plan office with floor-to-ceiling glazing can have 3–4× the cooling load of a well-insulated north-facing space of the same floor area. Never apply simplified rules-of-thumb to glazing-dominated buildings.
Duct leakage is the largest single source of HVAC system energy waste — ASHRAE 90.1 allows no more than 4% leakage, but field measurements routinely find 10–30%. Specify sealed, tested ductwork (SMACNA leakage Class A or B) in contract documents and require post-installation duct leakage testing before system commissioning.
Variable Air Volume (VAV) systems save 30–60% in fan energy compared to constant-volume systems because fan power scales with the cube of airspeed. Reducing airflow to 70% of design reduces fan power to 34% (0.7³). Variable frequency drives (VFDs) on supply and return fans are essential to realise these energy savings.
Thermal comfort depends on six factors: air temperature, mean radiant temperature, air velocity, humidity, activity level, and clothing — not just air temperature alone. Chilled ceiling or radiant systems can achieve equivalent comfort at 26°C instead of 22°C by reducing mean radiant temperature, saving 10–15% cooling energy over a conventional all-air system.
⚠️ Engineering Disclaimer
Results are intended for preliminary design and educational purposes only. All calculations must be verified by a licensed Professional Engineer (PE) before use in any construction, manufacturing, or safety-critical application. Local codes, material standards, and site conditions may vary significantly.
❓ Frequently Asked Questions
More HVAC & Building Services Tools
Cooling Load Calculator (HVAC)
Calculate cooling load in BTU/hr for rooms and buildings using heat gain analysis.
Heating Load Estimator
Calculate building heat loss and heating requirements using degree-day method.
Duct Sizing Calculator
Size HVAC ducts using velocity and static pressure methods for air distribution.
Psychrometric Calculator
Calculate dew point, enthalpy, humidity ratio and wet bulb temperature from air conditions.
Energy Efficiency (COP / EER)
Calculate COP and EER cooling efficiency metrics for HVAC equipment.
U-Value / R-Value Calculator
Calculate thermal resistance (R-value) and transmittance (U-value) for building insulation.
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